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

70LonghornCST · 05-01-2009, 03:35 AM

I recently installed an American Auto Wire engine compartment harness. I then talked, by chance, to a tech that helped design the harness, and he said that I will need to run an inline tach filter if using the factory tach in order to get the proper RPM readings since I am using an HEI distributor. Has anyone who has installed the American Auto Wire harness come across this problem. I have not yet installed the new cluster with the tach into my truck so I am not sure what reading I will get. What do you think? I'd hate to be hasty and run out to purchase a tach filter only to find out that I did not need it.

Andy4639 · 05-01-2009, 03:47 AM

I don't have the wiring harness you speak of but my factory tack plugs right to the cap on my hei and doesn't have anything to do with the wiring harness in the dash or under the hood.

70LonghornCST · 05-01-2009, 03:59 AM

True, but the battery part of the harness plugs into the HEI and may have an effect on the spikes in voltage, possibly. I guess I should have been more clear.

Longhorn Man · 05-01-2009, 05:30 AM

sounds like you spoke to more of a salesman than a tech.

70LonghornCST · 05-01-2009, 08:46 AM

Actually, he's an electrical engineer. I am looking for someone here with actual experience that has used the "American Auto Wire" harness. American Auto Wire does not sell the inline tach filter, and I had purchaed the harness 2 years before talking to him, so, no, he was not trying to sell me anything except one NOS Woodgrain Longhorn molding.

Longhorn Man · 05-01-2009, 04:39 PM

We have hundreds of guys here who have wired up straight from the HEI to the tach. The maker of the wire doesn't matter one bit, its still copper strands wrapped in plastic/rubber/poly cover. Guys have been hooking them up like this since the late 70's.

70LonghornCST · 05-01-2009, 05:06 PM

Thank you Longhorn Man. I do know that they work with factory wires, and one would assume that they would work. However, unless you have actually hooked up this exact harness up, then you cannot help me. I appreciate your oppinion, though, but I am looking for someone who has actually purchased an American Autowire harness and hooked it up to an HEI distributor and OEM tach.

Anyone?

ksbrktracer · 05-01-2009, 06:30 PM

For what it is worth my 83 monte SS had a tach filter that went bad and I got a new one from the local chevrolet dealer.........maybe a good source for one. I dont know the purpose of it and I cant help with your question either. Is the factory wire a resistor wire and maybe the aftermarket wire is not ??????? Hmmmmmmmm

Tommy

Longhorn Man · 05-01-2009, 07:28 PM

factory is not, it's just a wire.

70LonghornCST · 05-01-2009, 07:37 PM

http://www.americanautowire.com/faq/?p=400

cparman · 05-01-2009, 07:48 PM

Longhorn man, I am not looking for a fight, but do you remember on the late 70 Corvettes there was small square transistorized gizmo the bolted to the intake manifold near the hei distributor? I know I am old as crap, but think I remember that having something to do with the tach?? Maybe this is what the guy is talking about? I too would like my tach to read accurately, I have a very long 5 mile drive to work, and I REALLY need to know how many RPM i am turning while driving 25 MPH on Seminole Rd. ha ha

70LonghornCST · 05-01-2009, 07:53 PM

I went to the website and found the link above. It appears that what they are claiming is this:

"HEI and other electronic distributors create a different signal or pulse then do original points style distributors. For this reason, your OEM tach that was circuited to read the latter, does not recognize and cannot read the signal it’s receiving from the newer style distributor. The fix for this situation is to purchase a tach filter or interface device. They are available through several aftermarket manufacturers. They can be found on most GM cars in junkyards that have factory tachs in them. The factory style filter looks like a condenser with a wire and connector on each end, one of which plugs onto the HEI distributor, and the other onto the tach lead wire. Hooking this filter device in series between your distributor and tach should allow your factory tach to work in harmony with your new electronic distributor."

