My stock Tachometer has always been off and would bounce around to some degree but to day when I started my car it bounced once then pegged out and stayed there ,it even stays there when I turn the car off. I know these Tachometers are known for being extremely temperamental but is there any reason I should be concerned about a ground/etc. Or is this just the tack finally totally giving up on me?

 

I have my tach out of my 89 RS LO3 and am awaiting the arrival of a capacitor to see if that is the problem

 

There is a connection just off the coil on a white wire that appears to be leading to the Tachometer. If I disconnect that and the gage still stays pegged that would mean there is a ground. If it drops than the problem is in the tachometer. Dose that sound right?
I really need to know because the Chilton manual says that a ground can damage the computer control module .

 

Is this were you disconnect the Tach. from the coil and if so car I run the car with it unplugged ? I want to test the side going to the coil for 12 volt. Does that sound right? If these sound like dumb questions I'm sorry but I don't want to mess any thing up.

 

The wire in the photo, from the coil to the tach driver module, is the tach signal. It is simply the PIP signal from the ICM to the coil. It changes frequency with engine speed. Connected to any tach, it will produce an RPM signal. The problem could be a ground but it would be a ground in the cluster circuit, not affecting the ECM. The main grounds for the ECM are at the rear of the cylinder heads. If you have any doubts, check those grounds. You might try accessing the two cluster connectors, by removing the cluster, and see if you get any improvement by cleaning and checking contact on those two connectors. They can become damaged.

 

If I can not find the problem can I / should I disconnect the Tach. at that connection or should I leave it as is? I know I should have a Tach. but if I cant find the problem it will have to wait . Thanks again for the help.

 

When you turn the car off it stays pegged right? Start car, key in "ON". Turn off car, key in "OFF".

What happens when you switch the key back to the "ON (car not started)" position?

With mine, sometimes it will stay pegged when I turn the car off. It'll always return to 0 when I switch it back to "ON".

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The part that is strange is if it stays pegged all the time. Meaning that disconnecting the white wire shouldn't doing anything because no signal is being generated. Unless the coil itself could be grounded? Or the wire going from the coil to the dist? Make sure none are cut or touching. This has the coil mounted separately from the dist right? Not sure on the later years. If the coil is at the top of the dist, go ahead and take it apart and make sure nothings touching, the coil should be grounded with it's own wire if I remember correctly.

On my 83, it's just 1 black wire from the dist to the instrument circuit board connecter and from those contacts to the tach.

http://www.austinthirdgen.org/mkport...ine_wiring.gif
http://www.austinthirdgen.org/mkport...uges_part1.gif
Would "to instrument cluster" (first image, single white wire coming off the ignition coil) mean to the tach?
If so, then it is similar to mine:
http://www.austinthirdgen.org/mkport...ine_wiring.gif

It should be fine to run with that white wire disconnected. I don't belive that the ECM uses the tach to know the RPMs.

Go ahead and unhook it and check continuity.

 

Yes the coil is external. The Tachometer stays pegged at all times except when I turn it over/start .It stays pegged even in axillary also. When I turn it over/start it jumps to a halfway point then Peg's again.

 

The signal to the tach is AC. It goes from battery to ground and back each time a cyl fires.

So no, the issue here is not a "ground", as such, and won't damage anything.

The ECM doesn't run the tach. The tach is driven by its own little circuit independent of the ECM, just counting those pulses.

Sounds to me like just a classic case of a broken tach. Certainly not something that would prevent the car from being used. And, once that white wire is unplugged, that cuts it completely loose from everything else anyway, which totally eliminates any possibility of it affecting anything else, even if it does carry some infectious electronic disease.

 

There ya have it.

No ECM reads from the same tach line in a thirdgen, right?

 

That's correct. No connection between the Tach and the ECM. You may want to disconnect the white Tach signal wire so that, in case the Tach circuit shorts to ground, it won't kill the engine. As I said, it is the PIP signal to the coil from the ICM and if it shorts to ground, it will shut the engine down. Since the tach is gone, may as well disconnect it. You might try checking the cluster like I said to see if you find a bad connection that might be causing it to peg.

Just noticed in that photo what looks like some recent wiring work. You might want to make sure that wirings all good. That may be part of your problem.

 

Common problem. There is a resistor that goes bad on the tach driver board.

Head over to the electronics board here at TGO. There is a long thread on this issue that also shows how to fix it.

Not sure if it is a sticky or not. But it is usually near the top as the issue is so common.

RBob.

 

Thanks to everyone ,That makes me feel much better as I have had major electrical problems in the past with a "different" vehicle. As far as the wiring in the background photo. that's just some wire lumen that has been taped in place and over the coil wires to shield them from my open air element that sets "not touching" but very close to the coil. I even have electrical tape on the bottom side of the element tray that sets closest to the coil. Its just precautionary. I'm going to try to test everything this afternoon. Thanks again.

 

OK, I unhooked the white wire at connection and started the car. The wire to coil is 13.45 and the wire to Tach. is -.10 and jumps up and down from there to -.08 .Since its been unplugged the needle no longer jumps to a half way mark than pegs on start up. It just stays pegged. I understand the wire to coil but am unsure about the wire to Tach. results. Is the .10 telling us any thing ?

 

Not really...

That wire doesn't "do" anything, when unplugged from the coil. The tach doesn't "inject" anything into the coil circuit. Think of the tach as being like a multimeter, and the white wire as being like one of the leads.... if you hook up a meter to a circuit and measure something, it's not supposed to "do" anything to the circuit under test; and when you unhook that meter lead and measure it with some other meter, you'll find .... basically, nothing. Same deal here.

The problem with your tach is entirely self-contained within the tach and its little driver board.

 

In that case I'm just going to leave it unplugged and tape off both ends until I get a chance to fix it.

 

ok where it the fix for us 82-89 guys????? i got a spair tach that is MINT no fade or nothing, it just pegs, id like to replace my working one with it since it looks better

 

^ You're better off getting one of those tach face decal cover thingies.

 

do they have them in the stock look, i want bone stock looking

 

That's an excellent thread. These guys amassed a wealth of information on the tach driver. There is a solution there for every third gen tach failure.

 

All I could find is white. Type 89 camaro gauge overlay