Zfbod94

Ive had this problem for a while. Ive checked all fuses, hot wires, grounds, ohmed out serial data line from DLC to pin at PCM, all to tfind everything has checked out. Ive had many shops try and scan it with multiple scanners, all without luck. I got a new pcm and had it flashed with my vin. I had my alternator rebuilt because it was giving me erratic readings, so just to cover all bases so i dont fry the new pcm. I still cannot read the pcm.

The oddest part is both before and after changing the pcm, there are no drivability issues at all. It still starts, idles, goes into closed loop all like its still new. Has plenty of power and shifts when and how it should. Fans work properly.

I only came across this issue because the check engine light has come on a few times and i wanted to see what it was. Ive traced as many systems as I can to try and find a fault in a wire.

Does anyone have any ideas? Am I overlooking something stupid?

Injuneer

Have you tried data logging software (like DataMaster or Freescan) and a cable to connect to your laptop?

Some of the newer combination OBD-I/OBD-II scanners seem to be looking for a 12V power supply in the ALDL connector, and there is none. I've seem people post that they have successfully connected with an Actron CP9145 and the correct Actron OBD-I cable. There was a cheap Auto Xray, produced maybe 20 years ago that worked great. Might be able to find one on ebay.

What specific scanners have you tried to connect?

Zfbod94

I have not tried any data logging yet. still saving up for that. Ive tried the newer snap-on scanners, a specific obd1 handheld, the older ones that flash the codes and just straight jumping the A and B terminals. I got the codes to flash out once on the old comp, but after that it never did it again.

I was told there might be a hidden fuse or diode that might be out. somewhere under the dash or near the comp. Do u know if there is one inline with the connector?

Injuneer

Something is screwed up if you got the codes to flash on the SES light by shorting the ALDL pins. That is not possible on a 94 PCM. 93 ECM was the last year that worked.

I have the PCM wiring diagrams from the 94 shop manual. There is nothing in the data line between the PCM pin and the ALDL connector pin. The tan serial data wire runs from pin D30 on the PCM to pin M (lower row, right end) on the ALDL connector. The wire also branches off to feed the ABS computer and the SIR system.

There is a black/white wire at pin A of the ALDL connector (upper row, right end) that connects to a chassis ground. There is a white/black wire for output/field service enable that runs from PCM pin D20 to ALDL connector pin B (upper row, second from right end). There is a black/white wire connected to ALDL pin G (lower row, left end) that runs to the keyless entry system/

Check to see if someone has wired another device into the wire to the ALDL connector. I have my ScanMaster connected to the tan wire, and in order for any device to successfully connect to the ALDL connector, the Scanmaster has to be disconnected.

Zfbod94

the SES light only did that once when jumping it, it may have just been a fluke. I have tried connecting to wit with an older hypertech 3 without any luck either. I made sure that it wasnt tied into another cars tune.

Im gonna go back through every wire on the connector and ohm them out, and try to clean up any grounds. Im so frustrated with this, im about to pay GM an hrs labor to diagnose it for me

Zfbod94

ok..so as an update, i am now able to read my computer. i found that the serial data line splits off the the EBCM (electronic brake control module) and the DERM (diagnostic energy reserve module). so i disconnected the plugs at both modules and tested for continuity. on both with my DMM set to 2000 under the OHM section the reading was 3. assuming that means 3ohms. i pulled the pins out of the connectors and plugged the connectors back into the modules. connected my scanner and YES i can read my comp now. no codes because of the new computer and not driving it. but it can connect. so i put the pin back into the ebcm, it can still connect. reconnect the pin on the derm, and i can still connect to the comp. i still dont know what was causing the connection failure, it had to be something in one of the connections at either module. but now the problem is fixed. i am going to hook up the old comp and pull the memory codes out of it( if there still there after being disconnected for a couple weeks).

thanks for the help Fred. i have finally fix a problem weve had with this car for almost 10yrs!!! youre the man