# 1966 GTO Tach Issue



## 21553 (Sep 30, 2012)

Hello,

New GTO owner and new to the site. Thanks to all for any help and I look forward to helping others where I can.

Well here is my tach issue. It is an in dash tack and being used with a HEI distributor. I checked the wire from the distributor to the tach. It is good and has continuity. The other is an orange/black stripe that is also connected to the cigarette lighter. I have searched long for a wiring diagram or tach connections with no luck.

The issue is that the tach works at first start-up for under a minute. Then it will just start dropping off then go to zero. Here is a link to a short video I took this morning before heading out to a show. 

https://www.box.com/s/okmjxw3nbsqtr7a8bw0j

The file is about 70MB. I have read several places that if folks are having tach issues the car doesn't run or not well. Car runs great and no issues. Any ideas or has anyone experienced this? 

Thanks.:confused


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## chuckha62 (Apr 5, 2010)

66GTOMN said:


> Hello,
> 
> New GTO owner and new to the site. Thanks to all for any help and I look forward to helping others where I can.
> 
> ...


This is more likely the source of your issue. Fairly well known issue...The factory tach is not compatible with the HEI distributor. There are adapters you can buy.

Read up on it and you'll find the right solution.


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## 21553 (Sep 30, 2012)

chuckha62 said:


> This is more likely the source of your issue. Fairly well known issue...The factory tach is not compatible with the HEI distributor. There are adapters you can buy.
> 
> Read up on it and you'll find the right solution.


There is a MSD Tach adapter on the firewall. I connected this as well and it doesn't work at all with this connected. Any way to test the MSD part? I hate to purchase a new unit if that isn't the problem. Thanks.


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## chuckha62 (Apr 5, 2010)

Not sure about the MSD adapter, it may be MSD specific for a factory tach.


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## leeklm (Mar 11, 2012)

I have a similar issue with my 66 and stock-type HEI, and have read a number of posts and online articles, but have found no clear answer. My factory tach works all the time, seems to be accurate at low rpm, but then is very eratic from about 2k+ rpm.

I purchased a generic inline tach capacitor/filter, but now my tach does not go above 1K rpm! When looking at the MSD site, it specifially states that their adapter is only for MSD ignitions. So, i ended up spending $40 on a new tach instead of the MSD or other "adapters". My next step is to find a working 66 tach and test it with my HEI to determine if HEI is in fact the problem. In my case, I may simply have a bad factory tach. 

I will be watching this post to see if anyone has more ideas!


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## joesweeting (May 8, 2011)

I just got rid of my points system last weekend. I installed a petronix ignitor III. My hood tach was working before and after the install. If you still have the old distributor, with the above system, you just pull the points out and install the unit. Run the red wire to the positive side of the coil and the black wire to the negative side and that's it, then reinstall distributor. Hope this helps. 

Sent from my ADR6425LVW using AutoGuide.Com Free App


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## chuckha62 (Apr 5, 2010)

Poke around this site... Tachometer Repair Restoration for Antique Classic Cars Auto


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## 21553 (Sep 30, 2012)

66GTOMN said:


> There is a MSD Tach adapter on the firewall. I connected this as well and it doesn't work at all with this connected. Any way to test the MSD part? I hate to purchase a new unit if that isn't the problem. Thanks.


Well last night I went to troubleshoot it again. I was able to connect the MSD Adapter and it acts exactly the same as without it. The tach works for about a minute perfectly and drops to zero and won't come back on until the next day. 

I purchase an old engine analyzer at a swap meet this weekend since it had a tach gauge. I connected it up directly to the HEI connection and everything works as it was suppose to with and without the MSD adapter. So back to the tach in the dash. I am going to see if I can get to all of the connections without having to remove the dash. My thought is to run a new power wire and ground connection.

Really has me stumped since it works for what seems to be the same amount of time each time then fails like it looses a connections. Well I will continue to troubleshoot and report back if I find anything. Thanks for all the help and suggestions and please keep them coming.


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## joesweeting (May 8, 2011)

Any luck with your tach?

Sent from my ADR6425LVW using AutoGuide.Com Free App


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## chuckha62 (Apr 5, 2010)

66GTOMN said:


> Well last night I went to troubleshoot it again. I was able to connect the MSD Adapter and it acts exactly the same as without it. The tach works for about a minute perfectly and drops to zero and won't come back on until the next day.
> 
> I purchase an old engine analyzer at a swap meet this weekend since it had a tach gauge. I connected it up directly to the HEI connection and everything works as it was suppose to with and without the MSD adapter. So back to the tach in the dash. I am going to see if I can get to all of the connections without having to remove the dash. My thought is to run a new power wire and ground connection.
> 
> Really has me stumped since it works for what seems to be the same amount of time each time then fails like it looses a connections. Well I will continue to troubleshoot and report back if I find anything. Thanks for all the help and suggestions and please keep them coming.


It's probably not losing a connection. More than likely the circuit is heating up due to an incorrect voltage getting to the circuit.


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## allpawl66 (May 9, 2008)

joesweeting said:


> I just got rid of my points system last weekend. I installed a petronix ignitor III. My hood tach was working before and after the install. If you still have the old distributor, with the above system, you just pull the points out and install the unit. Run the red wire to the positive side of the coil and the black wire to the negative side and that's it, then reinstall distributor. Hope this helps.
> 
> Sent from my ADR6425LVW using AutoGuide.Com Free App


Petronix is a point conversion unit not a true electronic conversion , that is why it is still working without a tach. adapter . M&H has there version of the petronix that works as well without a tach. adapter . MSD has several tach. adapters for different ignition systems . Check your adapter for application 1st.


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## jmt455 (Mar 26, 2012)

As stated above, tHe OEM tach is not compatible with the HEI system.

I had a Firebrd hood tach converted to HEI by H&H Auto (The Tach Man).Tachometer Repair Restoration for Antique Classic Cars Auto

There are others who offer this same service.
It's not inexpensive...but you might fry your tach if you leave it as is.


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## 21553 (Sep 30, 2012)

joesweeting said:


> Any luck with your tach?
> 
> Sent from my ADR6425LVW using AutoGuide.Com Free App


Still no luck. It literally operates for an exact same amount of time each time it starts up. With the adapter and without. 

I haven't had time to work on it lately since I am installing a heater and insulating my garage so I can wokr on this winter.


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