7 replies to this topic

[mister bridger]

The fuel gauge on my mk1 has suddenly started reading full all the time. I did knock one of the wires to the sender off but put it back on again straight away - they can only go one one way as the spades are different sizes. Can't find any obvious shorts. Any ideas?

[HarveyT]

usually its because the connectors are rusty, that was the problem with mine, apparently if they are not properly cleaned its because there is a dodgey connection

[KernowCooper]

Take the connector of the centre pin of the sender unit if the gauge falls back to empty the senders faulty, if the gauge stays at the full mark then the gauge is being given a earth other than sender. Check the 2 wires on the tank sender are not touching together or shorting as this would give the same effect..

 

I would if you have a open back to the speedo measure the voltage stabliser voltage which should be a 10v output to the gauge, I doubt this is the fault as it would just make it read higher and not stay there, but worth checking.

[dklawson]

The fuel gauge system changed around September of '64.  If you have a "bolt-on" sender and matching gauge the troubleshooting is different than the gauge system that uses the locking-ring sending unit.  Which type does your car have?

[mister bridger]

I'm glad you've appeared DK - you helped me out with issues due to mismatched parts on my old mk1. This one's a 67 and I presume the parts are  original but not had a chance to look at the gauge end yet.

[dklawson]

If the gauge and sender are both from 1967 they will be the later type that uses the voltage stabilizer.  For that system, earthing the green/black sending unit wire will make the gauge read full as Kernow said.

 

Try a couple of quick tests.

Turn on the ignition and confirm that the gauge is still going to "full".

Disconnect the green/black wire on the sending unit.  Do not let its terminal touch anything.

If the gauge needle drops to "empty" there is a short in the sending unit or somehow its float arm is stuck "up".

If the gauge needle stays on "full" proceed to the next test.

 

With the ignition on, open the bonnet and find the green/black wire on the back of the fuel gauge.

Pull the green/black wire off the fuel gauge and turn on the ignition and watch to see if the needle drops to "empty".

If the gauge needle drops, there is a short in the green/black wire which you will need to find and repair or replace the wire.

If the gauge needle stays "high", this indicates somehow the gauge has an internal short and the gauge needs repair/replacement.

 

If on the other hand you find that the gauge is working... but just seems to always read "too high" (like 40-50% too high) then the problem may be the voltage stabilizer.  A meter is typically useless trying to measure voltage stabilizer output as the voltage is rapidly switching between 0V and about 14V (with the engine running).  The stabilizer is supposed to be delivering an average of 10V.  However, if the stabilizer has stopped working or has lost its earth connection it will pass full system voltage (again... upwards of 14V) to the gauges making them read quite high.  If you have an electric temperature gauge you would observe that the car also appears to be running very hot when it is not.

[mister bridger]

Thanks Doug - the Gauge Guru! I've printed that off and will check these things if it ever stops raining!

[mister bridger]

Follwed your advice, Doug, and it turned out to be just a faulty sender. As I had a spare it was a quick fix to put it right. Many thanks. New thread following shortly on a similar subject!