So, in retrospect, it appears that what they are claiming is that it is the HEI, not the wire itself. So, Longhorn Man is certainly correct there. I guess the only way to find out for sure is to hook it up and see. I'll be back in a day or two to let you know the results.

ksbrktracer · 05-01-2009, 07:58 PM

Im wrong.......I was thinking about the coil wire being a resistor wire and having to run a new power wire to the coil on the HEI , not the tach.....oops.....Im not sure of the reason you would need the tach filter, But I know my 83 Monte Carlo SS tach wouldnt work without it , It had all factory components in it too, HEI and all, I believe my later corvette had it also.......

Andy4639 · 05-01-2009, 09:02 PM

I guess the factory tach in my 71 hasn't worked correctly in the last 20 years then!
I have no filter on it and it works fine. I don't know what could be in the harness or the HEI to make it not work correctley.

Longhorn Man · 05-01-2009, 11:39 PM

I don't know anything about the corevettes, I just know that I've hooked up 6 or 7 stock tachs to HEI with nothing but a wire, and have never had an issue, I know guys have been doing this since I was still in diapers, and as far as acurate, a stock tach wasn't very acurate back in the day... nor are the repro ones.

**leddzepp** · 05-02-2009, 02:51 AM

FWIW, my stock tach is hooked up with nothing but a wire to the HEI. I also have a 700R4, and my gauges read real close to Andy4639's. I suspect he has slightly higher (numerically) gears than I do. I have 3.08.

Andy4639 · 05-02-2009, 03:54 AM

I have the 4:10 gear set in mine. I'm not claiming anything about the tach except what I know and I have hooked up at lest a half dozen and never had a problem. They aren't that acrurate as stated but they are nice to have. The factory shop manuals don't say anything about a filter either.

Mthird · 05-02-2009, 04:11 AM

Thank you to everybody that answered to this post.... I have been planning on purchasing a complete dash cluster for My 71 C-10 and I wanted to buy the one with the factory tach. it said in the description, cannot be hooked up to an HEI distributor, which kinda turned me off to putting a factory tach in it, cause im not getting rid of my HEI, this brings hope to my situation....

**Classic Heartbeat** · 05-02-2009, 11:42 AM

I don't know for sure about US Autowire, I haven't purchased the tach harness from them specificly. However I have from M&H and after looking at some of US Autowire's harnesses and compairing them to M&H's harnesses I suspect they are the same. However I am not absolutly possitive on this point. I do know that M&H (as well as us) sell a tach harness that has been converted to plug strait into the GM style HEI distributer. I have installed them, used them, and they work without any issues. The only problems that I have found is with the aftermarket tachs. Some of them have the "V" shape of the terminals flipped upside down. Causing the tach to be wired backwords. WES

Skirkpat · 05-02-2009, 02:32 PM

Don't know about tach's but I do know that when we ran amplifiers, you would run a filter, did'nt have to but it sure worked better when we did. It may have something to do with electrical "noise" from the HEI? unless it is expensive it might be worth while to follow the instructions? Just my.02

Huck · 05-02-2009, 03:10 PM

A few years back, one of the members (an engineer) of web site Colorado K5 did an analysis of factory tach's hooked up to both stock ignition and then to HEI. He came up with the signal wave length of the HEI was different then the stock ignition and he recommended some kind of filter.
AND NOW A TASTE OF REALITY---I HAVE PERSONALLY INSTALLED DOZENS OF FACTORY TACH'S IN BOTH STOCK AND HEI IGNITION AND THE DIFFERENCE IS SO SMALL YOU CAN'T REALLY TELL. I have also tested them against a new fully electronic tach for a test of accuracy---the factory tach is not as smooth or as accurate when installed in either original ignition or in HEI ignition. The factory tack is not fully electronic---it has a mechanical component that is influenced by temperature, dirt, vibration etc.
So if you want to run a new dash cluster with an old styple tach--if the tach is good, it will work fine with HEI ignition (relatively speaking). It's accuracy will be close enough for most drivers--giving rpm within a very small % or accurate (probably less then 3%deviation).
I have sold probably 30-40 Tach Dash's with never a problem due to relative accuracy. Further note---the factory had at least 4 different tach designs installed in their 67-72 trucks --I know because I disassemble evey one and inspect it before I sell it.
If you want to pursue a filter for greater accuracy, go to ColoradoK5 and do a search on Tach accuracy and perhaps the writeup is still around. One more thought, the wiring harness has basically nothing to do with the tach connection which comes directly off the distributor coil. When you change over to HEI, you do require a heavier gauge wire from the ignition but assuming the new harness you are buying is correctly built for HEI---you should be good to go.
Huck

