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?