Functional Tests of Factory GM Electric Gauges

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FUNCTIONAL TESTS OF FACTORY GM ELECTRIC GAUGES

by bd

     The procedure described below will functionally test OE analog gauges, verify gauge calibration, and help diagnose sending unit problems.  It will not, however, substitute for skilled voltage drop measurements, methodic inspection of connections, tracing circuitry, or pinpointing short circuits!  This procedure cannot effectively test an unpowered or improperly grounded gauge circuit; therefore, it is strongly recommended that gauge power and ground be fully verified if the procedure indicates that the gauge under test does not function correctly.  Refer to the Wiring Manuals in the 73-87(91) GM Pickup Manuals thread located in the General Information and Maintenance section of the Technical Forum for model-specific wiring schematics to aid your diagnosis.

Important Usage Notes:

• All sender resistance specifications are exact values expressed in ohms (Ω).
  
  
• Safegaurd your meter — when using an ohmmeter to measure the resistance of any circuit component, make absolutely sure the circuit is powered OFF before and while the meter is connected!
  
  
• Practical meter skills, a decent multi-scale volt-ohm-milliammeter (VOM), digital volt-ohmmeter (DVOM), or digital multi-meter (DMM) and an instrument panel gauge tester or variable resistor (potentiometer - Fig. 1) are needed to perform this procedure.
  
  
• What do you do if you don’t have access to an instrument panel gauge tester?  Build one....
  
  For nominal cost, a useful tool can be constructed that will test any OEM electric gauge in GM trucks that requires an external sending unit (circa 1967 to ~1990).  Construction is simple using a few components obtainable from any reasonably stocked electronics supply.
  
  "Basic" gauge tester:  To test gauges only rarely, connect a 2,200 ohm (2.2K Ω) resistor in parallel^(A) with a 5,000 ohm (5K Ω) potentiometer^(B) (aka, pot), effectively creating a zero to ~1,500 ohm (1.5K Ω) variable resistor.  Specifically, connect a 2.2K Ω resistor between the two outside terminals of a 5K Ω linear taper pot^(C), then jump the center terminal of the 5K Ω pot to one of its outside terminals (Fig. 2).  Attach 3-foot-long, 18-gauge test leads to the center terminal and to the ‘un-jumpered’ outside terminal of the pot.  Solder the electrical connections using 60/40 rosin core solder.
  
  
  

  Figure 1.  Basic potentiometer.	Figure 2.  Recalibrated potentiometer.

  
  
  "Improved" tester:  If you anticipate testing many gauges, you can augment the precision of the 'basic' tester by connecting the ‘1.5K Ω pot’ in series^(D) with a 100 Ω pot and even a supplemental 25 Ω pot.  To safeguard against potential electrical overloads, a fuse or circuit breaker can be incorporated.  Just as with the basic tester, jump the center terminal of the 100 Ω linear taper pot to one of its outside terminals; repeat with the 25 Ω pot.  Mount the pots (and optional fuse) in a small plastic project box.  Chain the two (or three) pots together, end-to-end.  Connect one 3-foot-long, 18-gauge test lead to each of the endmost pots, OR, if installing an optional fuse, jump the lowest value pot to the fuse then connect the test leads to the fuse and the 1.5K Ω pot (Fig. 3).
  
  
  

  Figure 3.  Tester internal wiring.

  
  
  Run the test leads through a grommet in the side of the box and add an alligator clip to the free end of each test lead.  Solder the connections and finish assembly (Fig. 4).
  
  
  

  Figure 4.  Completed gauge tester.

  
  
  The 1.5K Ω pot is used for coarse adjustment of the tester’s total resistance, while the 100 Ω (and optional 25 Ω) pot is/are used for fine adjustment.  Paired with a sensitive ohmmeter, the tester will effectively substitute in place of the specialized professional tools typically specified by the OEM (Fig. 5).
  
  
  

  Figure 5.  Commercially available gauge tester.

  
  
  The schematic wiring diagram for the gauge tester is illustrated in Figure 6.
  
  
  

  Figure 6.  Gauge tester schematic wiring diagram.

