I got a 91 f800 truck and when I first got it the speedometer wasnt working. I bought a new speedometer and that fixed the problem but just a couple days ago it just stopped working. With no power, the needle will go to 0, but when the key is turned to run it will dip down below 0 and stay there. I took apart the dash and unplugged the wiring going to the guage and there are 4 wires, from the left- black with pink stripe, green, black, red with green stripe. The manual says that the red with green stripe should have 12 volts which it does, and the green and black wires will have ac because they go directly to the sensor. And the manual says the left most wire supposedly goes to the key. I stuck my multi meter into the green and black while the truck was moving and the ac volts moved up and down accordingly. The circuit board on the guage looks fine and has no burns or anything that looks bad. Does anyone have an idea of what could be going on?
Grounding out as you hit bumps somewhere or wire broken on the inside. Put meter on it and wiggle wires going to it all the way down, look for chafing.
The wires going down to the sensor are perfectly fine. I get continuity from the guage to the sensor. And as the truck moves I get a change in ac voltage like it should at the plug for the guage
The unit has to have a ground somewhere that you haven't mentioned, it sounds like all the wires in the connector are accounted for and are not ground. Does the gauge cluster have a foil circuit board? I've had cases where the contact to the foil has carboned up and lost connection. Most electrical gremlins can be traced to a ground problem.
Sorry I forgot to say the black wire in the 4 pin connector is the ground wire, it has a good connection to ground. I'm starting to think the guage may have gone bad. I unplugged it and using 2 pins I connected ground to where the black wire connects and 12 volts to the spot where the red and green wire connects. Right as I applied 12 volts to those 2 connections the guage dipped below 0. Since my truck doesnt have a 2 speed rear axle I am now imagining the wiring would be different from what the manual says. I now believe that the red and green wire is the wire that is connected to the key because 12 volts only appears when the key is on, and I know for sure the green and black wires go to the sensor. But the left most black and pink wire is just there. It seems to not do anything.
I just checked for connectivity. In the female plug which is on the guage there is no connectivity between any of the 4 pins. With the key off in the Male connector in the truck only the rightmost red and green wire has connectivity with the black ground wire. With the key in the run position, the Male connector has no connectivity. Just to make sure the red and green wire wasnt the problem I cut the wire and added a male connector to the end and ran a new switched 12 volt wire to it and the guage still dipped down. I also added a female connector so I could reconnect the red and green wire.
find the sending unit check it out that's most likely the problem
I replaced the unit, and it sends proper ac voltage to the guage when the truck moves.