POOR PERFORMANCE/DRIVEABILITY DETONATION SENSOR DIAGNOSIS

Model and Year 1982-83 MALIBU, 1982-84 IMPALA, CAPRICE, MONTE CARLO AND EL CAMINO

If subject condition is encountered on above listed vehicles, the warm driveability work sheet should be completed as well as the CCC circuit diagnostic check to determine if any codes are present. Any items found in those tests that are out of specifications must be corrected first.

If all functions are within specifications, and no trouble codes are stored, the condition may be a detonation sensor picking up noises from the engine or transmission which will retard the timing as much as 19 degrees. To determine if it is occurring, the following diagnosis should be used.

Diagnosis with ALCL Scan Tool, all with ESC (1982-83) (single board ECM):

Drive the vehicle at normal city and highway speeds with the scan tool in the knock retard mode, and monitor the degrees of retard. "0" degrees of retard should be seen on the scan tool (with no spark knock). If reading other than "0" is seen on the scan tool (up to 19 degrees) and no spark knock is present, disconnect the detonation sensor, and road test the vehicle again at the same speeds as before. If no retard is seen on the second test, the detonation sensor is picking up noise from the engine or transmission.

Diagnosis with ALCL Scan Tool for 1984 3.8L engines:

If a 1984 3.8L equipped vehicle exhibits this condition, different readings will be seen on the ALCL scan tool in the knock retard mode.

These readings are no longer direct degrees of retard and are best explained as counts. Figures or counts as seen on the scan tool may be any figure between 0 and 255. If the number on the scan tool stays constant (the same number) then the engine has 0 degrees retard.

However, if the numbers seen are continually increasing such as (example) 60, 90, 118, 150, 195, 230, 10, 40 indicates that the ECM is seeing a signal and is retarding the timing in large amounts (190) which will usually set a code and is most likely an electrical disconnect to or from the ESC module. Small number increases indicate smaller or very little retard is taking place.

To locate the noise causing spark retard, the vehicle should be run on a lift at the speeds the most count changes were seen, and listen closely from underneath to the engine and transmission for any unusually loud knocks, pecks, or buzzing, such as, rocker arms, lifters, transfront pump pressure regulator valve, etc.

If retard is still seen with detonation sensor disconnected, retard is due to a false signal on wire from knock sensor to controller or ESC connections. Rerouting wire away from other wires, such as spark plugs, etc., is required.

Diagnosis without ALCL Scan Tool:

Perform CCC circuit diagnostic circuit check to determine if any trouble codes are present. If any codes are present, use of the appropriate diagnostic chart and correction is required before proceeding.

If no trouble codes are present, drive the vehicle to determine if a performance condition exists (compare with another known good-like vehicle for comparison, if required). If a driveability condition does exist, Code Chart 43 voltage at ECM terminal 'L' low, should be used first. If no trouble is found, using the Code Chart 43, disconnect the detonator sensor, and drive the vehicle again. If improvement is noted, the problem could be engine or transmission noise being picked up by the detonation sensor.

To locate the noise, the vehicle should be run on a lift at the speeds the performance was at its worst, and listen closely from underneath to the engine and transmission for any unusually loud knocks, pecks, or buzzing, such as, rocker arms, lifters, trans-front pump pressure regulator valve buzz, etc.

If a performance condition still exists with the detonation sensor disconnected, engine timing may still be retarding due to a false signal on wire from the knock sensor to the electronic spark controller. Rerouting wire away from other wires, such as spark plugs, etc., is required.

The following components should also be checked; distributor shaft end play, camshaft timing, spark plug wires and distributor cap.

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.