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**painterljp** · 07-06-2018, 07:15 PM

This is on 72 GMC. Switched to HEI. Tachs wasn't hooked up before so don't know previous if it worked or not.

Tach is reading about 1100 rpm at idle when truck is actually running 550-600 rpm at idle. Then the tach only goes to 3500 rpms. Then it bottoms out at 3500 rpm and wont rev any higher. Then it's at about 1600 rpm. A friend at a shop put it on an oscilloscope. He's says the tachs is just faulty and may be but wish to check here before I get an aftermarket. Truck is all original survivor, we rebuilt motor and tranny and put the HEI in it. We tried a tach filter and no change in tach, still same.

Any ideas?

07-06-2018, 07:15 PM	#1
painterljp Senior Member Join Date: Nov 2014 Location: Albuquerque NM Posts: 903	Tach problems?? This is on 72 GMC. Switched to HEI. Tachs wasn't hooked up before so don't know previous if it worked or not. Tach is reading about 1100 rpm at idle when truck is actually running 550-600 rpm at idle. Then the tach only goes to 3500 rpms. Then it bottoms out at 3500 rpm and wont rev any higher. Then it's at about 1600 rpm. A friend at a shop put it on an oscilloscope. He's says the tachs is just faulty and may be but wish to check here before I get an aftermarket. Truck is all original survivor, we rebuilt motor and tranny and put the HEI in it. We tried a tach filter and no change in tach, still same. Any ideas?