Fuel & Temperature Gauge not working

[adams.neil]

I've recently picked up an 88 T25 and neither the fuel or temperature gauge are working. Actually, they did work once for one journey when it was hot, but they've now stopped again.

I've reviewed the following article which was great, and following tests on the voltage regulator it's showing that there is no feed coming in (fails the first test in the article below)
https://club8090.co.uk/wiki/VW ... uel_gauges" onclick="window.open(this.href);return false;

I've now got to try to trace the wiring to find out where it's going wrong. My suspicion is that it's a bad earth. The fuel and temperature gauges were working when it was parked up 6 years again, and given they both worked at the same time on one occasion I suspect a dodgy connection. So my main question is where would I find the earth?

Any thoughts would be much appreciated.

Neil

[MidLifeCrisis]

If your turn indicator light is working then you have a good 12v connection to the gauge cluster?
If your high beam headlight warning light (the blue one) is working then the gauge cluster has a good ground connection?
Neither of these tests mean that the voltage regulator is getting 12v and/or ground but at least that proves that the supplies are getting to the cluster!

Let us know if these tests prove anything?

My guess is that either the blue plastic pcb is broken somewhere or the voltage regulator mounting connections are dirty/rusted/broken or something like that ??

(Edited: to add then remove something that wasn't right!!)

[California Dreamin]

Could be ^^^^ however, back in the day.....we used to change quite a few voltage stabilisers for this very reason (small three pronged component on the back of the printed circuit board).

Martin

[ghost123uk]

I've reviewed the following article which was great, and following tests on the voltage regulator it's showing that there is no feed coming in (fails the first test in the article below)
https://club8090.co.uk/wiki/VW ... uel_gauges" onclick="window.open(this.href);return false;
Neil

Just to clarify, are you saying there is no 12 Volts to the input terminal of the 10 Volt regulator ? (terminal "A" on this picture)



If you are NOT getting 12 Volts there, then looking for bad earths ("My suspicion is that it's a bad earth") is not the way forward, you need to find out why you are not getting a 12 Volt supply to the regulator.


.

[adams.neil]

To ghost123 - That's right - I'm not getting a 12v input to the regulator. Any thoughts on how I can trace why not? I'm wondering whether the fact that it did work on one occasion when it was a hot day may be a clue.

[ghost123uk]

Hi Neil, just noticed. you are new here = welcome in

Tracing the lack of 12 Volts input is just a case of working backwards with the red probe of your meter (the black probe can be attached to ANY earth, not just the earth on the regulator = makes it easier / safer). You can see the copper tracks in the blue circuit board, just trace it back by eye to the next place you can get at it with your red meter lead. If no 12 Volts there either, just keep working back until you get 12 Volts and that makes tracing the break pretty easy. You could if it comes to it, cheat, find 12 Volts somewhere convenient on the panel, then carefully solder a wire from there to the input of the regulator. A bit of a bodge but it would get it working. Oh, an ignition switched 12 Volts, not a permanently live 12 Volts

[MidLifeCrisis]

I've reviewed the following article which was great, and following tests on the voltage regulator it's showing that there is no feed coming in (fails the first test in the article below)
https://club8090.co.uk/wiki/VW ... uel_gauges" onclick="window.open(this.href);return false;
Neil

Just to clarify, are you saying there is no 12 Volts to the input terminal of the 10 Volt regulator ? (terminal "A" on this picture)

If you are NOT getting 12 Volts there, then looking for bad earths ("My suspicion is that it's a bad earth") is not the way forward, you need to find out why you are not getting a 12 Volt supply to the regulator.


.

Confusingly - the way that the original 'Voltage Regulator' test is written means that the test needs both a good 12V and a good Ground to 'pass' - so it could still be either that is at fault;

A more complete test might be to put the positive probe on the 'A' terminal and the negative probe on any bit of metal and test if you are getting 12V, and then
Put your multimeter into 'Continuity Mode', put positive probe on 'B' terminal and negative probe on any bit of metal and see if you have ground continuity.
From the results of this you'll be able to figure out if the 12V is bad or the ground is bad.

[marlinowner]

Just make sure the ignition is off when doing the continuity test if you value your meter.

[MidLifeCrisis]

Just make sure the ignition is off when doing the continuity test if you value your meter.

Good point!!
(And make sure the ignition is on when you do the 12v check!)

[adams.neil]

Right - I've done some more tests as suggested. Looks like their is an earth by doing the connectivity test, but no 12v input when connecting between terminal A and to good earth on the car body.

I've established that there is a 12v supply coming into the dashboard on the multi-point connector. That's on the assumption that I've got the right input on the multi-connector ribbon cable. The workshop manual says the fuel gauge supply is pin # 8 and that has 12v.

Given there's no obvious break on the blue circuit board, I think I may now be having to look at getting a different complete instrument panel.

[MidLifeCrisis]

I've established that there is a 12v supply coming into the dashboard on the multi-point connector. That's on the assumption that I've got the right input on the multi-connector ribbon cable. The workshop manual says the fuel gauge supply is pin # 8 and that has 12v.

Did you confirm that the indicator green light on the dash flashes when you have the indicators on - if so then 12v is getting into the instrument panel and there must be a break somewhere in the pcb?
If all else fails you could splice a wire into the wire that goes to pin#8 and connect directly to the leg of the voltage regulator (that would save you the cost of a new panel or pcb ....)

[adams.neil]

You're right in that there is 12v to the panel as the indicator green light does flash.

I have run a cable from another 12v supply to the input and the fuel gauge works. I'll have a go at soldering something together a little neater and use that as a temporary (or more likely permanent) solution.

Many thanks to everyone for their input (pun intended!)

Cheers

Neil