Engine Controls - System/component Tests - Gasoline: Diagnosis GMC Cab & Chassis C2500

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 connected, measure voltage between mode selector switch Dark Green wire (Dark Green/White on "L", "M" and "U" Series) 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 Dark Green/White wire and ground. Voltage should not be present. If voltage is present, replace mode selector switch.

System Test (Vacuum-Controlled Valve)

Start and run engine to normal operating temperature. With engine at idle, RPM should drop as EGR valve is opened by pushing up on underside of EGR diaphragm.

A/C REQUEST SIGNAL CIRCUIT DIAGNOSIS

Turning on A/C system supplies battery voltage to A/C compressor clutch and Engine Control Module. ECM in response increases idle speed. ECM does not control A/C compressor clutch. If A/C compressor clutch is malfunctioning, repair basic A/C system problem. If A/C system is operating properly and idle speed drops too low when A/C compressor turns on or flares too high when A/C compressor turns off, check for an open A/C signal circuit to ECM. If circuit is okay, repair faulty ECM connection or replace faulty ECM.

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Auxiliary Cooling Fan Circuit Schematic ("C" & "K" Series W/ A/C). Scheme 5

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Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - 4.3L, 5.0L & 5.7L With A/T - 1 Of 3). Scheme 14

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - 4.3L, 5.0L & 5.7L With A/T - 2 Of 3). Scheme 15

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - 4.3L, 5.0L & 5.7L With A/T - 3 Of 3). Scheme 16

Wiring Diagram (Pickup, Sierra, Suburban - 4.3L & 7.4L With M/T - 1 Of 3). Scheme 17

Wiring Diagram (Pickup, Sierra & Suburban - 4.3L & 7.4L With M/T - 2 Of 3). Scheme 18

Wiring Diagram (Pickup, Sierra & Suburban - 4.3L & 7.4L With M/T - 3 Of 3). Scheme 19

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - 5.0L & 5.7L With M/T - 1 Of 2). Scheme 20

Wiring Diagram (Blazer, Pickup, Sierra, Suburban & Yukon - 5.0L & 5.7L With M/T - 2 Of 2). Scheme 21

Wiring Diagram (Pickup, Sierra & Suburban - 7.4L With A/T - 1 Of 3). Scheme 22

Wiring Diagram (Pickup, Sierra & Suburban - 7.4L With A/T - 2 Of 3). Scheme 23

Wiring Diagram (Pickup, Sierra & Suburban - 7.4L With A/T - 3 Of 3). Scheme 24