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Old 04-21-2022, 08:44 AM
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Default Tach rebuild/replace

The tach in my ‘70 Trans Am does not work properly. In addition to the clock not working (and the little ‘second’ hand being broken off) the tack bounces around, sometimes it sticks, and then falls back. Yesterday, under hard acceleration it spun all the way up to 8000 RPM, but obviously the engine didn’t spin that high.

Not that’s it’s relevant, but the fellow I bought the car from had the tach rebuilt by a place called Palo Alto Speedometer back in 2011. In looking at the repair order, he also complained of the tach sticking between 2000-2500 RPM.

I was also given an 8000 RPM tach that the previous owner told me he picked up at a swap meet years ago. Is there any way to identify that this is indeed an OEM tach (it was given to me in an OER box, and I didn’t think they made a tach for the ‘70).

Is my original tach rebuildable and if so, where do I send it (is Pete Serio still doing this kind of work?)

Will the extra tach I have actually work in a ‘70 TA (I thought the 70-72 cars were different from the ‘73 and up, and that the tachs couldn’t interchange.

Here is my extra tach:



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Old 04-21-2022, 10:23 AM
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I can’t help with the tach, though I’m interested to see the responses because mine sticks occasionally around 1800 rpm, but as far as the clock, the second hands should swap easily. They just pull off/press on. You may also find the electromechanical clock movement just needs lubricated. If the contacts are worn, that’s another story. If you’re not worried about originality they do sell a quartz conversion kit for that clock. I picked one up several years ago and it works well enough although I may have a pice of dirt or something in mine because it does stop occasionally. I just haven’t taken the time to pull everything apart again to look at it.

I’d already bought the conversion though when I took my clock apart and found that the mechanism still worked after being lubricated.

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  #3  
Old 04-21-2022, 11:15 AM
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Also meant to ask, I would like to change my distributor from points to an electronic system (not HEI, but something like M&H Breakerless). Would something have to be changed in the tach in order for this to work?

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The frogs take up where they left off.
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Old 04-21-2022, 11:19 AM
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Quote:
Originally Posted by bhill86 View Post
I can’t help with the tach, though I’m interested to see the responses because mine sticks occasionally around 1800 rpm, but as far as the clock, the second hands should swap easily. They just pull off/press on. You may also find the electromechanical clock movement just needs lubricated. If the contacts are worn, that’s another story. If you’re not worried about originality they do sell a quartz conversion kit for that clock. I picked one up several years ago and it works well enough although I may have a pice of dirt or something in mine because it does stop occasionally. I just haven’t taken the time to pull everything apart again to look at it.

I’d already bought the conversion though when I took my clock apart and found that the mechanism still worked after being lubricated.
Thanks Brian. In looking at my extra tach, I'm pretty sure this is an original GM tach, and not a reproduction, I just don't know how to tell if it is the correct year or not.

The clock in the tach that is in the car still "works" in that the broken second hand still rotates, but the clock definitely does not keep accurate time. Also, I think I rad in another post that they called this a 'tic-tac' clock because it makes a ticking noise, but I don't think mine does. Is it possible it was already converted?

Also, how much work is it to remove the tach? I'm planning on pulling the engine and going through it, should I wait until I do that, or is this something I can remove and repair/replace relatively easily without disassembling the dash, wheel, steering column, etc.

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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone.
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Old 04-21-2022, 11:54 AM
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I cannot speak to who you could send the tach to for a rebuild - I had understood that the broken clock rod means that clock is NFG - the clock kits require you to reuse that part.

The 1970 & 1971-1976 8'000rpm tachometers all interchange in your car;
The 1970 tach's are unique because they were the only ones made with the single "coil" connection on the back (seen in your picture);
Starting in 1971 a unitized distributor could be ordered, and the tach's received a second positive plug on the back - this one read (iirc) "12V";
That "12V" plug was used for 71-74 Cars with the unitized distributor and 1975-1976 cars (all HEI), while the other 71-74 cars with a points distributor used the "COIL" plug.

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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.


1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
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Old 04-21-2022, 01:01 PM
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Quote:
Originally Posted by unruhjonny View Post
I cannot speak to who you could send the tach to for a rebuild - I had understood that the broken clock rod means that clock is NFG - the clock kits require you to reuse that part.

The 1970 & 1971-1976 8'000rpm tachometers all interchange in your car;
The 1970 tach's are unique because they were the only ones made with the single "coil" connection on the back (seen in your picture);
Starting in 1971 a unitized distributor could be ordered, and the tach's received a second positive plug on the back - this one read (iirc) "12V";
That "12V" plug was used for 71-74 Cars with the unitized distributor and 1975-1976 cars (all HEI), while the other 71-74 cars with a points distributor used the "COIL" plug.
Thanks unruhjohnny
So, it sounds like it might make the most sense to have the 'extra' tach/clock unit rebuilt since that one does not have a broken clock rod, but will the tach still work with a modern electronic ignition since it won't have the "12V" plug?.

