SERVICE MAN. UPDATE SEC. 6E3 A/C CONTROL CIRCUIT DIAGNOSIS

Subject: SERVICE MANUAL UPDATE - SECTION 6E3 - REVISED AIR CONDITIONING CONTROL CIRCUIT DIAGNOSIS

Model and Year: 1990 "Y" WITH 5.7L L98 (VIN 8) AND LT5 (VIN J) ENGINES

Revised A/C Control Circuit Diagnosis --------------------------------------- The following information updates the A/C clutch circuit diagnostic chart and facing page information found on pages 6E3-C10-4 and 6E3-C10-5 in Section 6E3 "Driveability And Emissions" for the 1990 "Y" 5.7L L98 (VIN 8) and LT5 (VIN J) Corvette Service Manual.

Specific information within this bulletin will be identified as follows:

L98 - "(8)" LT5 - "(J)"

The "Scan" tool diagnostics in this bulletin apply only with the use of a Tech 1 "Scan" Tool and the 1987-1990 or 1987-1991 Tech 1 ECM Cartridge.

CHART C-10 A/C CLUTCH CIRCUIT DIAGNOSIS 5.7L (VIN 8) & 5.7L (VIN J) "Y" CARLINE (PORT) ------------------------------------------------- Circuit Description:

The A/C clutch control relay is ECM controlled to delay A/C clutch engagement after the A/C is turned "ON." This allows the ECM to adjust engine rpm before the A/C clutch engages.

The ECM will engage the A/C clutch any time A/C has been requested unless any of the following conditions exist.

o High coolant temperature. o High oil temperature. o Wide open throttle. o Sustained high vehicle speeds (LT5). o High engine rpm.

The ECM can determine A/C request by sending a voltage signal to the A/C control head. When the A/C control switch is closed, the A/C request voltage signal is grounded. This is shown on the Tech 1 as A/C request "YES" (8) or A/C clutch "ON" (J).

When a request for A/C has been detected by the ECM, the ECM will ground the A/C clutch control relay drive circuit, the relay contacts will close and energize the compressor clutch portion of the circuit. If the system is properly charged, the circuit will be complete to the A/C compressor clutch.

When A/C request has been detected by the ECM, the cooling fan(s) will be turned "ON" unless vehicle speed is too high.

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. Refer to ELECTRICAL DIAGNOSIS (SECTION 8A) "FUSE BLOCK DETAILS" for fuse circuit power distribution.

2. Checks the ECM's ability to control the A/C clutch control relay.

3. Checks to determine if the ECM is capable of detecting the A/C request signal being grounded.

4. Before replacing C68 A/C control components, refer to ELECTRONIC AUTOMATIC AIR CONDITIONING (SECTION 1C1) for further A/C diagnostics.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.