# 1969 GTO tachometer wiring



## zgzmithr (Jan 1, 2020)

so i have a set of gauges for my 1969 GTO, i bought the car with the dash removed and dissembled... trying to understand how the tachometer is wired. There is one post on the back of the tachometer that i assume is for power? ground is right to the body of the tachometer? and where does it receive signal?


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## 1970GTO. (Dec 12, 2019)

My dash pad has cracks in it. How difficult is it to remove and repair


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## BLK69JUDGE (Jun 10, 2010)

Zg
the tab on the rear is the fused PULSE wire from your coil .... - side
IF
your changing to oil water gas gauges in the left pod from a single fuel gauge
there is also sending units on the engine to swap out'
and
wires on the master plug at the circuit board and ignition switch also at least in 68 for sure
you will also need a Y light harness if you didnt have a clock originally for the illumination bulbs,,gray wire

you might consider spending 100 on some manuals ,,,,,

we like to help when we can .........

Scott

hey 1970 lets start a new chat about your dash pad ....
so this thread wont get hijacked from Zg


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## zgzmithr (Jan 1, 2020)

great thank you blk69judge.

so from what i have read, these pontiac tachometers are "single wire" units, theses tachs do not use a secondary power source as most gm tachs do. ia this correct?

which brings me to my next question, can i bench test this tachometer with a battery charger as i would other tachometers, AND what could i use as a single generator to complete a more comprehensive bench test?


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## BLK69JUDGE (Jun 10, 2010)

NO
your not testing a light bulb ...
if your tach is an original it should be checked by a qualified speedometer shop

often original tachs are way off calibration .... and need rebuilt....
I have an NOS GM hood tach that works perfect at idle but at 5000 rpm it reads 3300
the originals are just not reliable,,, unless checked first on another vehicle with a known calibrated tach
and running them side by side ... still kinda ruff on engines testing tachs at 5000

buy a new repop ... from AMES ... and the harness ,,,, You do not want to pull the dash back out very many times
and a lazy tach can cost you 1000's in engine repair over a 200.00 tach

just my opinion

Scott


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## BLK69JUDGE (Jun 10, 2010)

BLK69JUDGE said:


> NO
> your not testing a light bulb ...
> if your tach is an original it should be checked by a qualified speedometer shop
> 
> ...


also
if your running anything but a points distributor ,,,, you will want the aftermarket tach for its internal updates ..


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## 29585 (Aug 4, 2013)

zgzmithr said:


> so i have a set of gauges for my 1969 GTO, i bought the car with the dash removed and dissembled... trying to understand how the tachometer is wired. There is one post on the back of the tachometer that i assume is for power? ground is right to the body of the tachometer? and where does it receive signal?


 as other posters have said, the single terminal on backside of tach is for the (brown) fused signal wire coming from the negative side of the coil. The mounting tabs on the tach case serve as the ground. Separately , if your car had a tach in it already, there should be a light harness with the 2 bulbs (gray) or, if your car had a clock, you may be able to use the same light harness, or at least buy a harness from one of the suppliers and it will plug right into the gray wired 12v connector for your clock wire. Ive installed two of these in the last few years, one was original, one I bought from PartsPlace I think. I also bought the light harness from the, Here's a wiring diagram of sorts out of the Zazarine GTO book. Note, if your car did not have a tach, you may not have the Brown wire included in your original harness, in that case I just ran a fused wire from the coil thru one of the holes in the firewall and attached it to the tach.


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## desertman (Jan 2, 2020)

Wow what a great forum!!! Great advise from very knowledgeable people 
Thanks


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## zgzmithr (Jan 1, 2020)

BLK69JUDGE said:


> also
> if your running anything but a points distributor ,,,, you will want the aftermarket tach for its internal updates ..


getting close to reinstalling the dash and would really like to not need to remove it again(we will see if this works out or not), so I am back to questioning weather or not to use the factory tachometer. So yes this is a self exciting factory tachometer and i realize it is voltage sensitive, but the distributor i plan to use is a msd 8528 rtr distributor. Now it has a wire coming out of it (gray wire) that produces a 12v square wave signal. as i understand it the factory tachometer operates between 11.2v - 14.6v. so in my simple mind this would work.

what do you all think

the main hold-up for me getting a repop tach with the updated electronics, is that i despise chinese parts for the lack of quality and would hate to put one in if i did not need to.


