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Wiring Issues - 93 Fox Temperature Gauge

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    Wiring Issues - 93 Fox Temperature Gauge

    Hi all,

    I am doing some work on a 93 Fox and have hit a strange issue.


    1/ With Engine/Ignition OFF, if you then switch on the lights (i.e. leave the lights on when you switch off the engine), the TEMP GAUGE starts to rise and will eventually go to max. The hotter the engine the faster it rises - the hot engine symptom is also duplicated by shorting the temp sender wire too.
    2/ With engine ON and running and at any temperature (more noticeable the hotter it is), the TEMP gauge will rise and fall about 1 notch as you put the lights on and off.
    3/ Other gauges seem to move VERY slightly when lights switched on and off - but nothing really noticeable.

    The TEMP issue ONLY does it if the cluster is lit. Remove the bulbs or bypass the switch to run the lights with no cluster (or turn down or remove the dimmer totally) and the TEMP problem goes away.

    I have checked the following and all are perfect:

    1/ All earths to speedo cluster good (added another for good measure) - register a ZERO ohms resistance to earth
    2/ Engine and body earths good (problem persists even with jumper cables added from battery to block)
    2/ Light switch good
    3/ Dimmer switch good
    4/ Temp sender good (replaced anyway)
    5/ Disconnected alternator - made no difference
    6/ If you remove ALL the "ILL" cluster bulbs (all the ones that illuminate when the lights are on) it DOES NOT do it

    This issue is therefore only noted when the cluster is lit. I am even thinking of switching to LED cluster lights as these will (1) draw less current and (2) act as a diode and not allow any reverse current etc.

    I have tried another speedo cluster, but the problem was identical. The replacement cluster MAY also be faulty as this appears to be a very common issue without any known cure. The common fix is to add a second aftermarket temp gauge and ignore this one - but that is not an option in this case.

    I am a qualified spark, so assume I have a pretty good knowledge of electrical things etc. I will add a wiring diagram with all interconnected circuits shown to help any bright ideas you may have.

    One idea is to wire the sensor directly with new separate wires, but the diagram suggests that the circuit has nothing interconnected that could cause this issue.

    I am stuck with the idea that the gauge is faulty - but I am still trying to find a relationship between the cluster bulbs and the TEMP gauge - other than a shared (and perfect) earth.

    Any help appreciated before I start pulling hair out (I don't have that much left).



    Wiring Diagram (with all TEMP interconnected wires shown in various colours):


      I'll leave the above info for anyone else in the future to search and hopefully find some helpful info within. BUT, I think I've cracked it.

      I decided to bite the bullet and try LED bulbs. These offered two distinct advantages (1) lower power draw and (2) do not allow any "reversed" drain/draw.

      And they work!!!! So, if you find you have this issue on a 87-93 Fox cluster - replace the 6 main illumination bulbs with LED's and you should have the same success.

      I hope that helps.

      I will test it fully over the next few days and report. But at the moment, no matter what I do with the lights, the temp stays nicely just under halfway (that's with cooling fan removed to help the tests).


        Well done for getting it sorted. Where did you get the LED bulbs from? Do you have a link?
        I think there is a small voltage regulator in the dash somewhere, maybe its faulty!?!

        1980 Notch + 1990 GT hatch


          Pre -87 there was a regulator in the dash, but from 87-93 there was no regulator for the general gauges but there is a "slosh" module for the fuel gauge alone.

          The only regulator on a 87-93 is on the alternator, but even with that disconnected it made no difference in this case.

          Some have reported some success in replacing the alternator, but as it made no difference with it totally disconnected I didn't see this gamble as being worth the expense.