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Thread: S3 diesel running hot when instrument lights are on

  1. #1

    Default S3 diesel running hot when instrument lights are on

    I usually drive my 1983 S3 diesel tuesday to meet friends for pizza and beer. Last week when I left Round Table Pizza I had to use headlights and instrument lights for the 2.5 mile drive home. A mile and a half into the drive I noticed that the temperature was nearing the red zone and the glow plug indicator light was glowing ( not bright like when starting but a low level glow) ......I finished the drive home. The next morning I checked fluids and the fan belt....all was good. So this got me to thinking why did the diesel heat up with the lights on.....I have never seen it get hot before....but usually only drive it during the day. Then I took it for a short drive and after a long idle the temperature was normal. The only thing I could come up with is that the glow plugs were somehow "on" with the lights on. I checked voltage at the most forward plug with various light switch positions. When the headlights were on there was two volts at the plug. When the instrument lights were on ( and the headlights) there was just over 3 volts at the plug. When I turned the key ( with all lights off) to the glow plug position there was a little over 11 volts at the plug. Has anyone experienced this issue? What was the cause? I know I must pull out the instrument cluster out to check wiring. I have the wiring diagram but am perplexed how this situation could occur if the wiring behind the instrument panel is correct. The previous UK owner installed a new wiring harness. I looking for ideas on exactly what to look for. The glow plugs are the newer type. Comments welcomed and thanks in advance.
    Jim

  2. #2
    Join Date
    Jul 2020
    Location
    UK
    Posts
    300

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    In essence there should be zero volts at the glow plugs at all times other than when the switch is in the on position for the plugs. Looking at the wiring diagram the one common denominator, the " ignition/ starter " switch. Check the wiring to & from it for a bad connection, and see if it is working correctly and not has an internal fault. If the voltage is wrong at the switch it will affect the fuel and temperature gauges. I presume you have a steering column lock, the electrical switch, on its own, is still available as a spare No. 579084

  3. #3
    Join Date
    Dec 2010
    Posts
    395

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    if the glow plugs are thermostatically controlled i'd go to that controller. this controller simply eliminates turning the glow plugs on if the engine is allready warm. If your engine was really overheating you should be able to notice.
    you can also use a heat gun ......(point and pull trigger to read temps) anytime to measure engine temp for comparison (measure at top of block, preferably on the thermostat housing.
    if you have a thermal controller in the circuit i'd zoom on that.
    if it doesn't P480 is on the path too,

  4. #4

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    As a follow up question: would 3volts cause the glow plugs to heat up enough to raise the engine temperature into the red zone in a short drive? Is there a minimum voltage required to activate the glow plugs?

  5. #5

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    I figured it out. After checking the ignition switch ( by leaving the key in the off position) I turned on headlights and the instrument lights - glow plug light glowed.....removed the ground wire at the light.....as expected it did not glow......disconnected the grounding end of the same wire and reconnected the ground wire at the light....still no glow.....reconnected ground end of the wire at where it had been connected and the light glowed. So it was somehow getting voltage via the ground wire. I found another grounding point and connected there.....all is good. The original grounding point was the post on the speedometer....wonder why it was getting voltage when the lights were on? Now I have to put everything back....that seems to be the hardest thing to do.

  6. #6
    Join Date
    Jul 2020
    Location
    UK
    Posts
    300

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    Quote Originally Posted by jvsimmons View Post
    I figured it out. After checking the ignition switch ( by leaving the key in the off position) I turned on headlights and the instrument lights - glow plug light glowed.....removed the ground wire at the light.....as expected it did not glow......disconnected the grounding end of the same wire and reconnected the ground wire at the light....still no glow.....reconnected ground end of the wire at where it had been connected and the light glowed. So it was somehow getting voltage via the ground wire. I found another grounding point and connected there.....all is good. The original grounding point was the post on the speedometer....wonder why it was getting voltage when the lights were on? Now I have to put everything back....that seems to be the hardest thing to do.
    I had forgotten that the warning lights on a S3 are all grounded at the same location so there was a back feed due to the resistance on the grounding point. Congratulations in sorting it.

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