Factory Tach help... Does NOT go to ZERO why? HELP - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72Levi · 12-22-2007, 06:33 PM

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

HuggerCST · 12-22-2007, 07:38 PM

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

Jtrux · 12-22-2007, 07:42 PM

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.

vtblazer · 12-23-2007, 12:31 AM

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...

**jaros44sr** · 12-23-2007, 01:36 AM

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...

Blazester · 12-23-2007, 01:25 PM

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

Jtrux · 12-23-2007, 04:00 PM

Quote:

Originally Posted by Blazester

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

Good idea

72Levi · 12-23-2007, 05:34 PM

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.

12-22-2007, 06:33 PM	#1
72Levi Active Member Join Date: Feb 2006 Location: sa tx Posts: 851	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 __________________ 1972 K5 Blazer ALL ORIGINAL paint & engine, A/C, Tilt, 4sp, AM/FM, Tow Hooks, Aux Battery Tray, Tach, Vacuum 2004 FORD FX4 SuperCrew 2005 CTS-V 12 sec family car 2008 Buick Enclave AWD, Nav, & DVD, Wife LOVES it!

12-22-2007, 07:38 PM	#2
HuggerCST Registered User Join Date: Jul 2001 Location: North Louisiana Posts: 4,501	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. __________________ Wynne 70 CST short fleet Hugger orange & white, 350/350, ps, pb, air, tilt, tach/vac/speedwarning, original buckets, AM/FM, oak bed floor, shoulder belts, 3.5"/5.5" drop, 20" American Racing VN425s. Build thread 53 Chevy shortbed 69 Camaro Click here to subscribe Chevy GMC International Truck Club Louisiana Charter Member http://www.louisianaclassictruckclub.com

12-22-2007, 07:42 PM	#3
Jtrux I'm a poor spectator Join Date: Aug 2006 Location: San Antonio, Tx Posts: 2,287	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. __________________ 2006 Jeep Unlimited IMPACT ORANGE 1993 Chevy 2500 4x4 ExCab LWB 454/NV4500 Tow rig 1977 Ford F100 2x4 LWB 1st truck I owned, still have it!!! 1979 Ford F150 4x4 SWB Built Ford Tough!!! 1971 Chevy Blazer 350 / SM465 / NP205 UNDER CONSTRUCTION Soon to have a LQ4 6.0!!!

12-23-2007, 12:31 AM	#4
vtblazer Registered User Join Date: Oct 2003 Location: Central Vermont Posts: 8,537	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...

12-23-2007, 01:36 AM	#5
jaros44sr Senior Member Join Date: May 2006 Location: Philadelphia, Pa. 19454 Posts: 9,780	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... __________________ Semper Fi...Uncle Sam, you da man All parts offered to help are free, unless otherwise noted Dont try this stuff in my build thread, unless you have 55 years of mechanical OTJ training SAFETY FIRST AS usual, off topic They say your mind goes second, can't remember the first Jim

12-23-2007, 01:25 PM	#6
Blazester Registered User Join Date: Jun 2007 Location: Duluth, GA Posts: 51	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

12-23-2007, 05:34 PM	#8
72Levi Active Member Join Date: Feb 2006 Location: sa tx Posts: 851	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. __________________ 1972 K5 Blazer ALL ORIGINAL paint & engine, A/C, Tilt, 4sp, AM/FM, Tow Hooks, Aux Battery Tray, Tach, Vacuum 2004 FORD FX4 SuperCrew 2005 CTS-V 12 sec family car 2008 Buick Enclave AWD, Nav, & DVD, Wife LOVES it! Last edited by 72Levi; 12-23-2007 at 05:37 PM.



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