70LonghornCST · 05-02-2009, 04:45 PM

Quote:

Originally Posted by Huck

A few years back, one of the members (an engineer) of web site Colorado K5 did an analysis of factory tach's hooked up to both stock ignition and then to HEI. He came up with the signal wave length of the HEI was different then the stock ignition and he recommended some kind of filter.
AND NOW A TASTE OF REALITY---I HAVE PERSONALLY INSTALLED DOZENS OF FACTORY TACH'S IN BOTH STOCK AND HEI IGNITION AND THE DIFFERENCE IS SO SMALL YOU CAN'T REALLY TELL. I have also tested them against a new fully electronic tach for a test of accuracy---the factory tach is not as smooth or as accurate when installed in either original ignition or in HEI ignition. The factory tack is not fully electronic---it has a mechanical component that is influenced by temperature, dirt, vibration etc.
So if you want to run a new dash cluster with an old styple tach--if the tach is good, it will work fine with HEI ignition (relatively speaking). It's accuracy will be close enough for most drivers--giving rpm within a very small % or accurate (probably less then 3%deviation).
I have sold probably 30-40 Tach Dash's with never a problem due to relative accuracy. Further note---the factory had at least 4 different tach designs installed in their 67-72 trucks --I know because I disassemble evey one and inspect it before I sell it.
If you want to pursue a filter for greater accuracy, go to ColoradoK5 and do a search on Tach accuracy and perhaps the writeup is still around. One more thought, the wiring harness has basically nothing to do with the tach connection which comes directly off the distributor coil. When you change over to HEI, you do require a heavier gauge wire from the ignition but assuming the new harness you are buying is correctly built for HEI---you should be good to go.
Huck

Huck,
The tach harness I have has the coil connection. Are you saying that the harness designed for the old style coil connection may have a different gauge wire than one designed for an HEI? Or, are they the same, just possessing differnt end terminals?

Huck · 05-02-2009, 09:10 PM

The wire from the coil to the tach is not that critical for either the HEI or the original ignition system. Just use the tach wiring harness that comes with the tach gauge cluster or with your new custom wire harness.

70LonghornCST · 05-10-2009, 12:24 AM

Well,
I finally got around to digging out the instrument cluster and checking out how the tach works without using a filter as suggested by American Autowire. All I got to say is that nothing beats the experience that is on this board. YOU GUYS ROCK. Thanks to all who posted. As you can see by my two pictures, the 1970 tach read 750 RMP at idle. When I compared this to my 2006 tach, which read 700 RMP, I can say that it's pretty much dead on.
Thanks again.

Bluegrass · 07-27-2012, 12:51 PM

Ok I might be the one an only with a tach that is not working correctly after I installed an HEI dist. Guess I need to buy and try a filter.....my terminals on the back of the factory 1967 tach are "V" shaped...I have a repo tach harness from LMC....and everything plugs in nice and neat. I have made sure the polarity has not been reversed.
The tach reads low and erratic. Any ideas other than the filter that may or may not do the trick? Thanks in advance.
Posted via Mobile Device

05-01-2009, 03:35 AM	#1
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Tach Question I recently installed an American Auto Wire engine compartment harness. I then talked, by chance, to a tech that helped design the harness, and he said that I will need to run an inline tach filter if using the factory tach in order to get the proper RPM readings since I am using an HEI distributor. Has anyone who has installed the American Auto Wire harness come across this problem. I have not yet installed the new cluster with the tach into my truck so I am not sure what reading I will get. What do you think? I'd hate to be hasty and run out to purchase a tach filter only to find out that I did not need it. __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal Last edited by 70LonghornCST; 05-01-2009 at 03:37 AM.

