1991 GMC Forward Control 4.3 Z, AutomaticFACTORY OEM SERVICE PROCEDURE

A/C On Switch System Test

  1. Turn ignition switch to RUN position. Move mode selector switch to OFF position. With A/C control assembly connector connected, measure voltage between mode selector switch Dark Green wire (Light Green/Black on Astro and Safari) and ground. For wiring schematics, see mini-schematics in A/C CLUTCH under MISCELLANEOUS ECM CONTROLS.
  2. Battery voltage should be present. If battery voltage is present, mode selector switch is operating normally. If battery voltage is not present, check wire from selector switch to fuse for an open circuit. Also check A/C high and low pressure switches for open.
  3. Check voltage between mode selector Dark Green or Light Green/Black wire and ground. Voltage should not be present. If voltage was present, replace mode selector switch.
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.

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