Tach problems - The 1947 - Present Chevrolet & GMC Truck Message Board Network

busterwivell · 09-18-2002, 01:55 PM

Anyone know why my tach would work accurately sometimes, and be way off others? Is it just screwed up, or is there something I can do to make it accurate all the time. 55 MPH is 3000 RPM, normally. Sometimes, though, it's more like 4000 RPM. Would rather fix than replace, if I can.

red71cheyenne · 09-18-2002, 01:58 PM

Is this a stock tach? Sounds like it is a loose connection. I would check to see if you have all the connections tight. Good luck, Jeff.

busterwivell · 09-18-2002, 02:01 PM

Yah, factory in-dash tach

BobbyK · 09-18-2002, 02:34 PM

Are you running HEI or the old points.The orig. tach was designed with the points in mind.
A filter can be made and placed "inline" on the tach wire.This will help with the erroneous readings.

palallin · 09-18-2002, 03:50 PM

OK, my high school electronics shop is a long way behind me: can someone interpret the schematic for me?

busterwivell · 09-18-2002, 04:17 PM

I have an HEI. I don't understand the schematic (I'm old). Where can I get a filter? just connect it in the HEI wire for the tach? Thanks!

Michael A. K. G · 09-18-2002, 05:09 PM

That's a double low pass filter, one with a pass at 12 Hz and the other at 1 MHz. The right one isn't going to do anything; the signal it gets will already be smoothed at such high frequencies.

Why did you put in the second filter?

red71cheyenne · 09-18-2002, 05:22 PM

You dont' need the filter. I run a factory tach with my Tune Port and have never had a problem. Just hook it up to the tach pin on the distributor and run the power lead to a pink wire. Jeff.

Stout72 · 09-18-2002, 06:05 PM

Busterwivell, mine does that, too. Factory tach, it seems to indicate approx 800 rpm higher when it's cold outside (like on the way in to work) compared to when it's warm out (like on the way home). I'm also running HEI, no filter. Was a bit curious about why.

Longhorn Man · 09-18-2002, 08:08 PM

Its becouse tehy were not acurate 30 years ago, and they are getting worse with age.

BobbyK · 09-19-2002, 04:12 AM

The tach schematic was posted on another board(coloradok5.com) that's where I stole it from.
I personally don't have a filter with my factory tach and don't have any problems( I run a G.M. TPI unit as well).
I know guys have problems with factory tach's and HEI.I heard it's has something to do with the shape of the wavelength in the HEI signal.I think,don't quote me though,the HEI signal is square where-as the points signal is more round.I think that is why the factory tach has problems with the HEI signal.
Plus,as longhornmail has stated,the built-in inaccuracey of the factory tach's,along with age could have something to do with it.
I believe the filter rounds off the HEI signal so it's looks more like a points signal to the tach.

busterwivell · 09-19-2002, 06:09 PM

Anyone with a factory tach and an HEI that's NOT having a problem, and if so, did you do something we should do?

Gene · 09-19-2002, 07:36 PM

I am running a factory tach. and a GM HEI. Ran a wire to the tach side of the distributor, and the other(red wire) to the fuse box, plugged it into a spare lead. Oh, are you using a factory wiring harness from a tach. truck? Mine was a used tach. dash and it came with the wiring harness with it. The wires are pretty heavy guage i'd say 10 gauge at least they are almost 1/4 inch diameter. Maybe your wires are too small for the tach. to work right? I dunno just a guess...

09-18-2002, 01:55 PM	#1
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	Tach problems Anyone know why my tach would work accurately sometimes, and be way off others? Is it just screwed up, or is there something I can do to make it accurate all the time. 55 MPH is 3000 RPM, normally. Sometimes, though, it's more like 4000 RPM. Would rather fix than replace, if I can. __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

09-18-2002, 01:58 PM	#2
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Is this a stock tach? Sounds like it is a loose connection. I would check to see if you have all the connections tight. Good luck, Jeff. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-18-2002, 02:01 PM	#3
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	Yah, factory in-dash tach __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

