Engine Controls - System/component Tests - 7.4L: Diagnosis GMC C3500 HD

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 and ground. For wiring schematics, see mini-schematics in «A/C CLUTCH»(ref-20094-S41499054742001010200000) under MISCELLANEOUS ECM/PCM/VCM 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.

C-2, Port Fuel Injector Balance Test

The injector balance test is used to pulse the injector for a precise amount of time, spraying a measured amount of fuel in the intake manifold. As each injector is pulsed, a drop in fuel rail pressure occurs. This pressure drop can be recorded and compared to other injectors. An injector with a pressure drop of 1.5 psi (.11 kg/cm 2 ) or more, greater than or less than other injectors, should be considered faulty.

Note. Allow engine to cool to avoid irregular readings due to "hot soak" fuel boiling. To prevent flooding, INJECTOR BALANCE TEST should not be repeated more than once without starting and running engine.

1. With ignition off, connect Fuel Pressure Gauge (J-34730-1) to pressure tap. Unplug harness connector at all injectors. Connect Injector Tester (J-34730-3) to one of the injectors.
2. Follow manufacturer's instructions when installing adapter harness. Ignition should be turned off at least 10 seconds to complete ECM/PCM/VCM shutdown cycle.
3. Turn ignition on. Fuel pump should run at least 2 seconds after ignition is turned on. Bleed air from gauge and hose to ensure accurate gauge reading. Repeat this procedure until all air is bled from system. Turn ignition off for at least 10 seconds.
4. Turn ignition on again to bring fuel pressure to maximum. Record initial pressure reading. Energize tester one time and note pressure drop at lowest point.
5. Disregard any slight pressure drop after low point is reached. Subtracting second pressure reading from initial reading indicates amount of injector pressure drop.
6. Repeat step 4) on each injector and compare pressure drop. Recheck injectors not within pressure drop range. Replace injector(s) failing second check. If injectors are all okay, plug in harness connectors and review SYMPTOMS in «TESTS W/O CODES - GASOLINE»(ref-20057) article.

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.

DIAGNOSTIC CHARTS

Note. In the following diagnostic tests, illustrations, schematics and flow charts are courtesy of General Motors Corp.

A/C REQUEST SIGNAL CIRCUIT DIAGNOSIS

Turning on A/C system supplies battery voltage to A/C compressor clutch and Powertrain Control Module (PCM) terminal E12 through circuit No. 59, increasing idle speed. PCM 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 circuit No. 59 to PCM. If circuit is okay, repair faulty PCM connection (terminal E12) or replace faulty PCM.

A/C Clutch Circuit Schematic - 7.4L VIN H ("C" & "K" Series) A/C Request Signal Circuit Diagnosis. Scheme 131

A/C Clutch Circuit Schematic - 7.4L VIN H ("G" Series) A/C Request Signal Circuit Diagnosis. Scheme 132

A/C Clutch Circuit Schematic - 7.4L VIN H ("L" & "M" Series) A/C Request Signal Circuit Diagnosis. Scheme 133

A/C Clutch Circuit Schematic - 7.4L VIN H ("P" Series) A/C Request Signal Circuit Diagnosis. Scheme 134

Auxiliary Cooling Fan Schematic (Models With A/C) Auxiliary Cooling Fan Circuit Diagnosis. Scheme 135

Auxiliary Cooling Fan Diagnostic Flow Chart (Models With A/C) Auxiliary Cooling Fan Circuit Diagnosis. Scheme 136

Scheme 137

Scheme 138

Scheme 139

Scheme 140

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 1 Of 3). Scheme 141

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 2 Of 3). Scheme 142

Wiring Diagram ("C" & "K" Series Pickup, Sierra & Suburban - 7.4L With A/T - 3 Of 3). Scheme 143

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 1 Of 3). Scheme 144

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 2 Of 3). Scheme 145

Wiring Diagram ("C" & "K" Series Pickup & Sierra - 7.4L With M/T - 3 Of 3). Scheme 146

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 1 Of 3). Scheme 147

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 2 Of 3). Scheme 148

Wiring Diagram ("P" Series - Commercial & Motorhome - 7.4L With 4L80-E - 3 Of 3). Scheme 149

Wiring Diagram ("P" Series - Cutaway - 7.4L - 1 Of 3). Scheme 150

Wiring Diagram ("P" Series - Cutaway - 7.4L - 2 Of 3). Scheme 151

Wiring Diagram ("P" Series - Cutaway - 7.4L - 3 Of 3). Scheme 152

Wiring Diagram (Van - 7.4L With 4L80-E - 1 Of 3). Scheme 153

Wiring Diagram (Van - 7.4L With 4L80-E - 2 Of 3). Scheme 154

Wiring Diagram (Van - 7.4L With 4L80-E - 3 Of 3). Scheme 155