DIAGNOSING INOPERATIVE DASH INSTRUMENT LIGHTING
by bd
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Refer to the appropriate year OEM wiring manual (http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.msg252586#msg252586) in the Technical Forum for greater detail that is specific to your particular vehicle.
Switch the running lights ON and rotate the dash instrument lamp dimmer rheostat all the way counterclockwise for all of the subsequent tests. For the greatest benefit, follow the diagnostic procedure in the order presented. DO NOT jump around or back-and-forth within the procedure!
Use an incandescent test light clipped to a verified good ground to probe both sides of the Instrument Lamps (INSTR LPS or INST LPS) fuse.
- If both sides of the INST LPS fuse are 'hot,' probe the GRAY wire at the instrument panel (I/P) harness connector using the test light:
- If the GRAY wire is 'dead' - verify that the INST LPS fuse is making good electrical connections to both of its socket contacts in the fuse block (see Supplemental Information below), then locate and repair the open in the GRAY wire between the INST LPS fuse and the I/P harness connector.
- If the GRAY wire is 'hot' - verify that all of the dash bulbs are good, check the electrical connection between the I/P harness connector and the instrument cluster printed circuit board (PCB), and check the instrument cluster ground connections provided by the BLACK wire(s) running between the I/P harness connector and the cab sheet metal or park brake pedal bracket at the left kick panel. [Note: perfect ground connections are crucial to the proper function of any electrically powered appliance. Read the technical article Vehicle Ground (http://forum.73-87chevytrucks.com/smforum/index.php?topic=35756.msg301879#msg301879) for explanation and details.] Using a soft pencil eraser, gently burnish the I/P harness connector and dash lamp socket connections to the PCB, being very careful to avoid peeling the copper foil loose from the flexible PCB backing - an all too easy mistake to make.
- If only one side of the INST LPS fuse is 'hot,' replace the INST LPS fuse and locate the short-to-ground in the GRAY wire running between the fuse block and the PCB. Make sure the PCB is not inadvertently grounding to bare metal edges behind the dash. Note that shorts-to-ground can be intermittent and difficult to locate in cases where the wiring subtly shifts due to expansion and contraction from ambient temperature changes and vehicle vibration or if the wiring becomes chafed or momentarily pinched by mechanical levers and linkages that move behind the dash.
- If both sides of the INST LPS fuse are 'dead,' probe the DARK GREEN wire at the headlamp switch with a test light:
- If the DARK GREEN wire at the switch is 'hot' - verify that the INST LPS fuse is making good electrical connections to both of its terminals in the fuse block, then locate and repair the open in the DARK GREEN wire between the headlamp switch and the INST LPS fuse.
- If the DARK GREEN wire at the switch is 'dead' - probe the ORANGE wire(s) at the headlamp switch:
- If the ORANGE wire is 'hot' - check all wire connections to the headlamp switch and/or replace the headlamp switch, as indicated.
- If the ORANGE wire is 'dead' - probe both sides of the Tail/Courtesy Lamp (T/L CTSY or TAIL/CTSY) fuse in the fuse block:
- If both sides of the TAIL/CTSY fuse are 'hot' - verify that the TAIL/CTSY fuse is making good electrical connection in the fuse block, then locate and repair the open in the ORANGE wire between the TAIL/CTSY fuse and the headlamp switch connector.
- If only one side of the TAIL/CTSY fuse is 'hot' - replace the TAIL/CTSY fuse and check for shorts-to-ground in the wiring between the TAIL/CTSY fuse and the PCB. The "short" can exist anywhere between the fuse block, the headlamp switch, and the cluster PCB.
- If both sides of the TAIL/CTSY fuse are 'dead' - check electrical connections through the firewall bulkhead connector, as well as the fusible links connecting the 12-gauge RED wires to the firewall junction block in the engine compartment and the 3/8" battery cable stud on the starter solenoid - repair as needed.