  
  
  
• In a pinch, a dash lamp rheostat will aptly serve as a variable resistance for testing the fuel gauge, only... Testing for an inoperative fuel gauge sending unit.
  

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^A parallel - electrical components that share connection points in such a way that current flow "divides" and flows through each component independently.  A parallel configuration is a powered electrical circuit element in which current flowing along a single path enters a junction, where it splits proportionally into two or more paths with each path carrying less than 100% of the total current, before recombining to flow along a subsequent single current path.  Each parallel-connected component carries a unique portion of the total current, so that all parallel components when added together, carry 100% of the total current.  This proportional divvying of current occurs, because the same voltage is applied across each of the ‘parallel connected’ components.  ...To reiterate:  when one path separates into multiple paths, that then recombine into one path, the multiple paths are said to be connected ‘in parallel’....
^B potentiometer - a type of variable resistor possessing three distinct terminals for wire connections.  Potentiometers are adjustable from a minimum value of about zero ohms up to a maximum value of 100 ohms or more (often, much more).  Potentiometers generally are used to control voltage.  However, as connected in the ‘gauge tester,’ both pots are wired to control current flow through the gauge circuit under test.
^C linear taper pot - a type of potentiometer with a resistance value that varies at a continuous rate of adjustment.
^D series - a powered electrical circuit element in which components are connected end-to-end such that there is a single current path so that each component must pass 100% of the total current.

TEST PROCEDURE - GAUGES

• Using an ohmmeter set to the appropriate scale, "zero" the ohmmeter if the meter requires it (a necessary step with analog meters).  Then, referring to the gauge-specific parameters listed below for the vehicle year, with the ohmmeter connected to the tester leads, adjust the tester to the stated resistance for testing the 'Low' index line of the gauge to be evaluated.
  
  
• Once adjusted, substitute the tester for the gauge sending unit by unplugging the wiring harness from the sender, connecting one lead of the tester to the sender wire, and the second lead of the tester to a verified good ground.  Turn the ignition ON.  The gauge needle should point to the lowest reading (leftmost index line) on the gauge face.
  
  
• Similarly, test the 'Mid-range' and 'High' index lines of the gauge by resetting the tester resistance according to the appropriate parameter.  Note that temperature senders are not linear.
   
  
• A correctly reading gauge indicates there are no problems with the gauge or its wiring, limiting any faults to the respective sending unit.
  
  
• An incorrectly reading gauge calls for further testing and verification of 12-volt ignition power, grounds, connections, and associated wiring.
  

TEST PROCEDURE - SENDING UNITS

• The subsequent parameters provide precise sender resistance values for three specific gauge readings (Low, Medium, and High), hence, can be used to estimate sender resistance when actual physical parameters are known (e.g., engine temperature that is verified with a thermometer, actual fuel tank level, engine oil pressure that is verified with a manual gauge, etc).  Therefore, a sending unit’s resistance can be measured and “subjectively” compared to its interpolated value between ‘Low,’ ‘Medium’ and ‘High,’ based on the parameters and the observable/verifiable physical conditions that actually exist on the vehicle.
  

COOLANT TEMPERATURE GAUGE PARAMETERS

For 1967 - 1987 (91) vintage trucks, engine coolant temperature gauge senders were manufactured based on a negative temperature coefficient (sender resistance is indirectly proportional to temperature).  That is, as temperature increases, sender resistance decreases.  Temperature scale marks are nonlinear.  The center mark is ~80% of full-scale temperature.

For Truck Years:  1967 - 1973

The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω


For Truck Years:  1974 - 1978

The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

See the topic, Correct temperature sender and connector for 1973-1978 C/K trucks [sic].


For Truck Years:  1979 - 1987 (91)

The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω

OIL PRESSURE GAUGE PARAMETERS

For Truck Years:  pre-1978

Prior to 1978, factory oil pressure gauges were of the mechanical direct reading type; OEM electric gauges were not yet available. 


For 1978 - 1987 (91) vintage trucks, engine oil pressure gauge senders were manufactured based on a positive pressure coefficient (sender resistance is directly proportional to pressure).  That is, as pressure increases, sender resistance increases.  Pressure scale marks are linear.  The center mark is ~50% of full-scale pressure.

