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Yes it is.. I am not sure but believe I gave you a link a while back to a service manual on cd, if so did you buy it?
If so, section 33-12-1 explains how to test it and gives a diagram of the circuit, actually 2 circuits. One is for a IAR and the other for a EVR Ammeter system.. What the difference in is the two I am not sure as I have never had to work on mine... section 33-51 addresses the printed circuit board and how to disassemble the instrument cluster.
It would be too many pages to post here and might be a copyright problem. But here is an example from the manual for a 1988 which is probably the same as your 1990.
To get the cluster out you may have to lower the steering column a little. That is done by removing the bottom cover and loosening the two bolts that hold the column to the dash frame.. Another problem will probably be the odometer cable. You can not pull the instrument cluster out from inside the dash till you gain some slack in the cable so you can pull the cluster out far enough to reach your hand in behind it and unlock the cable from the instrument cluster.
I would try to test the gauge first before replacing it
Here area a couple more examples of what is in the manual
Unless you MUST have the stock gauge I think you would be just as well off to go with an add on gauge. Most of the elec gauges are not really a guage but a reader of information off something else. I had an older ford ranchero, gas guage would not indicate right, had it changed, then they changed the resistor or whatever and still nothing. Have a ranger and oil press is non existant, put in a gauge and it works same for the water temp. On my f350 and the e450 MH I have scan gauge and get real time readings different form the 'gauges' in the dash, but it is OBD2 not a 1 like yours. So you would need to go direct reading with a guage.
SOP (Standard Operating Procedure): Be aware that during normal operation, the amp gauge needle barely moves to the right of center. You have to look very closely to see it move.
This is the case for every FoMoCo vehicle once alternators were introduced in 1965.
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