Chrysler Repair: fuel quantity indication problem: 89 minivan, plymouth grand voyager, fuel gauge

Question
I have a 1989 Plymouth Grand Voyager with a flex fuel 3.3L engine.  When the headlights are turned on the fuel gage goes to empty.  What can cause this to occur? eryjh

Answer
Hi James,
Unfortunately my closest manual to the '89 year is a '91. That was just around the time that the 'body computer' was adopted to centralize and rationalize functions like the fuel gauge, and the '91 manuals does show there to be that computer present. Were that the case in your '89 I would be hard pressed to explain the relationship between the headlamps and the gauge because information about those two systems would be digitized and communicated over the digital data bus between the body computer and the instrument cluster. On the other hand, if you had the good old-fashioned wiring, then I would have to suspect something in the cluster board which is  shorting the wire that goes to the fuel gauge to a 12v supply wire such that the fuel gauge has 12v on both ends of it coil and so it read 0. If you have any idea about whether or not you have a body computer we might focus in on this, but still withut a manual we may have to do some guessing. On the pre-computer model the fuel gauge wire is typically dark blue or dark blue/yellow. You could find such a wire on the cluster circuit board plug and see how it behaves voltage-wise when you turn on the headlamps as compared to not, and of course the ignition has to be in the 'run' position.
Please let me know what you can share about the vehicle's set up.
Roland