Monster Tach stuck at 4k

Ok so my friend brought over a tach that he has had sitting in his garage, he says it works . anyways its stuck at 4k, how could i fix this and test the tach? would hooking it up to a power source cause it to go back to 0?

Its a 5in autometer sport comp with no shift light if it makes a difference.

Thanks, Chris.

Hook it up to a power source - it should drop back to 0. Mine does this in the car sometimes.

Hook it up.

alright, ill hook it up in a bit ive never dealt with a tach before . Thanks.

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Hook it up to a power source - it should drop back to 0. Mine does this in the car sometimes.

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Yep...mine too.