For Truck Years:  1978 - 1987 (91)

The Oil Pressure Gauge Needle Should Point to:
Left Line (Low, 0 PSI) when sender resistance = ~0 Ω
Middle Line (30, 40, or 50 PSI) when sender resistance = 30 Ω
Right Line (High, 60, 80, or 100 PSI) when sender resistance = 60 Ω

Note:  although factory oil pressure sending units were available calibrated to 60 PSI, 80 PSI and 100 PSI gauges, 60 PSI was the most common calibration - 80 PSI and 100 PSI calibrations generally were restricted to select diesel applications.  To determine which sender you have, the pressure calibration is stamped into the hex nut at the base of the OEM sender adjacent to the mounting threads.  Regardless of pressure calibration, all oil pressure senders have the same resistance values, as indicated above.

FUEL LEVEL GAUGE PARAMETERS

For 1967 - 1987 (91) vintage trucks, fuel level gauge senders were manufactured based on a positive fuel level coefficient (sender resistance is directly proportional to fuel level).  That is, as the fuel level rises, sender resistance increases.

For Truck Years:  1967 - 1987 (91)

The Fuel Gauge Needle Should Point to:
Left Line (Empty) when sender resistance = ~1 Ω
Middle Line (Half) when sender resistance = 44 Ω
Right Line (Full) when sender resistance = 88 Ω

BATTERY / AMP GAUGE / AMMETER

For Truck Years:  1967 - 1975

The factory ammeter is a direct sensing shunt type^(E) – meaning that only a small portion of the total charging and load current actually flows through the dash wiring and the ammeter.  The majority of current bypasses the ammeter via the heavy-gauge battery/charging system wire (the shunt), which is electrically connected in parallel with the ammeter.  There is no external sender for the ammeter.  Calibration of the ammeter is dependent on the relative length and diameter (wire gauge) of the ‘shunt’ harness with respect to the ammeter harness.

The factory ammeter is over-current protected by one or two 20-gauge fusible links connected in series with two fast-acting fuses, one on either side of the ammeter.  You might ask, "Why two fuses and fusible links?"  Since both posts of the ammeter are connected to B+ power, if the ammeter or its dedicated wiring between the two fuses becomes compromised to ground, both connections to power must be severed to help safeguard from an electrical fire, hence, two fuses.  The fusible link protects the short-length ammeter feed wire connecting the starter solenoid B+ post to one of the two ammeter fuses.

Normal Ammeter Function:  Observing the ammeter needle at rest over its zero or middle mark (halfway between "D" and "C") is normal if every electrical load, including the engine, is switched OFF and there is no electrical activity.  An ammeter needle resting at the middle mark indicates that either zero current is flowing, or that battery load is exactly balanced by battery charging.  Generally, while the engine is running, the ammeter needle should deflect toward "C" (Charge), if only slightly.  The amount it deflects depends directly and entirely on the current output of the alternator, balanced against the electrical load and battery state of charge at the time.  With the engine OFF and any electrical device switched ON, the ammeter should deflect toward "D" (Discharge).  The amount of deflection depends on the amount of current drawn from the battery and the sensitivity of the ammeter.  With zero current flow, the ammeter needle should always return to its centered (neutral) position.  The high-current cranking circuit of the starter is universally excluded from ammeter registration.

Ammeter Diagnosis:  If the ammeter needle never moves, remove and inspect/test the two 4-amp fuses located either in the fuse box or in the engine compartment wiring harness in discrete inline fuse holders.  Both fuses must be removed for evaluation because both are connected to power and wired in series through the ammeter.  After both fuses are removed, make sure that 12 volts are available to both fuse locations.  If both fuses receive power, the fuses are good, and there is no corrosion on the fuse box/inline holder terminals, then the ammeter circuit between the two fuses may be "open."  Reinstall either one of the two fuses and check for power at both fuse box/inline holder terminals for the fuse that is still uninstalled.  Power at only one of the fuse clips indicates an 'open' in the ammeter or dash wiring connected to the 'dead' side (e.g., tarnished connections at the PCB, a broken wire or instrument cluster circuit foil, a faulty gauge, poor continuity through the firewall bulkhead plug, etc).  If you measure 12 volts at both fuse clips, then there may be a gauge calibration issue.