05-01-2009, 03:47 AM	#2
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Tach Question I don't have the wiring harness you speak of but my factory tack plugs right to the cap on my hei and doesn't have anything to do with the wiring harness in the dash or under the hood. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

05-01-2009, 03:59 AM	#3
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question True, but the battery part of the harness plugs into the HEI and may have an effect on the spikes in voltage, possibly. I guess I should have been more clear. __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal Last edited by 70LonghornCST; 05-01-2009 at 04:01 AM.

05-01-2009, 05:30 AM	#4
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Tach Question sounds like you spoke to more of a salesman than a tech. Last edited by Longhorn Man; 05-01-2009 at 05:30 AM.

05-01-2009, 08:46 AM	#5
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question Actually, he's an electrical engineer. I am looking for someone here with actual experience that has used the "American Auto Wire" harness. American Auto Wire does not sell the inline tach filter, and I had purchaed the harness 2 years before talking to him, so, no, he was not trying to sell me anything except one NOS Woodgrain Longhorn molding. __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal

05-01-2009, 04:39 PM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Tach Question We have hundreds of guys here who have wired up straight from the HEI to the tach. The maker of the wire doesn't matter one bit, its still copper strands wrapped in plastic/rubber/poly cover. Guys have been hooking them up like this since the late 70's.

05-01-2009, 05:06 PM	#7
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question Thank you Longhorn Man. I do know that they work with factory wires, and one would assume that they would work. However, unless you have actually hooked up this exact harness up, then you cannot help me. I appreciate your oppinion, though, but I am looking for someone who has actually purchased an American Autowire harness and hooked it up to an HEI distributor and OEM tach. Anyone? __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal

05-01-2009, 06:30 PM	#8
ksbrktracer TNT-MOTORSPORTS Join Date: Mar 2008 Location: TULSA OKLAHOMA Posts: 2,692	Re: Tach Question For what it is worth my 83 monte SS had a tach filter that went bad and I got a new one from the local chevrolet dealer.........maybe a good source for one. I dont know the purpose of it and I cant help with your question either. Is the factory wire a resistor wire and maybe the aftermarket wire is not ??????? Hmmmmmmmm Tommy

05-01-2009, 07:28 PM	#9
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Tach Question factory is not, it's just a wire.

05-01-2009, 07:37 PM	#10
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question http://www.americanautowire.com/faq/?p=400 __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal

05-01-2009, 07:48 PM	#11
cparman sharp as a marble Join Date: Jan 2007 Location: atlantic beach,florida Posts: 1,082	Re: Tach Question Longhorn man, I am not looking for a fight, but do you remember on the late 70 Corvettes there was small square transistorized gizmo the bolted to the intake manifold near the hei distributor? I know I am old as crap, but think I remember that having something to do with the tach?? Maybe this is what the guy is talking about? I too would like my tach to read accurately, I have a very long 5 mile drive to work, and I REALLY need to know how many RPM i am turning while driving 25 MPH on Seminole Rd. ha ha

