GMC 1976 ZEO 6083 Maintenance Manual page 632

Figure 4 1-Headlight Circuit Diagnosis
CHASSIS ELECTRICAL
12- 27
ONE HEADLIGHT INOPERATIVE OR INTERMITTENT
POSSIBLE CAUSE CORRECTION
Loose Connection Secure Connections to Sealed Beam Including Ground (Black Wire)
Defective Sealed Beam Replace
ONE OR MORE HEADLIGHTS ARE DIM
Open Gr . Connection at Headlight Repair Black Wire Connection Between Sealed Beam & Body Ground
Black Ground Wire Mislocated in Relocate as Shown in Service Circuit
Headlight Connection (Type 2
Sealed Beam)
ONE OR MORE HEADLIGHTS-SHORT LIFE
Voltage Regulator Misadjusted
Check and Adjust if Necessary to Specifications .
ALL HEADLIGHTS INOPERATIVE OR INTERMITTENT
Loose Connection Check and Secure Connections at Foot Switch and Light Switch
Defective Foot Switch
Check Voltage at Foot Switch With 12-Volt Test Bulb . If Bulb Lights
Only at Lt . Blue Wire Terminal, Replace Foot Switch .
Open Wiring
Check Voltage at Lt . Blue Wire With Test Bulb . If Bulb Lights at
- Light Switch to Foot Switch Light Switch Lt . Blue Wire Terminal But Not at Foot Switch, Repair
Open Wire .
Open Wiring Check Voltage at Light Switch Red Wire Terminal With 12-Volt Test
- Light Switch to Battery Bulb . If Bulb Fails to Light, Repair Open Red Wire Circuit to Battery
(Possible Open Fusible Link)
Circuit Shorted to Ground If, After a Few Minutes Operation, Headlights Flicker On and Off
and/or a Thumping Noise Can be Heard From the Light Switch (Cir-
cuit Breaker Opening and Closing), Repair ShorttoGround in Circuit
Between Light Switch and Headlights . After Repairing Short, Check
For Headlight Flickering After One Minute Operation . If Flickering
Occurs, the Circuit Breaker Has Been Damaged and Light Switch
Must be Replaced .
Defective Light Switch Check Voltage at Light Switch Red and Blue Wire Terminals With
Test Bulb . If Bulb Lights at Red Wire Terminal But Not at Lt . Blue,
Replace Light Switch .
UPPER OR LOWER BEAM WILL NOT LIGHT OR INTERMITTENT
Open Connection or Defective Check Voltage at Foot Switch Headlight Terminals With Test Light .
Foot Switch If Bulb Lights at Headlight Terminals (Lt . Green - U .B ., Tan - L .B .),
Repair Open Wiring Between Foot Switch and Headlights . If Bulb Will
Not Light at One of the Foot Switch Headlight Terminals, Replace
Foot Switch .
Circuit Shorted to Ground Follow Diagnosis Shown Above Under "All Headlights Inoperative
or Intermittent"
A-1810