73-87chevytrucks.com

Technical Forum (Read Only) => Instrumentation => Topic started by: bd on January 18, 2013, 01:55:22 AM

Title: Functional Tests of Factory GM Electric Gauges
Post by: bd on January 18, 2013, 01:55:22 AM
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 (http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.msg252586#msg252586) 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:
---------------------
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

TEST PROCEDURE - SENDING UNITS


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 (http://forum.73-87chevytrucks.com/smforum/index.php?topic=27971.msg233013#msg233013) [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.

---------------------
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)

Title: Functional Tests of Factory GM Electric Gauges
Post by: bd on August 15, 2017, 12:07:14 PM
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).


(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=37117.0;attach=50775;image)
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 * * *

---------------------
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.
Title: Re: Functional Tests of Factory GM Electric Gauges
Post by: VileZambonie on August 15, 2017, 12:13:40 PM
Amazing write up and highly detailed reference document. Thank you for your hard work on this.