05-01-2009, 07:53 PM	#12
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question I went to the website and found the link above. It appears that what they are claiming is this: "HEI and other electronic distributors create a different signal or pulse then do original points style distributors. For this reason, your OEM tach that was circuited to read the latter, does not recognize and cannot read the signal it’s receiving from the newer style distributor. The fix for this situation is to purchase a tach filter or interface device. They are available through several aftermarket manufacturers. They can be found on most GM cars in junkyards that have factory tachs in them. The factory style filter looks like a condenser with a wire and connector on each end, one of which plugs onto the HEI distributor, and the other onto the tach lead wire. Hooking this filter device in series between your distributor and tach should allow your factory tach to work in harmony with your new electronic distributor." So, in retrospect, it appears that what they are claiming is that it is the HEI, not the wire itself. So, Longhorn Man is certainly correct there. I guess the only way to find out for sure is to hook it up and see. I'll be back in a day or two to let you know the results. __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal

05-01-2009, 07:58 PM	#13
ksbrktracer TNT-MOTORSPORTS Join Date: Mar 2008 Location: TULSA OKLAHOMA Posts: 2,692	Re: Tach Question Im wrong.......I was thinking about the coil wire being a resistor wire and having to run a new power wire to the coil on the HEI , not the tach.....oops.....Im not sure of the reason you would need the tach filter, But I know my 83 Monte Carlo SS tach wouldnt work without it , It had all factory components in it too, HEI and all, I believe my later corvette had it also.......

05-01-2009, 09:02 PM	#14
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Tach Question I guess the factory tach in my 71 hasn't worked correctly in the last 20 years then! I have no filter on it and it works fine. I don't know what could be in the harness or the HEI to make it not work correctley. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

05-01-2009, 11:39 PM	#15
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: Tach Question I don't know anything about the corevettes, I just know that I've hooked up 6 or 7 stock tachs to HEI with nothing but a wire, and have never had an issue, I know guys have been doing this since I was still in diapers, and as far as acurate, a stock tach wasn't very acurate back in the day... nor are the repro ones.



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05-02-2009, 02:51 AM	#16
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 19,993	Re: Tach Question FWIW, my stock tach is hooked up with nothing but a wire to the HEI. I also have a 700R4, and my gauges read real close to Andy4639's. I suspect he has slightly higher (numerically) gears than I do. I have 3.08. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man Last edited by leddzepp; 05-02-2009 at 02:52 AM. Reason: added info

05-02-2009, 03:54 AM	#17
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Tach Question I have the 4:10 gear set in mine. I'm not claiming anything about the tach except what I know and I have hooked up at lest a half dozen and never had a problem. They aren't that acrurate as stated but they are nice to have. The factory shop manuals don't say anything about a filter either. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs. Last edited by Andy4639; 05-02-2009 at 03:54 AM.

05-02-2009, 04:11 AM	#18
Mthird Registered User Join Date: Apr 2009 Location: Kennedale, TX Posts: 8	Re: Tach Question Thank you to everybody that answered to this post.... I have been planning on purchasing a complete dash cluster for My 71 C-10 and I wanted to buy the one with the factory tach. it said in the description, cannot be hooked up to an HEI distributor, which kinda turned me off to putting a factory tach in it, cause im not getting rid of my HEI, this brings hope to my situation.... __________________ 71 Chevy Custom Deluxe 10 All original, although in need of some TLC. only 3 owners, my grandpa, my dad, and me.

05-02-2009, 11:42 AM	#19
Classic Heartbeat Project Junkie! Fishing Poor!! Join Date: Dec 1999 Location: Olympia,Wa. 98512 Posts: 10,770	Re: Tach Question I don't know for sure about US Autowire, I haven't purchased the tach harness from them specificly. However I have from M&H and after looking at some of US Autowire's harnesses and compairing them to M&H's harnesses I suspect they are the same. However I am not absolutly possitive on this point. I do know that M&H (as well as us) sell a tach harness that has been converted to plug strait into the GM style HEI distributer. I have installed them, used them, and they work without any issues. The only problems that I have found is with the aftermarket tachs. Some of them have the "V" shape of the terminals flipped upside down. Causing the tach to be wired backwords. WES __________________ On-line catolog Classic Heartbeat Pickup Parts WEHEPP@comcast.net Call us Toll Free (888) 338-2502 Like Us on Facebook No appointment necessary. 72 BB Chevy Custom Deluxe / Custom Camper 67 I-6 (Soon to be 5.7 LS1) Panel 68 BB Chevy Short Bed 72 6.0 LQ9 Short Bed Chevy 4X4 Olympia's fastest growing truck parts supplier. Olympia's home for Wayward Chevys

