ECU harness/communication

[fastenuf]

With apologies because parts of this are posted elsewhere, I am seeking specific advice and commentary on the subject of the ECU harness on my 94 Q45T. My car (2nd Q I've owned) has occasional sluggish throttle response and earlier troubleshooting led me to the knock sensors. The ECU returns the 34 code to confirm that it at least thinks I have a KS problem. I have replaced the knock sensors and checked the continuity and resistance of the new KS's all the way back to the ECU plug. I have 3 working ECU's of exactly the same part number, and all 3 still return the 34 code. I have purchased cables and software from ECUtalk and Blatz to see what's going on with the ECU and both fail to communicate with the ECU. Both companies have helped with with troubleshooting to no good end. I have done voltage and continuity checks with both of these and the consult connector, again back to the ECU connector. I have cleaned the connector pins (both halves) with contact cleaner and a toothbrush. The dealer tells me he is unable to operate the consult system on my car.

I read in one of the older forums something like "all knock sensors and harnesses will need to be replaced on all 90-96 Q45's". If I can check the wiring like I have done yet am unable to fix the problem, obviously I am missing something. By "all knock sensors and harnesses" do we mean the entire ECU harness or just that part from the KS to the ECU? What about my consult communication issue?

Any practical ideas about what I can do or try next? Thanks in advance.

[Lokim]

GROUNDS! Check your grounds. Start at the battery by cleaning the negative post/terminal, and clean the engine/body terminations of your negative battery cable. Secondly, check for ground resistance and voltage drop from all ECU ground pins, as well as all ECU system relays. Let us know what you find...

[Lokim]

Oh yeah! Also check for cross-continuity between the KS pins at the ECU and power & ground wires. If the KS circuit is shorted to power or ground, you may still have continuity in the wire, but also have continuity elsewhere.

[maxnix]

Just the KS sub harness is routinely replaced when the KS are.

[rkleff]

I was getting Code 34 Knock Sensor and after finally getting a good multimeter, I was able to get them to ohm in spec from the pins to the negative battery terminal. But still I was getting code 34 using ECU Talk so after seeing the above post about grounds, I decided to work on my grounds. I had already cleaned the negative battery terminal, but when I went to disconnect my negative cable to frame ground, I snapped the bolt. I had put Liquid Wrench on it but only waited about 30 minutes and did not tap the bolt to free it. I had to drill and tap a new hole in the frame to reconnect the ground. I cleaned it with a Dremel with a flexible extension to shinny metal and cleaned the copper tab with baking soda to get it back to copper color. I put dialectric grease on the connection and snugged it up. I also cleaned the C24 ground on page EL 277 of the 94 Q45 FSM and what I am trying to say is that IT WORKED!!!!!!!!!!!

I no longer have a code 34 and I no longer see my timing retard to single digits when I put a load on the engine. I am a newbie to the forum and have learned so much that I would like to thank everyone for their contributions.

[Q451990]

to NICO. Great first post! This is the first time I have head of a ground issue on a Q causing something like that - but with all of these cars approaching, or already 20+ years in service everything is fair game.

Heath

[rkleff]

I noticed that Lokim is from Seattle and I am from the Portland area where it rains a lot! This may not be an issue in drier climates but it is worth a check before you pull the plenum. Once again, thanks for all the information that is out here, it's a gold mine.

Bob