Engine Controls - Diagnostics For Internally Shorted IAC

89oldsmobile

SUBJECT DIAGNOSTIC PROCEDURES FOR INTERNALLY
SHORTED IDLE AIR CONTROL MOTORS


MODELS/YEARS 1988-89 CUTLASS CIERA, NINETY-EIGHT, EIGHTY-
EIGHT, TORONADO AND CUTLASS CALAIS WITH
3300, 3800 ENGINES
NUMBER: 89-T-72

CORP. REF. NO: 916505R

DATE: May 1989

SECTION: 6E
We have recently seen a few cases of internally shorted Idle Air Control Motor terminals on 1988 and 1989 models equipped with 3800 engines, and 1989 models equipped with 3300 engines, which may exhibit one or more of a variety of poor driveability conditions. Some of those conditions could include:

1. Engine miss at idle or rough idle (engine can be hot or cold).
2. Engine miss under light acceleration.
3. General engine roughness at all engine speeds.
4. Cycling whine (generator) accompanied by fluctuating RPM at Idle (3 or 4 times at each stop).
5. Chuggle at approximately 45 mph.
6. Excessive rattle from EGR valve (some noise is normal).
7. Transmission comments about 3-4 shifts (rough or repetitive 4th gear engagements).
8. Comments about T.C.C. operation (rough, shudder).

It should be noted that each vehicle may exhibit different symptoms and these sometimes can be intermittent. Although this condition is not very wide spread, it can be very difficult to diagnose unless the following procedure is used, since an IAC motor is not generally associated with these types of comments.

A quick test of the IAC motor can be performed by simply disconnecting the harness connector at the IAC motor and determining if the poor driveability condition is eliminated or not. The harness should be disconnected with the engine running and the IAC motor counts stabilized.

If the symptoms are still present with the IAC motor disconnected, DO NOT REPLACE THE IAC MOTOR. At this point, it will be necessary to reconnect the IAC motor and refer to the appropriate Service Manual for further diagnostics.