Factory Tach help... Does NOT go to ZERO why? HELP
 

Hello All, I installed a factory used tach I bought on eBay and it does not go to zero WHY? The needle does not go to zero, I adjusted the calibration in the back but it still stays around 400 rpm when shut off. I have factory points and coil. I connected it to the negative side of the coil, it works fine except does not go to zero. I think the tach may be stuck or bad, any ideas anyone? I also connected it to 12v keyed on the fuse box

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

You might want to check to see that the needle isn't rubbing somewhere. I've seen that make them stick.

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

I'm not sure about factory tachs but my aftermarket tach stays around 1000 when shut off but also works fine. I've seen several tachs do this.

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

Have you tried to 'readjust' the needle/pointer so it sits on zero when off?

Did they show pics of the tach in the e-bay add?
Was it sitting on zero then?
Could of got knocked during shipping maybe...

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

If you set up your points with a tach and dwell meter, you could see if it is reading correctly at rpm. If it is not rubbing as mentioned before, maybe you could 'tweak' it to the left a little...

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

NONE of the old O.E. tachs I've had ever go to zero when ignition is off !!!
Probably nothing wrong .....just the way they used to be made.

Try this trick, though..........
After you turn the ignition off..........bump the key back over to ON (don't start it) !!!

When I do that mine goes to zero !!!

Good luck..........but if it seems to tach correctly when you're driving it, I wouldn't mess with adjustments.

Blazester

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

Good idea

Re: Factory Tach help... Does NOT go to ZERO why? HELP
 

UPDATE... Thanks for all the tips guys. I got a hold of my brother, (out of town) he is an electrical contractor for the military. He knows all about transistors, resistors, diodes, gauges, etc... I took the tach out again and he told me to check if the needle is/was stuck. Possible foam, dust lodged against needle. Well, I took it apart and the needle does move to "zero" when you push it down. He told me that since it does that it def. is a resistor problem. A resistor on the back of the board is BAD. Instead of sending it to him, I simply bent the needle to zero. I do NOT recommend this as the needle can break. It did work, I got it to go down to about 50-100 rpm, instead of the 400 rpm it was on. It still works and registers close to accurate. The gauge is original Gm and 35 years old, some day I'll send it to him. He has everything to check, replace resistors. Thanks all, BTW when I bought it off ebay it was never at zero in the pic. I suspect the guy knew all along it was not in perfect condition. If you ever see one that does not go down to zero it may have a bad resistor. I had tried everything, adjusting the calibration, 12v, etc... In fact I was very concerned when I adjusted it all the way to the left that it did not even work. I centered the adjustment again and it was working again. Thanks all.