REPEATED ECM REPLACEMENT 6.0L DFI. Cadillac Seville 1981

DIAGNOSIS OF REPEATED ECM REPLACEMENT ON 1981 DFI CARS

Several instances have been reported where a vehicle required repeated ECM replacement immediately or shortly after installation. Two possible conditions could exist in these situations.

First, a vehicle system could have an intermittent short to ground or short to voltage which overloads the ECM. If the short is not found and corrected during diagnosis, the ECM will be diagnosed as the cause of the malfunction and will be replaced without finding the real cause. In this case, the owner will probably return with the same vehicle malfunction when the short recurs.

The second condition which could occur is an overload to the ECM existing in a circuit unrelated to the malfunction being diagnosed. Due to the fact that certain systems share common circuitry within the ECM, one vehicle system being shorted to ground or shorted to voltage (intermittent or hard) could create a malfunction in a different vehicle system.

To avoid unnecessary damage to replacement ECM's and customer dissatisfaction with comebacks, technicians must realize that an ECM malfunction may only be a symptom of a condition existing elsewhere in the vehicle. Field experience has indicated that for the following engine symptoms, certain circuits are worth investigating prior to ECM replacement.

Symptom #1 ---------- If a code 30 is being set (or possibly a code 27 if the ISC motor has retracted such that the throttle switch will not close) and it has been diagnosed as an inability of the ECM to deliver the proper signals, use the following diagnostic decision tree before replacing the ECM.

DECISION TREE #1 ---------------- (Code 30 Service Manual Decision Tree Should Be Followed First) Then:

o Clear codes and exit diagnostics

o Re-enter diagnostics and enter the output cycling test code 96

o To verify improper ECM signaling, back-probe PI (Blue) ECM connector pins 6 and 7 with a test light to ground | | |---------------------|------------------|-------------------| Light Flashes Light Flashes Light Does Not Light Flashes For 3 Seconds For 3 Seconds Flash on Either For 3 Seconds On Pin 6 Only On Pin 7 Only Pin On Both Pins | | | | Check for an Check for an Check for an ECM is OK intermittent intermittent intermittent short to ground short to ground short to ground on Circuit 426 on Circuit 425 on Circuits 425 | | |---and 426 -------------------------------- Check for an intermittent short between Circuits 425 and 426 (pin 6 to pin 7) | If OK. replace ECM

Symptom #2 ---------- If a code 20 is being set and it is found that the fuel pump relay is not energizing due to an improper signal from the ECM, use the following diagnostic decision tree before replacing the ECM.

DECISION TREE #2 ---------------- (Code 20 Service Manual Decision Tree Should Be Followed First)

o Turn ignition off for at least 10 seconds

o To verify an improper ECM signal, backprobe P1 (Blue) ECM connector pin 5 with a test light to ground

Symptom #3 ---------- If any of the following system or component malfunctions are found to be due to improper signals from the ECM:

o MD solenoid pair(s) activated or deactivated at all times (code 25)

o AIR switch solenoid on or off at all times

o AIR divert solenoid on or off at all times

o EGR solenoid on or off at all times

o Canister purge solenoid on or off at all times

o Cruise Control power solenoid on or off at all times,

use the following diagnostic decision tree before replacing the ECM.

DECISION TREE #3 ----------------

(If Code 25 Was Set, Follow Service Manual Decision Tree First) Then:

o Start engine and clear codes

o Stop engine and immediately return to ignition on

o Enter diagnostic "output cycling" code 96

o Verify improper operation of affected system and determine if any of the other systems listed above fail to cycle properly (MD is not checked in output cycling)

o Turn ignition off and disconnect all three ECM connectors. Turn ignition on. Using the High Impedence Meter (J-29125) set on the 200 ohm scale, probe P1 (Blue) ECM connector pin 8 (circuit 439 - ignition voltage) with the positive lead. For any of the following solenoids which fail to cycle, probe the affected circuit pin with the negative lead

1) Red Connector P2 - pin 8 AIR Switch Solenoid (Circuit 436)

2) Red Connector P2 - pin 19 AIR Divert Solenoid (Circuit 429)

3) Red Connector P2 - pin 2 EGR Solenoid (Circuit 831)

4) Red Connector P2 - pin 18 Canister Purge Solenoid (Circuit 428) | | All Readings----------------------One or More Greater Than Readings Less 20 Ohms Than 20 Ohms | | Does Cruise Control Follow this leg power solenoid cycle? of tree for each | circuit reading No Yes less than 20 ohms | | | Connect negative | Turn ignition off, lead of ohmmeter | Is reading now greater to ground and with | than 20 ohms? positive lead, probe | | P2 (Red) ECM connector | No----------------------Yes power solenoid (Cir- | | | cuit 402). Is reading | Disconnect sole- Repair short greater than 20 ohms? | noid connector. between solenoid | | Is reading now circuit & 12V | | greater than battery | | 20 ohms? No-----------------Yes--------| | | | No------------------------Yes Disconnect No hard shorts | | solenoid present at this Repair short between Replace shorted connector. time. If a solenoid circuit and solenoid Is reading replacement ECM 12V ignition now greater becomes damaged, than 20 ohms check all circuits | again for an inter- | mittent short | No------------------Yes Repair short | between sole- Replace solenoid noid circuit land ground

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.