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I have an old 85 Formula MX that I'm just gonna fix up for a spare sled because it doesn't really need any parts. Just a carb cleaning etc.
I rode it a bit last year, and when I got the sled the temp guage worked fine. Then, it just stopped working one day. I was just wondering where to start troubleshooting the temp guage problem. Basically, what I'm doing is signing the sled over to my buddy for free. He's gonna insure it so he has a sled to ride when he comes up north on weekends, and I've got an insured sled to use as a spare if mines broken down or something. He's not an experienced sledder, so it's important that the temp guage works so I can just say "Don't start driving the sled until the temp needle moves up." (to prevent a cold seizure, because he doesn't yet understand or know how long the sled should warm up for before riding.).

Also, is there any way to test the temp guage while I"m troubleshooting without running the engine? To see if I've been able to get the problem fixed? I really don't wanna run the sled for any period of time seeing's there's no snow yet. I have a voltage/ohm meter, that I can use. Where do I start? The main wiring harness looks good, with no signs of wear anywhere. I'm hoping it's a ground wire problem, but I don't even know what color the ground wire would be to follow it through the harness or anything.

Thanks.
 

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With the motor running jump the sender connection(violet) to ground and the gauge should read max temp. If this happens, the sender is faulty, if not run a redundant ground to the motor. The gauge has a yellow wire which feeds 12 volts to the meter and illumination coming from the voltage regulator and a green ground wire which changes to yellow/black at the dash connector it picks up from the chassis at the voltage regulator ground. My experience says it's a corroded connection at the dash connector or the regulator ground failed.
 
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