Ammeter Go-No-Go Bench Evaluation:  The factory ammeter has an internal resistance of much less than one ohm, which is too minuscule to evaluate using a typical ohmmeter.  As fortune would have it, momentarily connecting a fresh 1.5-volt battery (e.g., AA, C, or D cell) across the two posts of a good OEM ammeter will deflect the meter needle 90° CW or CCW, depending on the polarity of connection, from the zero or middle mark of the gauge scale.  This fundamental procedure serves as a simple and somewhat reliable go-no-go "test" of the ammeter proper while it is out of the vehicle, laying on a workbench.

Ammeter Calibration:  The factory ammeter displays full-scale needle deflection when the needle points directly at "D" or "C" with merely 1 ampere of current flowing through the ammeter at 0.3-volt drop, yielding an internal ammeter resistance of 0.3 ohm calculated using Ohm's Law.  Chronically driving the ammeter past 1.5 amps and 0.45-volt drop (~85° of needle deflection from the middle mark) risks ammeter damage.  Recalibrating the ammeter can be accomplished by altering the length and/or wire gauge of the main charge harness so that no more than 1-1.5 amperes will ever flow through the ammeter.  For example, a 40-amp charging system dictates that the main harness shunts up to 39 amps relative to the ammeter, which must carry no more than 1.5 amps at the maximum charging system output.  A 100-amp charging system requires that the main harness carries up to 99 amps relative to the maximum of 1.5 amps through the ammeter.   

If the ammeter is overly sensitive, that is, regularly exhibiting extreme needle deflections, an alternative to manipulating the wire gauge and length of the charge harness 'shunt' may be to incorporate a very small supplemental resistance in series with the ammeter.  Often, adding resistance can be accomplished by simply increasing the length of one of the 20-gauge fusible links that feed the ammeter.  Regardless of the approach taken, recalibrating a shunt-type ammeter can be a tedious and costly trial-and-error process.

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^E shunt - electrically connected in parallel (such as two distinct circuit paths that share the same beginning and end points, so that the burden of the total current flow is shared, but divided proportionately between the two circuit paths).

VOLT GAUGE / VOLTMETER

For Truck Years:  1976 - 1987 (91)

The instrument cluster voltmeter is a direct sensing gauge electrically connected between 12-volt ignition and cab ground using only two of the three gauge pins.  There is no external sender for the volt gauge.  Verify 12-volt ignition power, ground, and gauge connections to the flexible printed circuit using a handheld voltmeter and test light to determine the viability of the gauge and its circuitry.  Correct accordingly.


     (continued below)

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CLOCK

For Truck Years:  1973 - 1987 (91)

Two types of clock, both optional, were used in the 1973 - 1987 (91) light-duty trucks.  The factory conventional (non-quartz) clock is an electro-mechanical chronometer consisting of a simple mainspring and gear mechanism supported in a stationary frame, a pulsed solenoid used to wind the mainspring, and a set of traveling contact points (momentary switch) used to periodically energize and pulse the solenoid when the mainspring winds down.  The mainspring drives the conventional clock mechanism at a continuous rate, independent of the periodic operation of the solenoid.  In contrast, the motor in a quartz clock is also pulsed but at a precise frequency controlled by the uniform vibration of a piezoelectric quartz crystal.  The pulse motor of the quartz clock drives the clock mechanism in discrete digital steps, directly, without the aid of a mainspring. 

The only electrical connections to the clock are fused constant battery power (B+) and ground.  Both clock types share the same connector configuration (Fig. 7).

Figure 7.  Clock electrical terminals identification.

Generally, clocks fail either when the drive mechanism binds from an excess accumulation of dust and/or corrosion, the lubricant has thickened excessively, or when the solenoid contact points of the conventional type clock burn and/or fuse together.  Symptoms of clock failure range from erratic timekeeping to permanent stalling, to blowing the clock fuse, to a dead chassis battery.