09-18-2002, 02:34 PM	#4
BobbyK Registered User Join Date: May 2000 Location: Petrolia,Ontario,Canada but working in Port Huron,Mi. Posts: 1,771	Are you running HEI or the old points.The orig. tach was designed with the points in mind. A filter can be made and placed "inline" on the tach wire.This will help with the erroneous readings. __________________ 71 blazer,350SBC,approx.375HP,700R4,factory GM TPI.Dual electric fans,33x12.5x15 ATR on stock suspension. Petrolia,Ontario,Canada but working in Port Huron,MI. See ALL my Blazer pic's HERE

09-18-2002, 03:50 PM	#5
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,238	OK, my high school electronics shop is a long way behind me: can someone interpret the schematic for me? __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

09-18-2002, 04:17 PM	#6
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	I have an HEI. I don't understand the schematic (I'm old). Where can I get a filter? just connect it in the HEI wire for the tach? Thanks! __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

09-18-2002, 05:09 PM	#7
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	That's a double low pass filter, one with a pass at 12 Hz and the other at 1 MHz. The right one isn't going to do anything; the signal it gets will already be smoothed at such high frequencies. Why did you put in the second filter?

09-18-2002, 05:22 PM	#8
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	You dont' need the filter. I run a factory tach with my Tune Port and have never had a problem. Just hook it up to the tach pin on the distributor and run the power lead to a pink wire. Jeff. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-18-2002, 06:05 PM	#9
Stout72 Not my good side. Join Date: Mar 2002 Location: Fairfield, California Posts: 222	Busterwivell, mine does that, too. Factory tach, it seems to indicate approx 800 rpm higher when it's cold outside (like on the way in to work) compared to when it's warm out (like on the way home). I'm also running HEI, no filter. Was a bit curious about why. __________________ 72 Chevy K20 Custom Camper, 350/350, 4" lift, dual shock suspension front & rear. Daily driver and a work in progress. 00 Suburban LT 90 Camaro RS 79 Yamaha XS1100 Special (Ol' Reliable)

09-18-2002, 08:08 PM	#10
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Its becouse tehy were not acurate 30 years ago, and they are getting worse with age.

09-19-2002, 04:12 AM	#11
BobbyK Registered User Join Date: May 2000 Location: Petrolia,Ontario,Canada but working in Port Huron,Mi. Posts: 1,771	The tach schematic was posted on another board(coloradok5.com) that's where I stole it from. I personally don't have a filter with my factory tach and don't have any problems( I run a G.M. TPI unit as well). I know guys have problems with factory tach's and HEI.I heard it's has something to do with the shape of the wavelength in the HEI signal.I think,don't quote me though,the HEI signal is square where-as the points signal is more round.I think that is why the factory tach has problems with the HEI signal. Plus,as longhornmail has stated,the built-in inaccuracey of the factory tach's,along with age could have something to do with it. I believe the filter rounds off the HEI signal so it's looks more like a points signal to the tach. __________________ 71 blazer,350SBC,approx.375HP,700R4,factory GM TPI.Dual electric fans,33x12.5x15 ATR on stock suspension. Petrolia,Ontario,Canada but working in Port Huron,MI. See ALL my Blazer pic's HERE Last edited by BobbyK; 09-19-2002 at 04:14 AM.

09-19-2002, 06:09 PM	#12
busterwivell Registered User Join Date: Jan 2002 Location: Sahuarita, AZ USA Posts: 2,033	Anyone with a factory tach and an HEI that's NOT having a problem, and if so, did you do something we should do? __________________ Bill - Tucson, AZ 47 Chevrolet 2 dr sedan 56 Chevrolet 210 2 dr sedan 66 Chevrolet Malibu 2 dr 4 speed 71 Chevrolet C30 Longhorn 86 Chevrolet Monte Carlo SS (Sold) 01 Chevrolet Tahoe IM - BUSTERWIVE@aol.com

09-19-2002, 07:36 PM	#13
Gene Registered User Join Date: Aug 2002 Location: Muskogee, Oklahoma Posts: 2,914	I am running a factory tach. and a GM HEI. Ran a wire to the tach side of the distributor, and the other(red wire) to the fuse box, plugged it into a spare lead. Oh, are you using a factory wiring harness from a tach. truck? Mine was a used tach. dash and it came with the wiring harness with it. The wires are pretty heavy guage i'd say 10 gauge at least they are almost 1/4 inch diameter. Maybe your wires are too small for the tach. to work right? I dunno just a guess... __________________ 1971 SWB 4X4 Build BOARD MEMBER #5448 BOOMER SOONER!!!!

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