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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone.
The frogs take up where they left off.
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Old 04-21-2022, 01:44 PM
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Quote:
Originally Posted by racerboy View Post
Thanks Brian. In looking at my extra tach, I'm pretty sure this is an original GM tach, and not a reproduction, I just don't know how to tell if it is the correct year or not.

The clock in the tach that is in the car still "works" in that the broken second hand still rotates, but the clock definitely does not keep accurate time. Also, I think I rad in another post that they called this a 'tic-tac' clock because it makes a ticking noise, but I don't think mine does. Is it possible it was already converted?

Also, how much work is it to remove the tach? I'm planning on pulling the engine and going through it, should I wait until I do that, or is this something I can remove and repair/replace relatively easily without disassembling the dash, wheel, steering column, etc.
If the clock that works is an original electromechanical yes, it should tick slightly. About every 30 seconds or so you’ll hear a loud click which is the mechanism resetting. I’ll see if I can find and post a video of my mechanism. They’re also notoriously not great at keeping time. I forget what the acceptable margin of error was bud I’d imagine that was the reason for the switch to the quartz version in later 2nd gens. Anyway, if it doesn’t make any noise it’s possible someone converted it previously. Possibly last time the tach was rebuilt. Another telltale is the second hand movement, which may be hard for you to see since it’s broken, but a quartz movement will be smooth whereas the electromechanical will be jerky as it ticks away.

Pulling it all out isn’t difficult. Remove the cover under the column, 2 screws, then there’s a bracket under the column that I think I removed, another two screws. Then there’s the bracket that holds the column in place. 8 loosened those two nuts to allow the column to drop 1/4” - 1/2”. Makes it easier to get the bezel off and cluster out. On either side of the column facing upward are two long screws that go into the bezel. Those need to come out. Then 3 screws along the top of the bezel facing up into the dash pad. Unscrew the trim ring from the lighter and the bezel should come off.

From there, you can reach up behind the cluster and unhook the speedo cable and you should also be able to reach the large plug on the back. There’s a release tab to press and wiggle it out. You may also want to feel around for another “wire” connected to a bulb socket behind the speedo. That’s the ashtray fiber optic cable which you’ll want to remove from the back of the cluster. You’ll be able to see what screws need removed to pull the cluster out. I can’t remember them off hand to describe them but it’s pretty straight forward. Then as you pull the cluster out a bit you’ll be able to and want to remove the plug off the back of the tach. Cluster should come out then.

My car is a 77 so it may be a bit different than yours as far as removing the tach and clock so I’ll stop here. It’s not hard though.

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  #8  
Old 04-21-2022, 01:54 PM
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Big Bear Big Bear is offline
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Used a circuit board from The Cajun Tach Shop to repair my Z-28 tach. It worked good after the board swap. They also have a repair service. cajuntachshop@gmail.com

  #9  
Old 04-21-2022, 01:55 PM
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Here’s the video of my mechanism.

https://youtube.com/shorts/pTaNkUG9XFE?feature=share

You can also likely use your non broken second hand on the working mechanism or on a working quartz movement. My second hand was broken and I found a replacement that I swapped out.

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1977 Trans Am 400/4speed (swap)

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  #10  
Old 04-21-2022, 03:30 PM
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unruhjonny unruhjonny is offline
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if it matters, the same clocks were used in both 8k-rpm & 6k-rpm tachs from 1970-1978;
The only change from 1978 to 1979 on the tach was the clock had quartz movement.

I could be mistaken, and replacement clock rods are available, but I swapped the clock out in a spare tach where the rod was broken;
I'm a good old fashioned Mennonite, which means I can't throw stuff out, and wouldn't you know it, that clock with the broken rod is sitting infront of my keyboard here... the same place it has probably been for close to a year now!
lol

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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.


1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
  #11  
Old 04-25-2022, 04:55 PM
70GS455 70GS455 is offline
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From the pic of the back showing the meter movement, it looks to be an OEM tach.

The sticking is likely from magnetic material or debris inside the magnet gap and interfering with the coil as it sweeps by. It can be cleaned. I use a piece of doubled up masking tape. Be extremely careful, don't touch the clockspring or wiring or you will ruin the meter.

If the circuit board is faulty, as mentioned, Cajun tach shop sells a replacement. I also have one of my own design.

If the board is replaced, the tach will need to be calibrated

Sent from my SM-T817V using Tapatalk

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Old 05-24-2022, 09:31 PM
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I contacted a board member several years ago about restoring my three early 70's TA gauge clusters. This ended up turning into a project not only make them visually correct but to also make them super accurate and highly responsive at the same time. This member has been working with a reputable meter supplier and I have chipped in money for the tooling required to reproduce them. Last I heard, he had prototype pieces in hand and was testing them out.

I'll drop him a line and see where he's at.

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Old 05-25-2022, 07:38 PM
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Just got my spare ‘70 tach back from 70GS455. He cleaned it up, calibrated it, and did the quartz conversion. Looks great! Now I have to attempt to replace it. I’m sure there’s a ‘how-to’ somewhere on this forum. My understanding is the tach comes out the back, but o have to drop the steering column and remove the gauge panel first. Is that right?



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The frogs take up where they left off.
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