* * * APPLICATION NOTES * * *

• Good ground connections are crucial to every properly functioning electrical system.  ‘Ground’ is one-half of virtually every automotive electrical circuit and bears importance equal to that of the battery, wiring, wire connections, electrical controls, components and appliances.  Ground is the electrical baseline to which every electrical component is referenced.  If an error occurs in that baseline, proper function is broadly compromised.
  
  A poor ground connection can wreak havoc with electrical functions, especially when the ground connection is shared by more than one circuit.  If a multi-circuit ground wire loses its connection to battery negative through the body, frame or cabling, all connected circuits sharing that ground will attempt to complete through alternate paths that were never intended.  The result is obscure symptoms, as interconnected circuits back-feed one another.  To summarize, never assume ground connections are sound - always verify them!  Reference: Vehicle Ground
  
  
• Referring to the fuel gauge parameters, above, notice that the resistance of the fuel tank sender gradually increases to about 90 Ω as the fuel gauge needle swings toward "Full."  Logically, if the resistance of the fuel tank sender circuit increases above 90 Ω, the fuel gauge needle will continue to swing past ‘Full.’  As it happens, when the fuel sender circuit develops an "open" (which is a loss of electrical connection or continuity, such as a disconnected or broken sending unit wire, a worn out or corroded sender, or a severed sender ground connection (i.e., infinite resistance)), the fuel gauge needle swings well past ‘Full’ and finally settles, pointing roughly toward the 3:00 o’clock position. 
  
  Conversely, if the fuel sender wiring between the sender and the gauge "shorts to ground," the fuel gauge needle swings to "Empty."  So, an ‘open’ in the fuel gauge sender wiring, anywhere between the dash gauge and the sender ground connection to the frame, will cause the fuel gauge to pin to the right, past "Full."  Whereas, ‘grounded’ sender wiring anywhere between the fuel gauge and the tank sender will force the fuel gauge to continuously indicate "Empty."
  
  
• As a sidebar, troublesome corrosion of automotive fuel tank senders and accelerated deterioration of rubber and plastic fuel system materials have become commonplace since the introduction of alcohol into fuel formulations.  Be prepared for it.
  
  
• Wild fuel gauge needle fluctuations, which are coincident with fuel sloshing inside the tank(s), such as occurs when accelerating, braking, turning or traversing rough terrains, may be due to a loss of fuel gauge mechanical dampening.  The OEM fuel gauge needle is affixed to a rotating axle.  The axle is suspended in a support bearing that is anchored to the frame of the gauge.  As delivered from the factory, the axle bearing is charged with a viscous silicone or fluorocarbon gel dampening fluid (imagine a super tacky, clear, greaselike substance that exhibits ultra-high shear strength).  Dampening (aka, damping) fluid is used to impose a rotational drag on the fuel gauge axle in order to retard the needle deflection rate, which effectively impedes erratic swings of the needle.  Eventually, the original fluid dissipates and loses its effectiveness at which point the gauge needle dances in sync with fuel surging inside the tank.  Watch the linked YouTube video comparison of a used vs new fuel gauge.  The simple solution is to replace the gauge with an OEM equivalent new part.  An alternate solution is to recharge the dampening fluid using a syringe and hollow needle, assuming that you can locate a source for suitable fluid that exhibits the correct properties including a viscosity in the neighborhood of ~30,000+ centistokes (cSt).
  
  
• The diagnostic logic that applies to the fuel gauge, similarly applies to the oil pressure gauge.  An oil pressure gauge that pins to the right, past maximum reading, is quite often caused by an "open" somewhere in the sender circuit between the oil gauge pin and sender ground.  Whereas, a "grounded" oil gauge sender lead will result in a continuous "0 PSI" indication.  An "erratic" oil gauge needle often results from a failing sender, but can also be a sign of connection issues.
  
  
• Inverse logic applies to temperature gauge diagnosis because temperature sender resistance values are reversed.  An "open" in a temperature gauge circuit forces the gauge needle toward "Cold."  A "grounded" temperature sender lead will pin the needle past maximum temperature.
  
