Fuel Gauge Problem
Robert asks:
having a problem with my gauges the fuel gauge pegs all the way past full when key is on. and I have no voltage. fuel has 2 tanks and wont switch over . this is a 89 ford f250 7.3 can some one tell me were I might be able to start to look for the problem
Answer:
First lets cover the basic operation of the fuel gauge, this is not all of them since newer vehicles are handled by the Body Control Module (BCM), however we need a background for the operation.
In the gas tank there is a float that is attached to a rod, which moves a pointer along a resistor, a ground signal is sent across the resistor and then to the gauge, the resistor limits the ground that goes to the gauge, more resistance = less gauge movement, or less gas in the tank.
Now hopefully we are talking about a factory installed tank switch, if this is the case you will be able to find wiring diagrams to trace the wiring. If the switch is a aftermarket switch then you will need to contact the manufacture for wiring information.
If it seems I am concentrating on the switch it is because I am, with wiring always go with the most logical problem. In your question you mention that the tanks will not switch over, in most designs the switch also controls which fuel gauge is sending a signal to the gauge. So I would start with testing the inputs and outputs of the switch wiring and replacing the switch if it is bad. But again you will need to know what type of system you have.
You may want to stat at the dealer parts house, give them your vin number and see if you truck had factory installed dual tanks or aftermarket.
If it turns out that the switch is not the problem I would next check the GEM module, it is bit like a early BCM. The GEM is a fairly common item to fail on Ford products and finally the gauge could be bad, check all fuses and follow the wiring diagram.
This is a good time to announce a new service we are offering, finding wiring diagrams is difficult enough but then how do you read them? We now offer personalized wiring diagrams, pricing ranges from $39.95 to $99.95 depending on the problem and vehicle. You would receive a diagram of the circuit, what each wire color voltage should be, and how you can find the wire on your vehicle. So you need a volt meter but hey for less than an hour at the dealer you could fix your electrical problems. To get yours just send a note to sales@rushps.com
Now one final note, when working on wiring on newer vehicles never use a clamp on test light, you must use a volt/ohm meter. Test lights send too much voltage through the wiring and can damage electrical components.
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