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

**First c10** · 06-08-2016, 03:59 PM

Hello guys, i have been searching and found answers to if the factory tach works or what you have to get it to work with a filter.
my problem is
I have a new HEI Distributor and installed a factory tach but when at 2000 RPM's it reads about 300 or so?
Any idea what I need to do?

Tdrcreyn4 · 06-08-2016, 04:53 PM

Have you verified you have the right tach. Are you running a V8/V6?

**First c10** · 06-08-2016, 05:40 PM

Sorry yes it is a V8 tach cluster out of a c50. It has the red v8 description on the back. The problem I am having is that it moves when you rev it up but only slightly.

davepl · 06-08-2016, 10:00 PM

I don't know about the factory originals but all of the aftermarket ones I've seen disclaim "NOT compatible with HEI" for some reason. I do not know the reason - though mine works with Pertronix ignition, I can't swear as to how accurate it is.

Worst case Dakota Digital (and others too no doubt) make a cheap tach adapter you can buy that will take most signals and filter+adapt them and probably be able to drive the tach.

**1969guy** · 06-08-2016, 10:11 PM

Unless it is damaged it should work just fine with an hei just it may bounce a little, the filter I believe will help curb that.

Now from my own experience with a factory tach, mine worked just fine with an original, obviously newer, hei but only after I realized that I had it hooked up backwards.

Check and make sure you have the wires running to the correct place, should be labeled on the back of the tach, and go from there.

When mine was hooked up backwards, it did just about like you said yours does, stays low rpms and barely moves.

**Stocker** · 06-09-2016, 01:46 AM

Quote:

Originally Posted by davepl

I don't know about the factory originals but all of the aftermarket ones I've seen disclaim "NOT compatible with HEI" for some reason. I do not know the reason - though mine works with Pertronix ignition, I can't swear as to how accurate it is.

I had my old Sun tach (Super II, iirc) since before I converted to HEI back in the Eighties. No problems before the switch.... or after. Pretty sure it has the original AC/Delco module if that matters.

Rich69shortfleet · 06-08-2016, 10:55 PM

I've read on here somewhere that the module in the HEI can make a difference. The recommendation was to only us a GM (AC/Delco) module to get the tachs to read right. I have a non-AC/Delco module and my reproduction tach reads just fine, but you said yours is a factory one so I could be all wet. If you are interested, my module is Standard brand.

**First c10** · 06-09-2016, 01:10 AM

Thanks guys for the information. I am going to try and switch the wires and see if that works. That will be the first and easiest step. I will report back. But any other idea please post away. M

06-08-2016, 03:59 PM	#1
First c10 Registered User Join Date: Dec 2012 Location: Los Banos CA Posts: 2,705	Factory Tach question Hello guys, i have been searching and found answers to if the factory tach works or what you have to get it to work with a filter. my problem is I have a new HEI Distributor and installed a factory tach but when at 2000 RPM's it reads about 300 or so? Any idea what I need to do? __________________ 1956 Chevy Bel Air 2 door 1956 Chevy 210 4 Door 1970 K20 LWB project the Hulk build w/Supercharged 4.8L-ly2/4L60E HULK BUILD 1970 C10 Yellow/White Deluxe LWB w/ 5.3L-LC9/6L80E 1968 K10 LWB Dark green my son calls it "THE HULK Jr"HULK JR SOLD GO GO!!!

06-08-2016, 04:53 PM	#2
Tdrcreyn4 Registered User Join Date: Aug 2013 Location: Atlanta, GA Posts: 702	Re: Factory Tach question Have you verified you have the right tach. Are you running a V8/V6? __________________ 1968 C10 292/3 spd - sold 1969 K10 350/3 spd/205 - sold 1971 K5 350/4 spd/ 205 - sold 1970 K20 350/350/205 - sold 1969 K20 396/400/205 - work in process

06-08-2016, 05:40 PM	#3
First c10 Registered User Join Date: Dec 2012 Location: Los Banos CA Posts: 2,705	Re: Factory Tach question Sorry yes it is a V8 tach cluster out of a c50. It has the red v8 description on the back. The problem I am having is that it moves when you rev it up but only slightly. __________________ 1956 Chevy Bel Air 2 door 1956 Chevy 210 4 Door 1970 K20 LWB project the Hulk build w/Supercharged 4.8L-ly2/4L60E HULK BUILD 1970 C10 Yellow/White Deluxe LWB w/ 5.3L-LC9/6L80E 1968 K10 LWB Dark green my son calls it "THE HULK Jr"HULK JR SOLD GO GO!!!

06-08-2016, 10:00 PM	#4
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Factory Tach question I don't know about the factory originals but all of the aftermarket ones I've seen disclaim "NOT compatible with HEI" for some reason. I do not know the reason - though mine works with Pertronix ignition, I can't swear as to how accurate it is. Worst case Dakota Digital (and others too no doubt) make a cheap tach adapter you can buy that will take most signals and filter+adapt them and probably be able to drive the tach. __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

06-08-2016, 10:11 PM	#5
1969guy Senior Member Join Date: Mar 2012 Location: Royston, Ga Posts: 625	Re: Factory Tach question Unless it is damaged it should work just fine with an hei just it may bounce a little, the filter I believe will help curb that. Now from my own experience with a factory tach, mine worked just fine with an original, obviously newer, hei but only after I realized that I had it hooked up backwards. Check and make sure you have the wires running to the correct place, should be labeled on the back of the tach, and go from there. When mine was hooked up backwards, it did just about like you said yours does, stays low rpms and barely moves. __________________ Evan - 1969 C10 LWB fltsd 307 3spd man.steer 3.73 rear orig.white/turquoise poly with drk turquoise int - 1972 Custom Deluxe10 LWB fltsd 350/TH350 pwr. steer/brks A/C 3.07posi rr orig.med blue with blue cloth trim. Last edited by 1969guy; 06-08-2016 at 11:05 PM.

06-08-2016, 10:55 PM	#7
Rich69shortfleet Registered User Join Date: May 2016 Location: Waverly, NE Posts: 382	Re: Factory Tach question I've read on here somewhere that the module in the HEI can make a difference. The recommendation was to only us a GM (AC/Delco) module to get the tachs to read right. I have a non-AC/Delco module and my reproduction tach reads just fine, but you said yours is a factory one so I could be all wet. If you are interested, my module is Standard brand. __________________ 1965 Malibu SS L79 Restoration Page 1965 Two-Door Wagon Project Progress Page 2006 Pontiac GTO Preservation Project Rich Cummings

06-09-2016, 01:10 AM	#8
First c10 Registered User Join Date: Dec 2012 Location: Los Banos CA Posts: 2,705	Re: Factory Tach question Thanks guys for the information. I am going to try and switch the wires and see if that works. That will be the first and easiest step. I will report back. But any other idea please post away. M __________________ 1956 Chevy Bel Air 2 door 1956 Chevy 210 4 Door 1970 K20 LWB project the Hulk build w/Supercharged 4.8L-ly2/4L60E HULK BUILD 1970 C10 Yellow/White Deluxe LWB w/ 5.3L-LC9/6L80E 1968 K10 LWB Dark green my son calls it "THE HULK Jr"HULK JR SOLD GO GO!!!



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