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I recently replaced my temp sending unit on my 77 400. Now my temp guage works, but often I notice the needle goes way over "H" for a brief moment 5-10 seconds, then returns to normal (abour 5/6 of full scale deflection; close to the hot end). I also just replaced my thermostat.
My question is
Does anybody else notice this? Could this be something bad, or is the meter just out of wack?
I'm beginning to think that my meter is just out of adjustment. But I never knew how well it worked before, because I bought the truck with a burnt sending unit.
If it does this when you just start out in the morning, your thermostat may be causing this. Usually in the morning the thermostat will let the truck warm up and then when it opens, cold water from the radiator will rush in and then make it close. It will do this a couple of times before it settles down. But it shouldn't peg the needle while doing this, so maybe the thermostat is sticking a little.
If the above is not the case, and the guage pegs randomly even after it is warmed up, I would say you have a loose connection or damaged wire going to the sending unit. Follow it along the harness to make sure it's not damaged.
Yeah, I've heard that it works by causing different resistances, so if the wire is corroded, melted, or pinched that could cause a higher resistance. Thanks for the input, I'll check it out.