  
• The coolant temperature, oil pressure, and fuel level gauges require three electrical connections to function properly: 12-volt Ignition, Ground, and Sender.  12-volt Ignition and Ground are constant, providing a fixed reference voltage and current to each gauge.  But, Sender is actually a variable ground connection through the respective sending unit.  Pitted against fixed Ignition, Sender provides varying voltage and current to the gauge.  The magnetic interaction between ‘fixed’ and ‘variable’ currents within the gauge, forces the gauge needle to point to a specific reading on the gauge face.  If any one or more of the three gauge connections are compromised, the gauge will not register correctly.
  
  
• The ammeter and voltmeter each require two electrical connections to function properly.  The ammeter requires two battery connections, separately sourced from the engine compartment harness.  The voltmeter requires 12-volt ignition and ground sourced from within the cab.
  
  
• The factory tachometer (tach), though beyond the scope of this reference, requires three electrical connections to function properly: 12-volt Ignition, Ground and Distributor Primary Pulse Signal (ignition coil negative connection) to function.  The signal lead of the tach may incorporate an electronic filter to stabilize tach needle motion and decrease the introduction of electrical noise into the vehicle's primary wiring.
  
  
• The instrument cluster gauges (excluding speedometers, clocks and tachometers) establish electrical connection with the flexible printed circuit board (PCB) through shiny metal spring clips (Fig. 8) that snap through the back of the instrument cluster housing.  The PCB and clips are notorious for accumulating dust and tarnish to the point of degrading electrical connections, while the spring clips have a tendency to lose tension over time, degrading connections even further.  Hence, anytime the cluster is disassembled for gauge service, or new gauges are installed, the clips should be burnished along with the PCB where the clips make contact.  But, be careful - the copper foil of the PCB can peel easily from its plastic backing.  A soft pencil eraser or crocus cloth works well for burnishing the copper foil.  Once the spring clips have been removed from the housing, they can be gently reformed if necessary to increase contact pressure with both the PCB and gauge pins.  Furthermore, inspect and burnish the connections between the wiring harness plug and PCB.
  
  
  

  Figure 8.  Instrument cluster clips (obsolete GM P/N-8986000).

  
  
  
• Excluding the speedometer and conventional OE clocks, none of the OEM instrument cluster gauges contain springs to return the gauge needles to specific "at rest" positions.  With ignition power switched off, gauge needles will gradually and independently drift to 'at rest' positions that are governed by residual magnetism retained by the gauge coils balanced against minuscule asymmetric mass distributions around the gauge axles, which, however insignificantly, may influence the axles by imposing slight torque such that off-center masses naturally tend to become oriented downward.
  
  
• OEM gauges commonly have a ceramic film resistor installed between two of the gauge pins protruding from the back of the gauges (Fig. 9).  The resistors are color coded and vary in value by application, but seem to hover around 90 Ω ± perhaps 10 Ω or ~11%.  The resistor function is to slew^(F) the gauge for calibration purposes.  Typically, slewing resistors dissipate between ¹/₂ and 3 watts of heat energy depending on the relative resistance of the associated sender, which corresponds to thermal radiation in the range of 90° - 130° F.  In other words, the resistors become quite hot!  Don't be alarmed by this; it is a normal function of the gauge assembly.
  
  
  

  Figure 9.  Ceramic film slewing resistor.

  
  
  
• Factory gauges are not precision instruments!  Accuracy is in the range of about ±7% by personal observation.  Hence, the procedure described herein for any particular gauge is intended as a “Pass-Fail” indication rather than to determine precision calibration.  Up to ~7% variance is acceptable and should be anticipated.
  
  
• I cannot overstress the fact that operating circuits are dynamic environments subject to vibration, chafing, extreme variations in temperature, mechanical connections and oxidation!  Circuits incorporate a power source, individual control components, and a load (the primary appliance the circuit supports), all interconnected by wire and/or printed conductor pathways using a slew^(G) of mechanical connections, any one or combination of which can introduce unwanted resistance, not-to-mention unintended intermittent breaks in continuity, short circuits and grounds, all of which interfere with proper circuit function.
  
