Fuel gauge wiring

Discussion in 'Mech Tech' started by jim mcglynn, Sep 30, 2019.

  1. Doing some testing of my fuel gauge, and have ordered a new voltage regulator for it.

    However, looking at various picture on't tinterweb, am now wondering whether it is wried correctly? For example, should the wires on the gauge be the other way round?

    Thanks in advance.

    Jim[​IMG]

    1979 Super Viking, 2lt type 4, Automatic
     
  2. Yes - the Brown/black and red are reversed

    If youre having issues make sure you have a good contact from the regulator to the back of the dash (it looks OK, but mine needed a good wiggle and tighten to work)
     
    jim mcglynn likes this.
  3. ...although thinking about it - it might not make a difference, it's just "heating" the bi-metallic strip in the gauge so polarity probably doesnt come in to it...
     
    jim mcglynn likes this.
  4. bluerustybucket

    bluerustybucket Supporter

  5. As mentioned polarity of the gauge is not an issue. So long as you have the connections to the stabiliser the right way round. Worth sticking a ohm meter on the brown/black sender wire to see if you have roughly the right resistance from the sender unit if you have an idea of the fuel level in the tank.
     
    Day, snotty and jim mcglynn like this.
  6. Did you test the voltage stabiliser? If working correctly it should give a steady 5.2 volts between the red wire and earth. It gets lower as the zener diode inside dies, and the fuel gauge then reads low.

    For anybody interested, the stabiliser is repairable and the diode costs buttons.
     
    DubCat likes this.
  7. I dont think thats a diode type stabiliser - it looks like a bi-metallic one, so try to measure the output with a needle multimeter as a digital one will bounce numbers all over the place.
     
    snotty likes this.
  8. Good point - I think you're right as I've just gone and looked at an old one I have. In which case if Jim's bus is a '79, we may have incompatible components. Maybe.
     
  9. The vibrator type is fine on a later Bay. My 78 has one.

    If you’ve got the zener type, check the light bulb inside (used as a limiting “resistor”) hasn’t gone pop.
     
    Last edited: Oct 1, 2019
    andyv likes this.
  10. Indeed - my vibrator type failed so I purchased a zener type - both do the same job
     
  11. mikedjames

    mikedjames Supporter

    The bimetallic stabiliser output bounces slowly enough to confuse almost any digital or analog volt meter, but the main thing is the output should be changing from 'off' to be 'on' for a bit under half the time.

    You could use a 7806 type three terminal voltage regulator or a 7805 and a diode to replace the bulb and zener.
     
    jim mcglynn and Valveandy like this.
  12. Fantastic. Thanks all.

    So,

    1 - Good Testing the sender, resistence from brown wire to earth is 62 ohms.... About right.

    2 - GOOD testing the gauge, Earthing the sender tab sends gauge to full.

    3 - BAD testing the volt regulator, get a reading of 13v on BOTH tabs! It's meant to reduce to 6, and jump between 0 and 6 isn't it?

    4 - BADDER - point 2 may have burnt out the fuel gauge because of point 3? It caused a little smoke inside the gauge. Fingers crossed that it didn't.

    I have ordered a volt reg. Going to investigate whether I ordered the right one or not now....

    1979 Super Viking, 2lt type 4, Automatic
     
    grandmst likes this.
  13. Great tip. I have now ordered a (some) 7806 just in case the "proper" thing I ordered don't work.
     
  14. davidoft

    davidoft Sponsor

    Does the fuel gauge read anything? If you put a constant 12v to it the gauge will burn out, I expect that’s whats happened
     
  15. Yeah, you are probably right. Fuel Gauge does read Full when then the tank is full, for a few miles. And it also went to full when I earthed the sender tab, but unfortunately I didn't know the volt reg was sending 12v....so yep, it probably has burnt out.

    So, volt reg has probably been on the way out for the last year or two, then when it failed completely recently, it has probably taken out the fuel gauge too. Will see for sure when the new Volt Reg arrives.
     
    77 Westy and davidoft like this.
  16. 77 Westy

    77 Westy Supporter

    I bet you wished you’d checked 3 before 2 Jim.:)
     
    jim mcglynn likes this.
  17. Yep. Lesson learnt. Again :)
     
    77 Westy likes this.
  18. The gauge might not be knackered - I accidently put 12v through it and the wire lit up like a 5-bar fire.... but despite some charrign around the edges she lives for another day. Admittedly it was only for a short period of time, but you might be lucky...
     
    jim mcglynn likes this.
  19. You don't want to hear this, but I reckon you've let out the magic smoke that makes it work :( on a brighter note, it would be interesting to see which part inside let out the smoke, it might be repairable
     
    jim mcglynn likes this.
  20. Have you let the genie out :(
     
    jim mcglynn likes this.

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