I did an ohms test between the aldl ground at the aldl port and the ecm case. I read the post below by six_shooter

Results: engine off = .1 ohms resistance
key in on position = 12.4 ohms resistance
engine running = 5.6 ohms resistance

Could be a noise issue.

Grab a DMM and check for ground at the ALDL connector, referenced to the battery and the ECM. Do this with the engine off and running, and look for a change there should not be or very minimally, like a shift of only a couple ohms if any. You can also use the voltage setting, and this should be at zero and remain at zero in any instance, if there is voltage shown on the ground at the ALDL when using either the battery negative or ECM (case is fine) then you have an issue with the ground lead at the ALDL, most likely.

Also while you're there, check the data line, and make sure that it's 5V at KOEO, and also at KOER, if the voltage drops off at KOER, then you may have an issue with the ECM, or possibly the grounding of the ECM to the engine block/heads.