  When evaluating gauge circuits, it is anticipated that the diagnostician will apply the Functional Test procedure described herein to individual gauges while they are installed in the vehicle.  This approach evaluates on a “Pass-Fail” basis ~98% of the complete circuit from the point of connection with the sender, through the bulkhead connector, dash harness and PC board, while simultaneously verifying the gauge circuit from the battery positive connection, through the ignition switch, fuses, dash harness, and gauge, all the way back to the battery negative connection via the cab and engine ground path.  In other words, if a gauge “Passes” an in-vehicle test, it is reasonable to assume the complete circuit, including power and ground to the gauge, is functioning as designed, leaving the sender and sender ground as the only "untested" variables (the remaining ~2% of the circuit).
  
  If, on the other hand, the gauge “Fails” its in-vehicle test, you will need to physically verify every harness connection between the sender connector and the gauge, ignition power into the gauge, the gauge ground all the way back to battery negative, and the ground connection of the substitute test sender (the test resistance temporarily installed between the sender wire and vehicle ground), before condemning the gauge with any degree of reliable precision.  Thus, the Functional Test procedure serves as a “Pass-Fail” evaluation of the entire working circuit, less the sender.  If a fault is indicated by the test, additional diagnostic steps become essential to identifying the actual cause of the failure.
  
  Of course, testing individual gauges can also be performed on the bench, but in so doing you will bypass the “dynamic environment” in which each gauge normally functions.  Still... testing on the bench when paired with redundant testing in the vehicle has merit and can reveal gauge condition as well as the effective net condition of the actual working environment in which the gauge functions.  But, using this approach routinely will involve unnecessary extra steps more often than not, since it does not identify the actual point of trouble, but merely identifies whether the gauge is adding to the problem.
  
  
• If a gauge failure is indicated by the Functional Test, be prepared to refine your diagnosis with the appropriate voltage drop and resistance measurements to isolate the cause.  The Fundamental Test is simply a guidepost to help you choose the appropriate diagnostic direction.  It is not an end-all evaluation... rather, it is one step of many.
  
  
• Diagnostic tests should NEVER be assumed or expected to be foolproof!  Every diagnostic procedure has associated pitfalls.  The majority of diagnostic difficulties result from either making unnoticed assumptions or simply not understanding adequately the mechanics of a properly functioning system (within the context of this article, a correctly functioning circuit).  To avoid even the simplest pitfalls demands that a diagnostician is suitably familiar with the system (s)he is diagnosing, can aptly apply the precepts of Ohm’s Law, and routinely demonstrates an expert selection and application of the correct meter for the specific job at hand.  Learn to read and then use the applicable wiring diagram(s).  Study to understand the intimate relationship between voltage, current and resistance.  And practice using your meter with the knowledge gained from applying Ohm’s Law.
  
  
• Ohm's Law is the exact, if dynamic, relationship between voltage, current and resistance with regard to DC (direct current) circuits.  Mathematically, it is expressed as:
  
  
  E=IR ,
  
                 where E is the electromotive force measured in volts, I is the intensity of current flow measured in amperes, and R is the electrical resistance to current flow measured in ohms.
  

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^F slew > verb - to move or shift sideways.  A slewing resistor alters the response curve of a gauge by shifting sensitivity toward one end of the gauge scale.  This can be done to improve scale linearity or, conversely, to asymmetrically weight the gauge indication toward the high or low end of the gauge scale.
^G slew > noun - many, or a lot of something.




The proper gradient:

Curiosity, or a purpose to know, begs for study.  Study lacks from any absence of practice since practice is applied technique.  Application brings experience; experience develops knowledge; knowledge builds confidence; confidence ushers judgment; and judgment blends the ability to predict with keenly developed diagnostic acuity.  Yet, it all begins with curiosity and one's purpose to know. - BD




Grateful acknowledgment is expressed to VileZambonie for his constructive critique during the preparation of this reference.

[VileZambonie]

Amazing write up and highly detailed reference document. Thank you for your hard work on this.