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## PontiacJim (Dec 29, 2012)

zgzmithr said:


> getting close to reinstalling the dash and would really like to not need to remove it again(we will see if this works out or not), so I am back to questioning weather or not to use the factory tachometer. So yes this is a self exciting factory tachometer and i realize it is voltage sensitive, but the distributor i plan to use is a msd 8528 rtr distributor. Now it has a wire coming out of it (gray wire) that produces a 12v square wave signal. as i understand it the factory tachometer operates between 11.2v - 14.6v. so in my simple mind this would work.
> 
> what do you all think
> 
> the main hold-up for me getting a repop tach with the updated electronics, is that i despise chinese parts for the lack of quality and would hate to put one in if i did not need to.


You must have missed previous post #6 by BLK69JUDGE. No, the factory tach will not work. It is not designed for a square wave electronic signal.....but if you want to install it and don't care about its function, then install it OR get an updated tach that will work with the electronic ignition. There have been several posts in the past regarding this and the answer is to upgrade the tach, Chinese or not. You can also send the tach out to one of the many companies that can install new guts that will make the tach functional if you want to go that route.


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## zgzmithr (Jan 1, 2020)

PontiacJim said:


> You must have missed previous post #6 by BLK69JUDGE. No, the factory tach will not work. It is not designed for a square wave electronic signal.....but if you want to install it and don't care about its function, then install it OR get an updated tach that will work with the electronic ignition. There have been several posts in the past regarding this and the answer is to upgrade the tach, Chinese or not. You can also send the tach out to one of the many companies that can install new guts that will make the tach functional if you want to go that route.


 obviously i did not miss the post, I quoted what BLK69JUDGE said in my previous response. You guys say it will not work, and it may not, you also say it wont work with the square wave signal, but i proved to myself it does by using a signal generator. So if it wont work it must be for a different reason.

So I am looking for some first hand knowledge with someone that has used an electronic ignition system with a factory self-excited tachometer.


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## rctach (Jul 19, 2011)

The original tachs will work with a 12V square wave generator as that is what I have used to bench test them with for the last 33 years. However that does not mean the output of your MSD will drive it. There are thousands of aftermarket ignition systems out here produced over the last 50 years and I have no way of knowing if yours will drive an original tach. To know if your original tach will work on the ignition system you have, it would likely be best to just test it on your system. If that means is not possible then you can have a circuit board installed in your original tach that GM used from 72 through around 78 for HEI ignitions, that circuit board will work on any ignition system out there. I reproduce these circuit boards. It does require an additional lead added to the back of your tach for 12V power source.


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## zgzmithr (Jan 1, 2020)

rctach said:


> The original tachs will work with a 12V square wave generator as that is what I have used to bench test them with for the last 33 years. However that does not mean the output of your MSD will drive it. There are thousands of aftermarket ignition systems out here produced over the last 50 years and I have no way of knowing if yours will drive an original tach. To know if your original tach will work on the ignition system you have, it would likely be best to just test it on your system. If that means is not possible then you can have a circuit board installed in your original tach that GM used from 72 through around 78 for HEI ignitions, that circuit board will work on any ignition system out there. I reproduce these circuit boards. It does require an additional lead added to the back of your tach for 12V power source.


awesome response! that is the information i need! I scoped out your website and was going to call you tomorrow (monday) so yes, i need to upgrade this tachometer with that circuit board! Which leads me to ask do you have a '69 gto in dash tach ready to go that you can sell to me, or do i need to send mine to you for the upgrade?


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## rctach (Jul 19, 2011)

You can call today. I may have one but don't have one ready. 
Randy Watson


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## zgzmithr (Jan 1, 2020)

Just did, no answer. does email work for you


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## PontiacJim (Dec 29, 2012)

zgzmithr said:


> obviously i did not miss the post, I quoted what BLK69JUDGE said in my previous response. You guys say it will not work, and it may not, you also say it wont work with the square wave signal, but i proved to myself it does by using a signal generator. So if it wont work it must be for a different reason.
> 
> So I am looking for some first hand knowledge with someone that has used an electronic ignition system with a factory self-excited tachometer.



Whoa there neighbor. Hmmm. You just proved it to yourself that it'll work with a square wave generator, and now, you respond to *rctach*, "so yes, i need to upgrade this tachometer with that circuit board! Which leads me to ask do you have a '69 gto in dash tach ready to go that you can sell to me, or do i need to send mine to you for the upgrade?"


What did I tell you in post #10?, "There have been several posts in the past regarding this and the answer is to upgrade the tach, Chinese or not. * You can also send the tach out to one of the many companies that can install new guts that will make the tach functional*."

Where do you clowns come from? You want advice, it is given, and then you tell me I don't know what I am talking about. Here, giving you this back because I think you misdirected it at me. ?

No offense to you *rctach. *


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## Techno (8 mo ago)

I'm curious about driving with a 12v square wave. I can't seem to get mine to work with 12v - I've tried 25%, 50% and 75% duty cycle. What are you using? Thanks.


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