05-02-2009, 02:32 PM	#20
Skirkpat Registered User Join Date: Dec 2008 Location: K.C. Missouri Posts: 1,024	Re: Tach Question Don't know about tach's but I do know that when we ran amplifiers, you would run a filter, did'nt have to but it sure worked better when we did. It may have something to do with electrical "noise" from the HEI? unless it is expensive it might be worth while to follow the instructions? Just my.02

05-02-2009, 03:10 PM	#21
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	Re: Tach Question A few years back, one of the members (an engineer) of web site Colorado K5 did an analysis of factory tach's hooked up to both stock ignition and then to HEI. He came up with the signal wave length of the HEI was different then the stock ignition and he recommended some kind of filter. AND NOW A TASTE OF REALITY---I HAVE PERSONALLY INSTALLED DOZENS OF FACTORY TACH'S IN BOTH STOCK AND HEI IGNITION AND THE DIFFERENCE IS SO SMALL YOU CAN'T REALLY TELL. I have also tested them against a new fully electronic tach for a test of accuracy---the factory tach is not as smooth or as accurate when installed in either original ignition or in HEI ignition. The factory tack is not fully electronic---it has a mechanical component that is influenced by temperature, dirt, vibration etc. So if you want to run a new dash cluster with an old styple tach--if the tach is good, it will work fine with HEI ignition (relatively speaking). It's accuracy will be close enough for most drivers--giving rpm within a very small % or accurate (probably less then 3%deviation). I have sold probably 30-40 Tach Dash's with never a problem due to relative accuracy. Further note---the factory had at least 4 different tach designs installed in their 67-72 trucks --I know because I disassemble evey one and inspect it before I sell it. If you want to pursue a filter for greater accuracy, go to ColoradoK5 and do a search on Tach accuracy and perhaps the writeup is still around. One more thought, the wiring harness has basically nothing to do with the tach connection which comes directly off the distributor coil. When you change over to HEI, you do require a heavier gauge wire from the ignition but assuming the new harness you are buying is correctly built for HEI---you should be good to go. Huck

05-02-2009, 09:10 PM	#23
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	Re: Tach Question The wire from the coil to the tach is not that critical for either the HEI or the original ignition system. Just use the tach wiring harness that comes with the tach gauge cluster or with your new custom wire harness.

05-10-2009, 12:24 AM	#24
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Tach Question Well, I finally got around to digging out the instrument cluster and checking out how the tach works without using a filter as suggested by American Autowire. All I got to say is that nothing beats the experience that is on this board. YOU GUYS ROCK. Thanks to all who posted. As you can see by my two pictures, the 1970 tach read 750 RMP at idle. When I compared this to my 2006 tach, which read 700 RMP, I can say that it's pretty much dead on. Thanks again. Attached Images __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal Last edited by 70LonghornCST; 05-10-2009 at 12:25 AM.

07-27-2012, 12:51 PM	#25
Bluegrass Registered User Join Date: Apr 2006 Location: cedar falls IOWA Posts: 7	Re: Tach Question Ok I might be the one an only with a tach that is not working correctly after I installed an HEI dist. Guess I need to buy and try a filter.....my terminals on the back of the factory 1967 tach are "V" shaped...I have a repo tach harness from LMC....and everything plugs in nice and neat. I have made sure the polarity has not been reversed. The tach reads low and erratic. Any ideas other than the filter that may or may not do the trick? Thanks in advance. Posted via Mobile Device

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