* * * SUPPLEMENTAL INFORMATION * * *
Unseen oxidation lurking between a fuse and its fuse box contacts is an all too frequent and aggravating occurrence. Whenever power to or through a fuse is interrupted, be certain to check for loss of continuity stemming from hidden oxidation of the fuse-to-fuse block electrical connections. Even minor oxidation can interrupt continuity and effectively undermine the proper operation of lights and electric appliances. Because of this, when checking for electrical power it is good practice to probe the fuse socket contacts in the fuse block rather than the fuse alone. With regard to glass tube fuses, verify that the metal end caps of the fuse are not loose on the glass tube, which can result in internal separation of the fuse element from the loose end cap.
HEADLAMP SWITCH REMOVAL:
With the headlamp switch installed in the dash, the wire harness connections to the switch point upward. In order to access the switch connections, either the instrument cluster must be removed from the front of the dash (this is the preferred approach for an individual seeking to gain better access to the headlamp switch and wiring behind the dash) OR the headlamp switch must be removed from behind and dropped down beneath the dash for somewhat limited access.
As a precursor to headlamp switch removal, the switch knob must be released and separated from the switch. Begin by disconnecting the ground cable from the battery. To release the knob, pull it all the way out until it stops in the HEADLAMPS ON position. Next, reach under the dash and depress the small spring loaded button located on the underside of the switch body (following image).
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While depressing the knob release button, continue to pull the headlamp switch knob outward until it disengages and separates from the switch entirely. To gain free access to the headlamp switch retaining nut, remove the instrument panel bezel from the front of the dash. Once the bezel has been removed, unthread and remove the switch retaining nut. If the instrument cluster also was removed, pull the headlamp switch out through the cluster opening in the dash, otherwise, pull the switch rearward and down beneath the dash.
To reinstall the headlamp switch knob, insert the triangular steel shaft of the knob into the switch and push inward until the knob "snaps" into place and locks.
HEADLAMP SWITCH AND WIRE CONNECTIONS:
(coded to match the 1973 - 1987 (91) factory wire colors — factory wire sizes noted in blue)
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RED — fused battery power into the switch (12-gauge connection): constant battery power (B+) feed from one of the engine compartment fusible links into the headlamp switch to power the headlamps, only. A 16-amp, inaccessible and independently non-serviceable, Type 1 auto-reset circuit breaker is contained inside the headlamp switch for redundant circuit protection of the headlamps, only. Failure of the integral circuit breaker requires replacement of the complete headlamp switch assembly.
WHITE — switched ground (18-gauge connection): Full CCW rotation of the dash instrument lamp dimmer rheostat to its detent position completes the ground connection from the cabin interior dome and courtesy lamps, turning them on. The dome/courtesy lamp control switch portion of the headlamp switch is wired in parallel with the optional cabin entry lighting door jamb switches when present.
LIGHT BLUE (1980 and prior) or YELLOW (1981 and later) — power out of the switch (16-gauge connection): headlamp feed to the headlamp dimmer (high beam/low beam) switch.
DARK GREEN — power out of the switch (16-gauge connection): dash dimmer rheostat controlled feed to the instrument lamps fuse and dash lamps - the wire color leaving the instrument lamps fuse (between the fuse and the instrument cluster) is GRAY.
BROWN — power out of the switch (18- to 16-gauge connection): "A" - switched power feed to the standard equipment exterior running, tail and license plate lamps. "B" - switched power feed to the optional cab overhead clearance lamps. Often, a ~3" long BROWN pigtail that terminates in a single-wire, black, Packard 56 connector attaches to terminal "B" for the purpose of powering the optional cab clearance lamps.
ORANGE — fused battery power into the switch (18- to 16-gauge connection): B+ feed from the tail/courtesy lamps fuse into the headlamp switch to power the running, tail, dash instrument lamps and optional cab clearance lamps, only. A second ORANGE wire that shares the power-in terminal of the headlamp switch jumps to feed B+ power directly to the courtesy lamp(s) under the dash. The courtesy lamps illuminate along with the dome lamp by way of switched ground paths; see the description for the WHITE wire, above.
BLACK — ground (12-gauge connection): ensures effective grounding of the headlamp switch frame and proper function of the cabin interior dome/courtesy lamps override switch. The short black jumper wire, when installed, grounds the headlamp switch frame directly to the sheetmetal or park brake pedal bracket at the left kick panel, behind the dash.
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