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  1. #1
    FEP Member
    Join Date
    Dec 2012
    Location
    Jacksonville, NC
    Posts
    102

    Default Temp gauge reading too high

    Hey guys, having an issue with the coolant gauge in my '86. When I first start the car, the gauge reading normal, but within a 1/4 mile of normal driving it's sitting past the L of NORMAL, almost all the way to the top. I've verified the car is running at normal temp, hovering around 200, give or take a few degrees. How can I get my gauge closer? I know they aren't the most accurate, but this is way too far. I'm tested it a couple different ways, but I'm running out of ideas. I grounded the sensor wire, and it shot up even higher until removed. Combined with the fact that it works normally the first minute, I doubt i have a short. I've replaced the sending unit twice, both with quality units, no change. The oil pressure and fuel gauge work as expected, so I don't think it's the instrument voltage regulator. I verified all the ground wires, and even put a jumper cable from the engine to the battery to eliminate a grounding issue.

    Is there any way to test the gauge? I saw on a couple boards for different vehicles where they wired in a potentiometer to adjust the resistance of the sending unit, but that seems kinda hacky. Any thoughts?
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

  2. #2

    Default

    A high gauge reading indicates least resistance to ground, just like when you grounded the sender wire (*)... odd that there's a change (resistance decrease, gauge reading increase) after warmup and later down the road... teflon tape on sender's threads? ... though that would either create a full open (no connection) to ground (very low gauge reading) or increased resistance (low gauge reading)...

    So yes, it could very well be a short with the wire to ground, intermittent or heat/vibration related, causing direct to ground short circuit and high gauge range reading...

    Verified engine operating, thermostat-open temperature with a mechanical temperature gauge, other? Check the sending unit at ambient temperature and engine operating temperature. New parts do not = parts that are working correctly. It's resistance to ground at differing temperatures should mimic the following...

    Gauge/sender checking involves (from Haynes manual) removing the gauge wire and connecting a 10-ohm resistor to it's end and grounding that... the gauge should read in the hotter end of the gauge... (* warning in Haynes manual: "Caution: Do not ground the sending unit wire directly to the engine. This may damage the gauge or instrument voltage regulator.") ... and a repeat of the above with a 73-ohm resistor should indicate on the gauge in the colder end of the gauge range.
    Last edited by Walking-Tall; 07-30-2018 at 12:46 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  3. #3
    FEP Member
    Join Date
    Dec 2012
    Location
    Jacksonville, NC
    Posts
    102

    Default

    Yup, verified with a stand-alone temp gauge.

    That's good information, thank you. To test the gauge, if I shouldn't ground the resistor on the engine, where should I? Battery, fender, grounding strap?
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

  4. #4

    Default

    You're welcome. Just not supposed to ground only the sending unit wire, anywhere... testing the gauge (sending unit wire, disconnected) should be going through at least 10-ohms of resistance, to ground... is the point, so there's no zero ohms to ground and possible gauge/IVR damage.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  5. #5
    FEP Member
    Join Date
    Dec 2012
    Location
    Jacksonville, NC
    Posts
    102

    Default

    Figured I should update. The "high temp" issue started before the cylinder head swap, and actually indirectly caused it to happen. When testing one last sensor, I accidentally cracked the bung in the lower manifold. Since the exhaust gaskets were leaking, it wouldn't be much more effort to pull the heads.

    After getting it all back together, the temp still read high. After driving around for a few days, the motor spit up a big air bubble back into the radiator. After adding another half gallon or so, everything is reading properly. So it's possible that replacing that final sensor with one from a different manufacturer did the trick, or maybe making the switch from Teflon tape to liquid thread sealer was the fix.
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

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