Temp gauge pegs when turning on headlights

Discussion in 'Electrical Systems' started by High Country Z, Jan 4, 2011.

  1. High Country Z

    High Country Z Veteran Member

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    This is a new one. When I pull out the switch to turn on the headlights the temp gauge pegs all the way to right. I'm thinking poor ground but not sure where. Anyone else ever have this problem?
     
  2. ULTM8Z

    ULTM8Z Veteran Member

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    My guess is probably a short on your printed circuit behind the instrument cluster. Somehow the temp signal circuit is getting 12V when you turn the lights on.
     
  3. High Country Z

    High Country Z Veteran Member

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    Yet it only happens when the ign. switch is in the off position. With the ign. switch in the on position, the gauge operates normally.
     
  4. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    Damn good question!!!!!!

    I've seen mine do the same when I shut the car off with the headlights on or the stereo cranked up.
    (The stereo bypasses the ignition switch, same as the headlights.)

    I suspect the problem is an ignition controlled relay sticking and back-feeding the gauge but I have not located the relay or confirmed this to be the cause. (I'm grasping at straws here.)

    It isn't a consistent problem so I haven't dug into it further but I suspect it may be a contributor to my distributor's ignition module burning out multiple times. :mad:
    (I can't prove that hypothesis either, yet.)

    I don't have any other electrical issues in the car. All gauges, lights and power accessories function normally.

    I hope someone can explain this for us!
     
  5. IDLZRUF

    IDLZRUF Veteran Member Lifetime Gold Member

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    Mine does that every now and then headlights like to turn of and on when driving too. along with a parisitic drain...

    My printed circuit is a little rough I suspect the cause of most of my problems are there too.
     
  6. Rene Melten

    Rene Melten Veteran Member Lifetime Gold Member

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    I've only got a theory, if both circuits have the same color, ie dark green, could they somehow have been tied together by mistake? Example: the heat sending wire wouldn't go thru the fuse panel, just the bulkhead connector.
     
  7. frankz

    frankz Veteran Member

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    I realize this doesn't help...but I've managed to avoid these kinds of gremlins by using only mechanical gauges.
     
  8. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    We've all been taking guesses....Let's work this out logically! :brightide

    What makes the temp guage needle move?
    Resistance through the #35 circuit with power from 39 and ground through 150.... Right?

    [​IMG]
    The needle is actually registering a DIFFERENCE in the two paths for ground... Correct?

    For someone that understands electronics... Please confirm this much of my analysis is correct so far!

    Thanks!
     
  9. 73Camaro383

    73Camaro383 Veteran Member Lifetime Gold Member

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    Dont think thats how it works twisted. The sending unit has an internal variable resistor when it gets hotter it lets more voltage though. The only thing the gauge is doing is showing that voltage increase. Dont think a difference in ground is right. The gauge is sort of like a metal spring that move when the voltage increases. The headlight switch isnt part of the cluster. The only thing the headlight switch does is provide the backlighting I believe.
     
    Last edited: Jan 4, 2011
  10. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    I may be completely off base but... Here's why I explained it that way.
    Keep in mind... Additional resistance lowers voltage. They are inversely related. (Ohms law, IIRC.)

    1. The resistance through the cluster ground should be very close to zero. (12V)

    2. When the resistance through the sending unit is high (voltage drop).... the gauge reads "cold".
    (Cluster ground circuit is still near zero Ohms and 12 Volts)

    3. As the sending unit heats up, the resistance drops (Voltage goes up) while the cluster ground circuit stays near zero.

    Ergo.... The closer to zero the resistance in the sending unit circuit gets (higher voltage), the further right the needle goes.

    So.... The gauge is effectively calibrated backwards in relation to resistance but is directly related to the voltage though the sending unit circuit.
    Variations in system voltage won't have much effect on the gauge because the two grounded circuits are "compared" to get a reading and you wouldn't need the cluster ground if it was a direct readig through the sending unit ground.


    I'm quite sure it's not the headlight switch causing the problem. The back lighting circuits only have the 150 (ground) circuit in common with the gauge.

    With the key off, a heavy load on the battery (horn, stereo or headlights) all cause the temp gauge needle to peg to the right.

    It may be time to head to the garage and experiment a little......
     
    Last edited: Jan 4, 2011

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