1993 ECM Repair; DTC H51
With help from Gordon Killibrew and repair by K&B Special Products, I fixed the ECM for my 1993 LT1 6 Speed Convertible, all stock, 74,616 miles. I have owned the car for 10 years. Anyone experiencing DTC 51 and the following described problems may find this of help.
Since summer 2011 the Service ASR light on the DIC came on along with the Service Engine Soon light; intermittently the Security warning light will flash, while the SYS light in the speedometer will flash on and off with 0 MPH showing while the car is moving. Despite all this, it will buck for an instant, and then run normally with all these warning indicators still on. Initially, turning the engine off erased all the lights and everything worked fine, with no codes stored. The problem persisted despite turning the ignition off, and codes 43 (EST) and 51 (ECM) were stored using an OBD1 scanner. The cooling fans would come on with the ignition left on, when in this mode.
The diagnostic trouble codes (using a jumper wire on the ALDS STR Diagnostic Enable Terminal) were as follows, described from the Corvette Service Manual:
CCM (Module 1) DTC codes
H72 Serial Data Link Malfunction
When I removed the ECM from the car and opened the PROM cover, I discovered it was not an original GM part. Gordon pointed out these aftermarket PROMs are problematic. K&B verified the PROM was aftermarket and replaced it. They tested and inspected the ECM, and repaired it with new parts from Delphi. As you may know, GM no longer makes replacement ECM's for this year.
The car started right up with the installation of the repaired ECM and none of the DTC or Service Engine Soon lights came on. Of course I will need to drive it more to verify all is well, but so far so good. Hopefully anyone else with C4's having ECM issues can find this of help.
Since summer 2011 the Service ASR light on the DIC came on along with the Service Engine Soon light; intermittently the Security warning light will flash, while the SYS light in the speedometer will flash on and off with 0 MPH showing while the car is moving. Despite all this, it will buck for an instant, and then run normally with all these warning indicators still on. Initially, turning the engine off erased all the lights and everything worked fine, with no codes stored. The problem persisted despite turning the ignition off, and codes 43 (EST) and 51 (ECM) were stored using an OBD1 scanner. The cooling fans would come on with the ignition left on, when in this mode.
The diagnostic trouble codes (using a jumper wire on the ALDS STR Diagnostic Enable Terminal) were as follows, described from the Corvette Service Manual:
CCM (Module 1) DTC codes
- H41 (Loss of ECM Serial Data Communications);
- H72 (LCD Dimming Output Circuit open or shorted to ground);
- H74 (LED Dimming Output Circuit open or shorted to ground).
- H13 Bank 1 (Left) O2 Sensor open circuit
- H14, H15 Engine Cooling Temp (ECT) Sensor Circuits, High and Low Temp
- H16 Distributor Ignition System (Low Resolution Pulse)
- H21, H22 Throttle Position Sensor Circuit (signal voltage high and low)
- H23 Intake Air Temp Sensor Circuit (low temperature indicated)
H72 Serial Data Link Malfunction
When I removed the ECM from the car and opened the PROM cover, I discovered it was not an original GM part. Gordon pointed out these aftermarket PROMs are problematic. K&B verified the PROM was aftermarket and replaced it. They tested and inspected the ECM, and repaired it with new parts from Delphi. As you may know, GM no longer makes replacement ECM's for this year.
The car started right up with the installation of the repaired ECM and none of the DTC or Service Engine Soon lights came on. Of course I will need to drive it more to verify all is well, but so far so good. Hopefully anyone else with C4's having ECM issues can find this of help.
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