[sugar ray]

Wondering if anyone here has ever had success trouble shooting a tachometer before?

My stock tach in my 1979 Trans am just stopped working! IT would no longer read anything. The needle just sits at zero.

So i aquired a good used one from a parts car that we "KNOW" was working before we removed it. After installing the replacement, it works "BUT" is completely in-acurate. For instance at idle it bounces from 4000k to 6000k.

Since we know this previous tach was working what else can i check?
The HEI-cap is pretty much brand new. Its a Pro-form rebuild kit with high voltage coil and module. The original tach use to work just fine with this cap.

I have enclosed 2 pics. of the tach. the back connecter has a piglet wire that plugs into the tachometer prongs. with 2 seprate wires leading into the tach-plug-in-prongs can i assume that one of these wires acts as a ground and the other leads to the disributor cap?

I assume that im dealing with a possible wiring issue here but can't imagine how one tach would just die completely, replace it and am now getting wacky readings with the replacement tach?

the wacky reading confirms that the tach is getting some-sort of signal. just NOT correct?

any ideas or suggestions? thanks!

[BORTI]

It sounds like a grounding issue.Being that it reads some sort of signal.If there is an aftermarket ignition box you may need a tach adapter.If there is not just check all wires.

[sugar ray]

NO aftermarket ignition box involved. Just a standard HEI distributor that houses an aftermarket coil and module.

NOTHING had changed between the time the tach work and the time it croaked. So a little baffling.

I will look into your suggestion in the morning. thanks

[Jake_Dragon]

Do those cars have a tach filter? If its bad it could cause your problems.

[75gmck25]

The stock GM tach wiring on my '75 pickup has three wires: ground, 12 volt power, and tach signal. If yours has only 2 wires, it must be grounded through the case/frame. I would verify that this ground point is connected to the instrument panel wiring, and that you have 12 volts on the other terminal. On my tach the ground is black, power is red/pink, and the input signal is brown.

Most GM tachs used a tach filter. You can buy them off the net for about $50, or make your own for about $5-10. Lack of a filter may screw up the readings, but usually won't make the tach fail.

There is also a tach circuit board on the back of the tach. It counts the ignition pulses and converts them to an analog signal to drive the needle. These boards are soldered or jumpered different based on whether the car is a 6 or a V8.

Some of the circuit boards develop breaks in the solder traces, which causes erratic readings. My tach sticks at about 3750 on a cold start, until the truck is warmed up. I think this is due to a bad solder trace that finally warms up and makes contact.

Bruce

[lfmgtc59]

It looks like about 100 days since the last post - but I'm having a similar tach issue.

At idle the tach shows the 600 - 750 rpm it should... for about 5 minutes, then the needle starts bouncing around 3000 - 4000. If I turn on all the accessories it may calm back down, but may not either. At 70mph / 2300 rpm it also seem to even out.

I have a 1967 Mustang with a 351W out of a 69.
I replaced the points distributor with a Accel breakerless (71204e) about 4 years ago. I am running a matching Accel Coil. Accel 10.8mm wires. 100 amp single wire alternator.

First the module in the distributor burned out. I replaced that, but noticed that the coil was getting extremely hot. That is when I first noticed the Tach Issue.

As the coil has never run that hot before, I figured that maybe the coil had caused the module to burn, so I replaced the coil with another matching Accel coil. This did nothing to alleviate the tach issue. I tested the setup with a new AutoMeter tach - the new tach does exactly as the old did.

I have a new Painless wiring harness throughout, but checked the wiring anyway. I ran a new ground wire from the distributor to the Block eliminating connectors and shortening the length and increasing the size of the wire, as the one on it looked like it got a bit hot at one time.

I checked all grounds from block to body (I have 2 something like 2 gauge grounding cables).

I checked the ground to the tach and switched it to another ground just to make sure, but this made no difference.

I pulled the alternator and had it bench tested under load for about 10 minutes, but the alternator shop swears that the alternator has no issues.

So now I am at a loss - I mean that is really all there is on a 67: Alternator, coil, distributor/module.. and of course the Tach strung together with some high dollar wire...

Any help would be appreciated...

Thank You

Lee

[cahorne]

I found your post through a search, but am havuibng the same problem with a 67 as well. Any luck on a solution? What have you tried? Maybe we need to run a larger wire for the tach signal. I too run a mallory distributor with pointless ignition.

[P-Air]

Originally Posted by lfmgtc59

It looks like about 100 days since the last post - but I'm having a similar tach issue.

At idle the tach shows the 600 - 750 rpm it should... for about 5 minutes, then the needle starts bouncing around 3000 - 4000. If I turn on all the accessories it may calm back down, but may not either. At 70mph / 2300 rpm it also seem to even out.

I have a 1967 Mustang with a 351W out of a 69.
I replaced the points distributor with a Accel breakerless (71204e) about 4 years ago. I am running a matching Accel Coil. Accel 10.8mm wires. 100 amp single wire alternator.

First the module in the distributor burned out. I replaced that, but noticed that the coil was getting extremely hot. That is when I first noticed the Tach Issue.

As the coil has never run that hot before, I figured that maybe the coil had caused the module to burn, so I replaced the coil with another matching Accel coil. This did nothing to alleviate the tach issue. I tested the setup with a new AutoMeter tach - the new tach does exactly as the old did.

I have a new Painless wiring harness throughout, but checked the wiring anyway. I ran a new ground wire from the distributor to the Block eliminating connectors and shortening the length and increasing the size of the wire, as the one on it looked like it got a bit hot at one time.

I checked all grounds from block to body (I have 2 something like 2 gauge grounding cables).

I checked the ground to the tach and switched it to another ground just to make sure, but this made no difference.

I pulled the alternator and had it bench tested under load for about 10 minutes, but the alternator shop swears that the alternator has no issues.

So now I am at a loss - I mean that is really all there is on a 67: Alternator, coil, distributor/module.. and of course the Tach strung together with some high dollar wire...

Any help would be appreciated...

Thank You

Lee

The old cars used to have a resister in line with the coil or a resister wire going to the coil from the key switch. It droped the voltage when running so as not to over heat the coil. For starting a wire came from the starter selinode marked R (hot in starmode) and went to the + on the coil after you let go of the key, power went through the resister wire to the coil at around 7+ volts . Obviously this could mess up your tach.