Hi. I have a 1984 F250 Diesel 4x4 Supercab with an Auto trans and I'm having a problem with my water temperature gauge. Specifically, it will sometimes creep up to the edge of the Hot range and sometimes quite a bit over (fully pegged) but then it will move back down into the Normal range.
The truck runs fine and shows no signs of overheating (the coolant level is fine, there is no coolant in the oil and there is no "boil over" in the overflow tank) and what is even stranger is that sometimes the Oil Pressure gauge will move in synch with the temp gauge and go to the fully pegged position (the situation with both gauges moving together has only happened once or twice, but the pegged Water Temp gauge has happened a lot... mostly on extended highway driving and it only started happening within the last few months).
Anyway, if I bang on the front of the gauge it sometimes goes back to the Normal range. So that might indicate the gauge is not going to the pegged region because of the temp sensor but rather some other weird reason (that and having the two gauges move together would seem to indicate an electrical problem).
Just wondering if someone has experienced this before and can shed some light on what might be wrong? I'm thinking maybe some bad (or weak) wiring connections from the sensors to the gauges but I was hoping someone had seen this problem before.
As a side note, both temp sensors are new from last summer when I had to pull the heads to replace one that had cracked from being overheated (which is why I am so sensitive to the temp gauge readings).
Thanks for reading this long post....
Scott

 

On tne back of the cluster there is a IVR instrument voltage regulator a little silver box that regulates guage voltage....... Also there could be a wire with worn shielding touching a ground.

 

I've had simular problems and can say that the dash is most likely the problem. When I bought the truck in Utah I drove it home to Georgia with the oil pressure went from the "N" in normal when cold to almost the bottom of the normal range when hot. I asked the same questions in this forum and was told " the dash most likely will be the problem, not the oil pressure". My water temp gage varied from the "O" in normal and would drop to the cold line while driving down the interstate then would suddenly go back up to the "N" or "O". I bought an oil pressure gage to check the oil pressure and found that the pressure would be 45 to 50 PSI when cold at idle to to 15 to 20 when hot at idle. Running down the road it consistantly runs 40 to 45 PSI. I just replaced the water temp gage but in the short time since I replaced it the temp gets up to between 180 and 190. It has never dropped below 180 even when I stopped to go into a store like when I was watching the "FORD" gage. The "FORD" gage would drop below the bottom of normal even if I stopped for just an few minuets to pay for fuel. If in doubt, get a real gage. Also, if you do replace the thermostat, replace it with a FORD or IH thermostat, not an aftermarket thermostat. If you are losing antifreez or see steam from under the hood then there might be a problem but otherwise, think "gage"!

 

PLC7.3
I had my dash out over the weekend and didn't see the silver box you refer to. Is it mounted onto the dash itself or somewhere else?

 

Look under your hood. Along the firewall just to the left (looking aft)of the master cylinder is a large ground wire. Make sure that this line is securely contected and that it is not severed anywhere along its length.

When I bought my 88 7.3 the same thing was happening to my gauges. Fluctuating when I turned anything on like the fan for the heater, the high beams, the KC lights, wind sheild wipers, basically anythig that draws juice.

After a couple of months of trying to figure this out I happened to be installing a K&N airfilter and noticed that not only was the ground line nearly broken in half but the insulation was gone (still haven't figured that one out) figured it grounded out and became hot enough to burn it off. Anyhow as a temportary fix I installed a jumper. This immediatley solved the problem.

Hope this helps,
Eero

 

Thanks for the good advice people! Unfortunately, I won't have time to dig into this problem in the very near future. Some knucklehead just kicked in my trucks back window on Friday night sometime after I had parked it for the night. Anyway, until I get the window fixed, the truck is going to stay parked at my parent's house out of the elements (they live a bit of a way from where I am living). So until I get a chance to straighten all that out, I'll be driving the Taurus.
I will update this post when I get a chance to do some more digging. I am definitely interested in how that "voltage regulator" module affects the gauges. Especially since it seems 3 of the 4 small gauges (water temp, oil pressure, fuel)
have begun moving together intermittently (all to the "higher" end of their scale). Interestingly, the volts gauge doesn't really move at all.
Scott

 

Scott,
All my gauges. The oil pressure, temp, volt, where fluctuating with the amount of juice I was using, if I used the high beams they would go higher, add the KC lights and the would edge even higher. Until I found the faulty ground wire. Only saying that is what it might be, took me awhile to find it. Hope this helps.

Eero

 

This is a VERY common thing for these old trucks to do. Both my 86's will do it on occasion. Basically the gauges will rise as quickly as they can, and usually only temperature will reach max, oil pressure will reach high side of normal, and the fuel gauge will gain another 1/4 tank. A quick tap on the top of the dash resets all of them. The amp meter is unaffected.

The small silver box is barely more than an inch long and is located on the back of the cluster assembly case.

 

That might be because the volt or amp meter is tied into the wiring from the battery so it gets a true volt reading.
Fuel, oil and water temp go through the insturment voltage regulator.

If the volt meter was running through the regulator, it would never move till the regulator was failing.