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The gauges on my '87 F-250 6.9 are doing some wierd stuff. The temp gauge works fine until you turn on anything electrical. then the gauge will rise a little hotter. and the more electrical stuff you turn on the higher it goes. the oil press. is the same way. I know the oil press. and fuel gauge block have been replaced. I'm guessing i'm losing ground some where. theres a ground strap from the fire wall to one of the intake manifold bolts thats burnt into. which seems wierd(it must have happend with th previous owner). The fuel gauge bounces all around, I think thats in the sending units.
The fuel gauge is the only one that means something.
Put in a mechanical oil pressure, voltage and temp gauge and you will see what I mean.
I do not think I have ever seen my amp gauge move in 20 years.
If the pointers are on the scale anywhere on either the oil or temp gauge it means the key is in the ON position probably.
Sending units are known to die in the fuel tanks.
Also there is a lot of plastic under the dash, the insturment cluster is all plastic.
A lot of the metal under the dash is not grounded either.
The connection for the electric to the insturment cluster is a printed circuit on flexable plastic that goes into a socket in the cluster. The plug with the wires in it makes contact with the printed circuit in the socket. I have removed the insturment cluster and removed and replaced the plug and cured gauge problems.
Also there is a voltage regulator on the insturment cluster that regulates the voltage for the gauges that has been known to fail after lots of years.
I am in the process of building a new insturment cluster with gauges I can trust instead of trying to fix ones I know I can't.
After a lot of work getting the reading right I know the stock speedo is giving me right numbers.
The tach is also giving me good numbers.
Fuel gauge is giving good numbers.
Stock amp meter, oil pressure and water temp gauges are a worthless waste of dash space.
I'm going to try the voltage regulator. My gauges seem to change with voltage changes. The gauges need a constant supply of unchanging voltage to be accurate. sounds like i should replace the sending units also. Do you think it would be easier to just remove the bed than drop the tanks.
I would put in a extra set of gauges but it hard to mount anything to these dashes and make it look good even a CB.