Instrument cluster on the fritz, truck randomly downshifts...

Haha you have no idea how happy that makes me to hear that someone else has had an issue similar to mine! Will do, so long as if your issue resurfaces and you figure it out before me you do the same.

So I've unplugged every module in the cab. 4WD, ABS, GEM, PDM, air bag, all the relays from the previously unseen box (not that I believed they'd do anything), completely removed the alternator electrically from the vehicle, and still this effing light is flashing and the gauges are screwed. I verified the condition of the resistors on the back of the gauge cluster and they all check out. Besides the ECM I don't think there are any more modules, controllers, or computers left to eliminate.

I appear to be getting clean power from both the key on power and hot at all times power pins on the air bag signal plug to the gauge cluster to ground. I also get seemingly clean power from the AB signal ground to both positive power pins. What's weird is that I will get better voltage with the module unplugged. I do not understand where the signal comes from the turn that light on if it only runs from the light directly to the airbag module. The best I could come up with without actually looking into it is that when the AB module plug is removed from the module there is a spring loaded pin that grounds the pin to the light to illuminate it. Anyone know whether or not this is true?

Without pulling them apart I checked the grounds in the cab and they look okay but I intend on thoroughly checking them next time I have time to kill. The only thing I can think of is that the problem is possibly in the cluster itself, but I have a feeling that it is not. I'm going to try a salvaged one and see what happens. I've visually inspected all exposed wiring and connectors (which right now is damn near everything) but I'll pull the air ducts off to make sure. If nothing of this changes anything I'll pretty much be at a loss. I have yet to check continuity from the ground pins on various plugs to battery ground. After that the last thing I could think of is to try replacing the ECM, but I also doubt that is the problem as it does not have a whole lot to do with what's going on.

What's worse is I have no one else to take it to if I can't figure it out. I have little faith that anyone else would figure anything better out than I have and the bill for that would probably be outrageous. I even called a dealer and they said they had never heard of this problem and had no idea where to start. Their exact words were "it could be almost anything." That didn't help narrow it down.

Figured it out. It was really stupid.

Ground 101 (Behind, or I guess more accurately in front of the battery) was loose. I had checked this in my early shotgun-guessing and inspections of connections and it seemed fine. I re-checked it after finding out that everything I've been having trouble with is grounded through that point. When rechecked, the ground still seemed tight and corrosion-free, but I noticed something I overlooked before. one wire, the top one that appeared tight, ran directly to the battery. There was another that comes out of the wiring harness directly underneath that that was loose. I had to tug on the wire to realize that the top (battery) wire was tight between two nuts on the frame stud, but the bottom (harness) wire was just kinda floating in a bit of space between the two nuts and the frame. Attempts to tighten the nuts led to the stud spinning, so I wired the harness directly to the battery and the other wire to a new point on the frame. In my frustration, haste, and sleep-deprived state I missed what turned out to be the simplest cause of what seemed to be a complex problem.

The whole thing has been quite an experience though. I know a lot more about how everything operates now than I ever would have cared enough to know. The battery gauge is just a standard volt meter and the reason that read low was because it wasn't getting a good connection. What originally puzzled me before was why the other gauges read all whacky. All other gauges have two coils in them that oppose each other. One is powered fully when the key is on, whereas the other is powered through a sender that varies resistance. This makes sense because if they were simply powered through the sender only then depending on system voltage (which varies given load and whether or not the engine is running) the needle would not always indicate the same even if what the sender is reading hasn't changed. So this is why the oil pressure and coolant temp read high. The fuel gauge sender input is first sent into an averaging module (exactly how this works I do not know) then to the gauge itself, so I am assuming that there simply was not enough power for it to work like it should have and that's why the fuel would show much lower than what was actually there.

The GEM also receives a portion of its ground through G101, as does the ABS module, air bag module, and passive airbag deactivation module (the passenger side key off thing under the stereo). Why the AB light would come on the way it did I'm not sure. I would think it would have either not come on at all when the hey was first on, or have stayed on all the time to indicate a problem. I guess it's just funky electronics stuff. Similarly I don't know why the ECU would seem to be working fine when all this other stuff was going bonkers despite sharing the same final ground point. IIRC there is only one additional connector for all the modules and dash stuff, so I can't see the miniscule amount of extra resistance that junction would impart affecting anything.


Another stupid and embarrassing mistake made is apparently placing one of the fuse taps in the PCM power fuse spot. This is a 25A circuit. With my mind on so many other things at the time I had not considered these taps are probably not rated for that much, and sure enough after looking it up I found they have a 10A max capacity. So after I fixed the ground issue I took the truck out and drove around for a bit to make sure everything was working (which it was) when out of nowhere I gave the truck some gas to turn from an intersection and the engine just died and would not start back up. I figured out right away it was something fuse related to the PCM, but of course I died on a very busy road with no shoulder so I couldn't open the door to check anything. Luckily I was right down the road from where I used to work so my old boss was able to give me a tow with a truck to the garage and I realized quickly my mistake. She started right up and surprisingly the fuse tap seems to still be working in a circuit that is within its rated limits.


It took a lot of time, I made a lot of mistakes, couldn't see the forest through the trees with some of the wiring diagrams, probably lost some hair but kept all my blood this time, and over 80 miles later I seem to have the same exact truck I had 6 months ago. Now I can move on to installing my recently acquired 8.8 LS from an explorer...

All Right.

Way to stay with it.

Ray