If a gauge is pegged high, that only indicates one thing - for sure...that there is little or no current flow in its sensing leg. The sensing leg consists of 3 main parts:
1. The pink wire that runs from the instrument panel down to the brass pin on the sender:
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2. The variable resistance (that is what the sender really is).
3. The ground wire - connects the sender body to the frame/chassis:
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Any number of things could cause a loss of continuity in the sensing leg:
1. A broken wire - in either the pink supply lead or the blue/black (depends on model
year) ground wire.
2. A loose/disconnected/oxidized terminal connection - at the ends of either of those
wires.
3. A broken/worn out part in the sender's variable resistance - or its intank connections.
Here are the first diagnostic tests you should do. When you are done with these two simple steps, you may not know the exact problem but you will know which of the following the issue is with:
1. The gauge/dash/sender hot wire group
2. The sender itself
3. The sender grounding connection
The first test is for voltage available to the sender. It is best done by removing the pink wire’s connector from the sender head. This can sometimes be done by reaching in between the cab and bed with a long screwdriver and prying it off.
If that won’t work, you can try working your hand up under the cab floor and reaching up to the sender. It is at the very front of the higher level of the tank – just before the tank top slopes down to the lower level:
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If neither of those methods is possible, you can unbolt the front tank mount bracket and loosen the rear one. That should allow you to tilt the tank down in the front and gain access to the sender.
Or you could just use your meter probe to pierce the insulation - as near to the sender as possible.
Anyway, the actual tests are real easy.
1. With the ignition key in the RUN position, confirm that there is 12 VDC available at the terminal connector on the end of the pink wire.
2. Check for continuity in the grounding lead – from the crimped connection at the sender head to some convenient clean part of the frame.
If you get 12 VDC at the sender end of the pink lead...that means the power supply, the wire that runs to the sender, and the gauge mounting in the IP are all good. For reference, here are the fuel gauge connections at the PC board:
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If you confirm continuity from end to end - on the blue/black lead that is crimped to the sender body and both ends are securely fastened…that is proof that the sender has a path to ground.
If the results of the two checks are as described above (i.e.12 VDC & continuity), the sender is faulty and will need to come out for replacement.
No voltage on the pink wire at the sender:
First, check the pink wire for voltage at the IP plug. If there is voltage there but not at the other end, the next step is to:
Check the pink wire for continuity - from where it exits the IP, all the way to the sender end. If you don’t get continuity, the next step is to:
Trace the length of the wire looking for breaks or melted areas. If the wire is found to be damaged, repair or replace as necessary.
No continuity in the sender grounding lead to the frame:
Check, clean and tighten the crimped connection at the sender head
Check, clean and tighten the bolted connection at the frame end.
If you still don’t see continuity, the wire is bad. Replace it.
Battery voltage available at sender and sender has a clear path to ground:
Sender is faulty – replace it. It is possible to revive a tired old sender, but you might not want to get involved in that procedure. It involves re-establishing tight contact between the wiper arm and the coils of the resistive element.
