Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1990 GMC Forward Control 3.9 LFACTORY OEM SERVICE PROCEDURE
Electrical Component Locator: Miscellaneous
MISCELLANEOUS LOCATION
| Component | Location | |
|---|---|---|
| A/C-Heater Blower Resistors | On evaporator case, in engine compartment. | |
| ALDL Diagnostic Connector | ||
| 4.3L | Under left side of dash. See Figure. | |
| 5.0L & 5.7L - 1989 | Under left side of dash. See Figure. | |
| 5.0L & 5.7L - 1990 | Under left side of dash. See Figure. | |
| 7.4L - 1989 | Under left side of dash. See Figure. | |
| 7.4L - 1990 | Under left side of dash. See Figure. | |
| Cruise Control Servo (1989 ONLY) | On bracket, on right side of engine. | |
| Fuel Heater (Diesel) | At fuel filter. See Figure. | |
| Fuel Pump Prime/Test Connector | ||
| 4.3L | On firewall, next to fuel pump relay. See Figure. | |
| 5.0L & 5.7L - 1989 | On firewall, next to fuel pump relay. See Figure. | |
| 5.0L & 5.7L - 1990 | On firewall, next to fuel pump relay. See Figure. | |
| 7.4L - 1989 | On firewall, next to fuel pump relay. See Figure. | |
| 7.4L - 1990 | On firewall, next to fuel pump relay. See Figure. | |
| Hydro-Boost In-Line Diode | In harness, behind instrument panel. | |
| Set Timing Connector (Gas) | In harness, near distributor. | |
| Tachometer Connector | In harness, at ignition coil. | |
| Vehicle Speed Sensor (VSS) Buffer | Taped to harness, behind instrument panel. | |
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.