Weird gauge happenings

Hi - I've got a 1990 GMC 5.7L 4x2 pickup. When I turn it on, the temp gauge goes all the way to the left and stays there (and when I turn it off, it goes all the way to the right). Also, when I put my turn
signals on, the voltmeter moves left to right to left to right with the ticking of the lights. I'm guessing this is a crazy electrical thing, but where exactly under the hood should I be looking?
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Sounds like a ground problem I would check the ground wire connecting the engine to the body. Actually all grounds in general.

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Likely, it is a ground problem - in a round about way. The sender unit provides a variable resistance ground path, which is what causes the needle deflection. Sounds like his sender is bad, therefore the ground path is bad. In this case though the body ground is not likely to be the culprit.
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For the temp gauge problem you need a new sending unit. There are a couple (or a few) different temp senders for each of the engines and they all have a different resistance. You'll need to provide your VIN in order to get the right sending unit with the right resistance.
As for your directions and your meter - if all of your lights are working properly then don't worry about it.
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