I'm pretty sure it isn't an overheating problem. I can start the truck from completely cold, turn on the fan, watch the gauge peg, and put my hand on the radiator and it is still cold. It seems to be tied to the fan switch.

 

I would check the wiring on the fan switch. A gauge that pegs usually indicates a ground issue. Maybe the fan switch shares a ground with the cluster...

 

Why am I thinking temp sending unit?...

 

I'd check for a common ground, but I'd think the whole cluster would share a common ground, not just one or two of the guages in the cluster. What I mean is if it is a common ground problem I'd think all the guages would still be acting all out of whack.

The temp sending unit is a possibility, but that doesn't explain the connection to the fan switch (at least in my mine, sane as it is ha!).

I haven't worked on an '87-'91 truck lately, on the instrument clusters, do they have seperate sections to them as in the '92-'96 trucks? Are the temp guage and the gas guage on the same side of the cluster and on their own seperate section of the cluster? If so, I would find another cluster and swap out/in that section and check the operation. My '96 had a failing volt meter and it turned out to be a bad guage and I had to replace that section of my cluster.

By the way, I believe you can have a cold radiator and still have an overheating engine if the problem is a thermostat stuck shut. How long do you have it running while you are doing these checks to the temp guage with the fan switch?

Sorry for the longish post, hope something I've mentioned helps in some way.

 

I appreciate all of the input. I had the cluster completely apart yesterday. Yes, it comes in sections. Temp and volts are in one section, fuel and oil press are in the other. I have 2 other clusters to play with, one from another diesel, one from a gas truck; so I guess I will try swapping the temp section from one of those in. While I am at it I will take out the temp control panel and see if there are any obvious shorts (try to figure out why my fan only works on high...related problem???). I am pretty sure it isn't overheating. I drove the truck back to AR from Louisiana last weekend, 8 hours on the road at highway speeds with no problems, and like I stated before, I can get it to peg out within minutes of starting the truck after it has been sitting in below freezing temps overnight.

 

I finally installed an aftermarket temp gauge, runs steady at around 190 all the time, so no overheating.
Anyone have any ideas about where to look for the grounds for the dash wiring harness? The more I drive the truck, the more I think I have a bad ground somewhere. I was driving in the rain today and the wipers were in slow motion. At one point they actually stopped for a second, continued for 1/2 a sweep, stopped again then completed the sweep. Did this several times.

Thanks for the help.