Check the IVR (instrument voltage regulator. You'll need a VOM. Measure the voltage at the sending unit wire, without the sender connected. The meter should swing between zero and battery voltage, with the pulse width duty cycle ~40%. This produces an average of 5 volts. You can use a DMM that has frequency and duty cycle measurement modes to measure the duty cycle. Another way to measure the average voltage is to connect an electrolytic capacitor to the sender lead, and measure the voltage directly. + goes to the lead, - goes to ground; the capacitor should be rated 15 volts or higher, and the capacitance value is not critical, but larger is better. ~100 uF should be sufficient. An analog VOM is best for seeing the operation of the IVR.
The IVR is a rectangular metal can with 3 connector blades. Its plugged into the back of the instrument panel. You *may* be able to see/reach it if you do a headstand in the driver's footwell. If not, remove the 5 screws from the top of the bezel and 1 above the radio. Pull the bezel out of the dashboard. There are 6 clips on the bottom, so you may need to gently pry it. This exposed the instrument cluster. Remove the cluster screws and pull it out of the dashboard so you can reach behind and release the speedometer cable. Pull it further out to remove the two harness connectors. While the cluster is you, examine the light bulbs. If they have a shiny silver coating inside the envelope, replace them. The sockets have a locking tab that has to be lifted slightly before turning the socket, or they'll break off.
The IVR can be replaced with a mechanical one (either new or junkyard), or else install a solid state diy or aftermarket unit. I *think* Chrysler, GM and Ford all used the same IVR, so a junkyard search is easy. If you're comfortable with electronics, you can build one using a few Radio Shack parts. The parts count is so small that its not worth ordering from DigiKey or Mouser.
Ken
