I have a 90 IROC and the tach is revving about 2000 rpm higher than the motor is. The motor runs good, and the tach action is smooth, but too high. Any ideas? This has been driving me crazy, help needed!!!

 

Find another tach and hook it up to the engine. See what it reads. Also, have you ever removed the needle from the tach? maybe if you or someone else did, they put it back on wrong. Try removing the needle and put it back on in another position that reads the same as the 2nd tach under the hood. Just be careful removing it. they break easy

 

I've got the same problem on an old (second gen) Z28 I just picked up. I haven't had time to mess with it yet, but if they are set up similar I might eventually have something of use. Let me know what you find out and I'll do the same. Now that I think of it, I think I've seen tach problems like this listed before. Maybe a search can bring them up.

On a somewhat related note, poor charging systems can lead to tachs that waver and bounce a lot. I've had problems like that as alternators start to go or when if the battery connections are not good.

Good luck,
MikeS

 

Someone suggested it might have been replaced with a tach out of a V6 car or something. Gonna have to check on that. When the key is off, the needle buried WAY below zero, when the key is on it goes to 2500. When I start the car it is 1000 to 2000 high. According to my helm manual it is a pretty straight forward circuit, not sure what could go wrong and still work. Still open for ideas, and thanks guys for replying. I will post when I figure it out.

 

you sure its not a v6 tach?

 

Not sure at all. Have both cars in my stable, and outward apperances they both look the same. Gonna have to talk to a tech to see if there is a diff, or try and find a part number or something.

 

Why is it that absolutely nobody on this site has a tach that reads low in the upper rpm's? Everybody reads high. Damn. My car's a freak.

 

Trust me, if you put in a V 6 tach it will read lower, not higher, I had the same problem and got a tach that was supposed to have been out of an IROC. When I put it in it read way low, like 3k at WOT. I pulled it back out and looked at the back side of the face plate and the little GM tag was still there and it said P/N: (what ever) 6 cyl. The V8 dist sends the tach pulse once per rev, as does the V6, but the V8 is firing 2 more cylinders per rev so its pulse will come slower than the V6.

 

I guess I had it backwards. I was thinking the V8 engine was sending 2 more pulses than the V6 tach was wanting, resulting in a higher reading. Any ideas? Guess I'm gonna have to pull it out of the dash and see what I've got in there. Matbe it just went bad?

 

Trust me, but you're wrong. A V8 fires 4x per rev. A V6 3x per rev. So at 1000rpms indicated, a V6 tach would be recieving 3k ppm. A V8 running at 3k ppm is actually turning only 750rpms. So a V6 tach on a V8 engine will read high, not low.

Perhaps the tach you got was just way off, but i assure you that ratio holds true. If you don't believe me, think about it real hard before you try to retort.

 

Ed is right.... the tach counts pulses and divides them by the number per crank rev. In a 6-cyl there are 3 pulses per crank rev, but in a 8-cyl, there are 4 per rev. So a 6-cyl tach will read 33% high when hooked up to a V8.

Think of it this way... let's pick a nice easy RPM to work with, let's say, 1200. 1200 revs/min = 20 revs/sec. So if this was a 6-cyl, the dist would be firing 3 cyl per rev, so the tach would see 60 pulses/sec. Hook this same tach up to a V8, it now sees 80 pulses/sec. It thinks that 80 pulses per sec corresponds to 26.67 crank RPM, which is 1600 RPM.

The tach is calibrated with a little surface-mount resistor on the back side of the little circuit board. That resistor apparently increases in value over time. The higher its value, the higher the tach reads. It could easily be re-calibrated by simply selecting a new resistance value. I did a bunch of factory tachs (much older than the ones in these cars though) back when I used to work as a bench tech in a test equipment lab.

These tachs almost always fail by reading high.... I don't think I've ever seen one that reads low. The one I have in my car is the most nearly accurate one out of all the ones I have, it's only about 3% high (at least between 3000 and 6000 RPM, where I've checked it). That's unusually accurate it would seem.

 

Nobody said I didn't believe you. I was simply trying to explain what I was thinking. I very much appreciate the time you've taken to try and help me, but telling me to think about it real hard doesn't seem very nice. I only asked a question. I even said in my own post that I had it backwards. Eitherway I'm forced to believe that the tach is probably just going bad. Do agree? The fact that it points down when off, up with the key on, and too high while running is a good indication something is tweaked. Unless someone comes up with a better idea.

 

I was talking to morley, not you tmony. Notice how he said trust me, then gave a completely wrong answer. Spreading false information, especially when you act cocky and confident about it is potentially bad on the internet. Because then someone else repeats it. Then somene else. It's fine to be wrong sometimes, we all are. Thats why i explained the situation. The last bit was just to avoid needless bickering.

 

Now I feel stupid! Sorry and thanks again.

 

Well EXCUSE me, but I'm not the one coming off "cocky" here. Pot-kettle-black? Maybe you should PM me before something gets said that shouldn't?

And yes the tach I got was for a V6 and it did read low on a V8 but it seemed to read pretty close on a friend's V6. PFM?

One other thing to ponder, There was a small article someone posted in the Electrical section on how to convert a V6 tach to work with a V8 by changing a capacitor on the PC board. I did the capacitor swap and now the tach reads "acceptably" on a V8 (just a tad high, 100-200 RPM).

 

Down low my tach is accurate. The higher in RPM's I go the farther off (reads low) my tach gets. Would you be able to tell me what I need to replace in the tach and what resistance? I'm not looking forward to spending $80 to get it recalibrated.
TIA

 

Is there a magic wand answer on how to do that? Eitherway, thank you, that makes alot of sense. I've seen the little circuit things you are talking about. Just not sure what to do about it.

 

I'll look at one and see if I can scan it or take a pic or something.

 

In case anyone ever has the problem I had....
My tach was unbalanced. Apparently somebody had removed the needle before and didn't put it back on correctly. The counterweight and needle were not lined up.
After lining that stuff up I put it on a Snap-On tach analyzer and found that my stock gauge is now only between 25 and 45 rpm off all the way to 5K. I suppose that's accurate enough for me.

 

As it turned out, someone did stick a tach(gauge) cluster from a V6 car in mine. I swapped it out with a good V8 car one, and it works fine.