Hey guys, I've been putting this one off way too long. This post is going to be kind of long, but I want to make sure I include all the info I have to make troubleshooting easier.

I have trouble pulling any data from my ALDL connection on my 88. Here are the basics:

With the ignition on and the car off, my check engine light is solid like normal. The bulb works.
When I jumper A & B I can pull code 12. Also back when my MAF relays were going south, I was getting trouble codes and a check engine light. I could successfully pull them no problem using the paper clip trick. This tells me that circuitry is good.
Everything about my digital dash works, and I'm told a lot of that information is computed by the ECM/mem-cal.

When I hook up a scantool (not just a code reader) like the Autoxray, it says "Vehicle Not Responding" or something similar. The idle does change while it's querying it. I'm sure I made a good connection (no bent or dirty pins) and checked to make sure everything was good at the actual ALDL interface. I've tried the Autoxray on two other late 80's GM cars and it works great. I've even tried having a professional try to scan it.. he says that every now and then something will pop up, but usually it doesn't respond.

The car runs great for the most part.. I'm not in continuous limp home mode or anything like that, hence I'm just about certain that the problem exists only in the ECM's output. So I looked in my Helm's manual.. that's where I need help.

Chart A-2 in "Drivability and Emissions" says that based on these symptoms, I have either a faulty ECM or Mem-cal, or a problem with circuit 461. Since I changed the ECM, it's probably not that. Since all my digital dash output works, it's probably not the Mem-cal. Agreed?

So we get into the actual wiring.. Circuit 461 is the orange wire coming from the ECM, A8 Serial Data. It says it goes to "D" on the 10-way instrument panel Conn. This is my first problem.. where is this 10 way IP Conn? Is this the smaller of the two plugs that plug directly into the ECM under my kick panel? If so, there's no real "wiring" to speak of since it makes a physical connection when it plugs in. If not, I have no idea where it is and would appreciate if someone would nudge me in the right direction.

From there it splits at some point. One fork goes to to the IP cluster and one goes to "E" on my ALDL connector.

Now, as I said before my IP cluster is working great. Would that not mean then that the fork between D on the 10way IP Conn and the IP cluster is completely good? And the problem resides somewhere on the fork between D and the ALDL connector? That's what I'm gathering from reading it.. and if so, does anyone have any idea at what point (physically) it forks? All that wiring is taped up, and I'm afraid untaping it would make things quite unpleasant under there.

I know how to check for shorts or open circuits.. my problem is finding WHERE exactly to start looking.

One last thing.. if I was to temporarily take some electrical wire and run it all the way between port E on my ALDL, and port D on my 10-way IP Conn, would that in theory bypass the fork I mentioned above and give me a definite answer as to whether or not that's my problem? And would I have to ground it?

Sorry for all the questions.. this problem seems simple enough to me, but knowing my luck I'll burn the whole car down if I don't ask these questions.

 

If nothing else, I'd really like an answer to my last question about temporarily running a wire.

 

You can safely test the connection from the E terminal to the ECM terminal with an ohmeter, so there would be no reason to run a wire there until you prove that this circuit is open. Running a wire wouldn't hurt anything and if you want to do that, it is safe!

 

Thanks for the response.

The ground issue is what was worrying me.. I wouldn't think I'd need to ground just that one wire, but the manual says it could be bad ECM, bad memcal, bad ground, or open circuit.

Judging by the symptoms I listed above, it just about HAS to be an open circuit from the ALDL I would think. Changing the ECM with no results means it's probably not the ECM. Having a working digital dash means it's probably not the memcal (this is an assumption I'm making based on what I've read in the past.. if I'm incorrect, someone please correct me). If it was a grounding issue, I would think again it would affect the digital dash readout since the ground isn't isolated to the ALDL->ECM connection. That just leaves an open circuit.

 

Sounds like an open circuit. This circuit involves a spring finger connection which could be open. Break out the ohmeter.

 

The manual that came with my OBD1 AutoXray says to wiggle the connection at the ALDL or on the AutoXray itself if it shows "Vehicle Not Responding". When I reconnect or wiggle the connections on my '88, it corrects the problem and it scans correctly without any problems. I find that it pays to read all that stuff. I'm sure you just missed that in the book.
Good luck

 

Nah, I tried that already. I contacted the makers of the Autoxray and they told me to do what I had already read in the manual.

It was at that point that I brought it to a professional and he had the same problem with a different, professional scanning system (see above).