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Factory Tach Information, Repair, HEI
#1
Well, I couldn't stand it anymore and I broke into my virgin, working 8000 RPM factory tach. My main motivation was to discount claims the tach could not be repaired, and to find out exactly how delicate it is. Also, I want to create a module that will allow us to keep our factory tachs with an HEI upgrade.(I know about MSD tach modules, but non of these are for HEI systems).

Bottom line if your tach IS NOT MECHANICALLY SOUND, like the needle has fallen out, you probably will not be able to repair it. However, if the tach has stopped working, or is not working properly, you should be able to repair it. The cost to you $10 in parts, and you need to know how to solder.

Stay tuned for more in about a week, but here is the scoop up to now:

What I discovered:

1. Despite the green movement's claims to the contrary, plastic is not forever. There is a delicate detente where the plastic is thin at the back of the gage you should be careful with.

2. If there is a mechanical issue with the needle to your gauge, that probably cannot be repaired. The needle is attached to a 1/32" diameter axle. The axle is seated at the center of a pair of 1 1/2" round magnets. This would be difficult to replace.

3. Every electrical component in the tach could be replaced. The most vulnerable item I see is the transformer. If this item gets too much current it would get hot and melt the insulation in it's windings. I hope to be able to get a better idea of it's specifications later this week.

4. When assembled, the instrument is solid. However, in individual pieces it is very fragile for a couple of reasons:
-The wire from the circuit board to the tach is a thin ribbon of a wire, not stranded.
-The tach itself has the needle protruding from one side, and a delicate exposed axle and connection on the other. Inadvertently setting it down could damage the axle.

5. There are 4 phillip head screws that are very small, but virtually glued to the plastic for being fastened 40 years. You need to use a size P1 screwdriver to prevent stripping the threads of these screws. 2 screws are on the face of the gauge and need to be removed to get access to the tach circuit.

More later.

[Image: 11jmcuc.png]
351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
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#2
How hard to replace just the needle? Did it slide off the axle pretty easy?

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
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#3
will e;66295 Wrote:How hard to replace just the needle? Did it slide off the axle pretty easy?

I am not doing going that deep into my tach.

Looks like the needle snaps on the axle because the axle has a stop/nib at the end. Not sure how it is calibrated to the RPMS.

[img][Image: 213fafd.jpg][/img]

[Image: 11jmcuc.png]
351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
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#4
I tried to reverse engineer a 66 tachometer, using a good working tach, de-soldering each component on the circuit board, and attempting to measure the values of capacitance and resistance. The resistors should have been easily read, but some were not marked with standard markings. None of the capacitors were standard values, and I couldn't get decent readings. I gave up and sent it back to the Tachman for repair.
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#5
These guys will fix your tach, but $$$.

http://www.paspeedo.com

mike

[Image: 1_11_11_13_11_50_27.png]
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#6
Last I heard the closest fix for tach's was what rocketman was doing.....I heard that Tachman wasn't touching our tachs anymore because he can't get parts?

[Image: 1_14_01_12_10_54_29.png]

Ken

73 Convertible 302-4v-3spd yellow /white int

08 Bullitt 4.6-5 spd black/black int


All I ask is a chance to prove that money can't make me happy
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#7
whitesoc;66399 Wrote:Last I heard the closest fix for tach's was what rocketman was doing.....I heard that Tachman wasn't touching our tachs anymore because he can't get parts?

Heard the same thing today about rocketman being about the only one these days.

Doc

[Image: 6y14ea.jpg]

Project started 8-7-10
Completed: All new suspension, rebuilt 351C H Code bored .030 over with mild cam and intake, new 3.50 TracLok, custom exhaust system
Current "mini-project": interior upgrade Undecided
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#8
He converted mine to a 3 wire, and I'm happy with it. Good guy to work with.

Steve
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#9
Can't speak for the none 71-73 tachs, because I have only taken a 71 apart.

The resistors are easy, 4 of those but those generally don't go bad.

There is a capacitor at .01uf. If too much current passes through this, that might be a problem. Easy to find and replace.

There appears to be an inductor of some type which I will try to get figured out tomorrow. My understanding is these NEVER go bad.

And then there appears to be a pickup coil. This could go bad if too much current passes through it also. This would be difficult to replace.

But in all honesty, it is difficult for to understand how any of these components would wear out.

The tach is pretty fragile because the plastic is thin and brittle. The moving components appear fragile because they are thin and light.

Hope to be playing with it Thursday or Friday and understanding what it's limits are in regards to voltage and current.

[Image: 11jmcuc.png]
351C Bold Manners, Brash Attitude
Favorite Teams: Michigan Wolverines and Whoever Is Playing Ohio State.

When I drive past a herd of cows, the cows MOO at me
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#10
How much of a PITA do you think it would be to change out the face - mine's a little time-/sun-faded in places. Honestly, I have no idea if it even works aside from what Joe from Mach 1 Club told me: that he had it tested (I have no reason not to believe him, but still need to see for myself when it's time to plug it in and fire it up).

I remember ruining a gas gauge in my '78 K-5 Blazer because it was dirty... so I took it apart and cleaned it (long before I was a little smarter about those things). And those gauges were cheap... not like a '71 Mustang factory tach cluster, after all. Wink

Eric

[Image: mach1sig2.gif]
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