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The picture is too fuzzy for me. Can you get a better one? I'm intrigued.....

Yep. That relay must be an isolated ground relay, or it's frying time again, you're gonna burn GPs. (Apologies to Eddie Arnold.) Nice work! :beer: https://www.youtube.com/watch?v=VlbhG8tdOvU

Two quick thoughts... A test light is often better than a meter, except when it's not. ;) It's possible to have a partial ground, and it may be that you have enough ground to light a light, but not pull in the relay. I'm not sure how much current that relay needs to pull it in. Need to look...

The ground by the PARKING* brake is a major one, but just because you have cleaned it up, don't assume that your controller card has a good ground. You are showing weird behavior (well, not you, I mean the truck :mrgreen: ), and weird electrical behavior is usually a ground problem. Those...

:mrgreen:

Sir Richard, have you tried grounding the relay manually? If yes, and it works, check continuity from the relay to the controller card. Also check for a good ground source for the controller card. It can't provide ground if it doesn't have a good ground. Might be a partial ground, too.

There ya go. My bad - I forgot about the temp sensor, but now we know the GP relay and resistor bank is all okay. Now to find out why the controller is not providing the correct ground. Temp sensor, continuity problem with the wiring, or bad ground are all possibilities.

That's all good. That's not good. You should see 0 volts at that third (blue wire) terminal. If a working controller card did not fix your problem, it's probably the ground for the controller card, or the blue wire itself. Do this (with a helper): Turn the key to the RUN position (so...