Starting issue
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
If the relay is not working have you checked 86 and 85 ??...do you know how to check that "control" side ??...do you have power on terminal 86 and is the ECM grounding 85 ??...do you know how to check this ??...a relay "clicking" means nothing...when you say the "ECU is undiscovered" do you mean the scan tool will not communicate with the ECM ??...I'm a stickler for correct terminology...after checking this ignition feed have you checked the battery feed and ground for the ECM...no power and ground NO communication.
Last edited by C5 Diag; Apr 18, 2026 at 07:08 PM.
Thread Starter
Heel & Toe
Joined: Jun 2025
Posts: 22
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I have confirmed the 86 to 85 works fine as I tested them with a battery and with power coming out of the fusebox. The high amp side works fine too as if the relay is clicked that circuit closed and would send power to the fuse 4. I dropped wires into 86 and connected them to the correct side of the relay and grounded it off of my alternator, It clicked closing the high amp circuit. I then dropped a wire into the 85 side and connected it to the correct node on the relay and it would not click when I touched 86 with the hot wire. The relay works fine, just doesn’t seem to jive when it is inserted into the box. I narrowed it to the ground side being unhappy. That issue is interesting to me.
ECU being dead is U0100, again, OBD2 cannot talk to ECU. Same issue as before, just with troubleshooting and more info, but no dice
ECU being dead is U0100, again, OBD2 cannot talk to ECU. Same issue as before, just with troubleshooting and more info, but no dice
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,308
Likes: 3,924
From: Cape Coral, Florida
You are doing way to much work to be checking that relay...should take 5 minutes to check 4 terminals for the relay...so you're saying the ECM is not providing ground at terminal 85 ??...with test light connected to battery POSITIVE when ignition is on the light should illuminate when probing 85 at the relay....have you checked G100 which is the ground for the ECM ??
Thread Starter
Heel & Toe
Joined: Jun 2025
Posts: 22
Likes: 1
Mmm, maybe I am overkilling the relay, but I am doing my best to be thorough. Are you saying that 85 of the ECM relay is grounded by the ECM and not the car’s ground? After a second look at the diagram, this does appear to be the case. What ECU pin is G100? Is that X2-73?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,308
Likes: 3,924
From: Cape Coral, Florida
The cars ground is G100 and that will provide the ground for the relay...the ECM has no special ground inside of it just for relays and such.
