1994 Base LT1 SES light won't go off......

[coves4me]

Well, I've been chasing code issues for about a month now and I'm running out of ideas and help. I've had C72, H72 and C42 codes when I jumper between pins 4 and 12. I get the same codes when I use my Actron 9145 Scanner. I've erased them by either disconnecting the negative cable from the battery and also using the scanner. I've diagnosed the C72 code as being either the aftermarket radio I installed or the Vehicle Speed sensor/connector. The C42 code was diagnosed using the FSM diagnostic tree DTC 42 and it is pointing to the connector at the ICM as the ICM's I have all checked out OK when tested at O'Reilly Auto parts. The engine starts, idles fine, revs up just fine, throttles down OK and drives very well, BUT the SES engine light won't go out. This last weekend I replaced the PCM (and re-installed my old PROM) with a new remanufactured unit, Denso O2 sensors, the oil temp sending unit and cleaned the ground wires above the oil filter, also. They were already clean, but I made it better, I think. Anyway, I received some advice from JustAnswer.com (a Chevy mechanic with 27 years of experience) and he advised me to jumper between pins #2 and #4 on the OBD II connector and see if the SES light would flash. Well, nothing happened, it just burned steady and nothing flashed across the dashboard. Hmmmmmmmm......anybody got any thoughts? I'm at my wits end here.

Thanks in advance.

Mark

 

[coves4me]

Well, here's an update to my dilemma. I'm baffled. I replaced the ICM connector with a new one and soldered it into the wiring harness and used shrink tape to isolate the wires. I also installed a new Vehicle Speed Sensor in the tranny and installed a new connector. After double-checking all of my work, I inserted the paperclip into pins 4 and 12 on the OBD II connector and checked codes and erased them in 1.7, 4.7 and 9.7. Then I repeated the erase codes process with my Actron 9145 scanner just to be sure they were gone. I turned the key and the SES light went out, BUT reappeared 2 to 3 seconds later and stayed on steady. I re-checked the codes and got a C72, and H72, an H42 and a C42 in the "A" module, or DERM (airbag) module. I then disconnected the battery and repeated the whole process again with the same results.

I'm baffled. According to the FSM neither a DTC72 or DTC 42 is supposed to turn on the MIL light!! So, what gives? Will a C42 in the DERM module throw the SES light?

Any help is greatly appreciated. And YES, I did use the FSM diagnostic charts to determine the ICM connector was most likely bad.

Mark

 

[XLR8]

I can't help with your issue, Mark. But I can give your thread a bump to the top of the heap again in hopes some of our more experienced members will see it.

Good luck!
Jane Ann

 

[Hib Halverson]

Ok...."C72" is a fault with the VSS. The "C" means the fault is current and existed at the time you scanned the ECM. H72 is the same DTC but is a history code, ie: at some other time in the past 72 has set. C42 is an ignition system fault and, is also current and, because of the H, long-standing. Use your FSM to solve those codes.

Forget trying to get the service engine light to flash. 94 and 95 VIN P ECMs do not support flash code diagnostics. That a "Chevy mechanic with 27-years experience" didn't know that is demonstrative of the value of "JustAnswer.com".

Forget the DERM as those two codes are ECM codes not SIR faults.

I'm a little unclear on your VSS replacement. Did DTC72 reset or not reset after you replaced the VSS and the connector?

 

[coves4me]

Hib, the DTC72 reset after I replaced the VSS and the connector. The connector was a factory AC Delco type with pigtails that I soldered to the two wires coming from the VSS. I installed an aftermarket radio and installed the resistors and the diode as recommended in the TSB Chevy issued to correct the C72 code being thrown. I am going to recheck the connections at the wires.

The C42 code doesn't come up in module 1, 4 or 9, but it shows after the A module on the dashboard. Hmmmm......I performed the diagnostics per the FSM on the ICM and the connector. Very strange.

Mark