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: Switches
SWITCHES LOCATION
| Component | Location |
|---|---|
| A/C Pressure Cycling Switch | On rear of accumulator. |
| Auxiliary Fan Temp. Switch | In right cylinder head. |
| Back-Up Light Switch (A/T) | At base of steering column. |
| Back-Up Light Switch (M/T) | On left side of transmission. |
| Brake Pressure Warning Switch | On proportioning valve, at left frame rail. |
| Clutch Switch (Cruise) (M/T) | On clutch pedal bracket. |
| Coolant Temperature Switch | In left cylinder head. |
| Fuel Pump/Oil Press. Switch | On rear of intake manifold. |
| Glow Plug Inhibit Switch (Diesel) | On front of engine. |
| Hydro-Boost Flow Switch | In pressure line, at power steering pump. |
| Hydro-Boost Press. Warning Switch | In pressure line, behind master cylinder. |
| Stoplight Switch | On brake pedal bracket. |
| Throttle Position Switch (Diesel) | On fuel injection pump. |
| Transmission Kickdown Switch | At top of accelerator pedal bracket. |
Factory Service Tools
Use the Manual With the Right Hardware
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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.