I finally got an L69 tach to put in my 83 TA, but after I put it in the tach reads about 1000 higher than the one I took out. I notice that the tach had been taken out of the cluster previously so assuming someone turned the needle by finger, could that damage the tach motor. I only wonder because with the ignition on and engine off the tach reads zero, but when I turn the key off the needle creeps back up to about 750rpm. The tach I had in before seemed to read accurate and didnt creep back up when key was turned off, so what would cause a tach to read high and creep up when off? It would have to be something on the tach itself since the other tach reads normal, so I know the circuit from distributor is ok.

 

The tach doesn't have a "motor"; it's just a super-cheeep pivot & jewel type meter movement.

No, the movement probably it isn't damaged. From my days some decades ago as a bench tech in a test equipment lab fixing industrial process control stuff that had the exact same meter in it, I can tell you that it may be a total piece of crap; but its crappiness doesn't involve being delicate. It's actually a fairly rugged meter. Just not very linear, and hard to get the friction out of. Which isn't too much of a problem in an automotive application because the vibration keeps the pivots from sticking in the jewels.

More likely, the same thing is wrong with it that's wrong with about all of them. The little circuit board on the back has a component that changes value over time, and cause it to read steadily higher.

I'd be willing to bet that your tach doesn't read "1000 RPM" high; like, it reads 1000 when it's really at 0, 2000 when it's really at 1000, 3000 when it's really at 2000, and so on. More likely, it reads some PROPORTION too high, say, 25% or something; so at 800 actual it reads 1000, at 1600 it reads 2000, and so on in some pattern of that sort, such that it's only off by EXACTLY 1000 RPM at EXACTLY one engine speed. That being how they always fail.

The thing about the "creeping up" is how they all do; has to do with a little fluid chamber they added to the meter to damp its motion. That part is OK. They usually read anywhere from 500 to 1500 with the key off, it seems.

The ckt from the dist is just a wire and some connections; no components. Not much to fail there.

 

With the old tach it read about 3000 exactly at 80mph, with this one it reads about 4100 at 80mph. Your right, its not 1K high all the way. at idle it reads about 2 or 300 high.

Do you know which component on the circuit board it is that falls out of spec. I am in an electronics class so I have access to any tools I'd need and have no problem making repairs, but I'd rather not dismantle the entire board(which isnt much)just to find one thing. I'm sure its probably one of the resistors since the board appears to have slight heat damage under where a set of resistors sit. I thought about wicking out the solder today and pulling out all the resistors and checking them, but like I said, I'd prefer to pinpoint it first.

 

Do a search. I have the same problem and found a post that told me more than I ever wanted to know about the tachs in our cars. Told you step by step how to check yours and repair if needed. I think I'm just gonna buy a new one... cause it seemed like a lot of trouble to fix the stock one.

 

If this was just the stock tach I would just get a new one too, in fact I originally did. This is my 3rd tach, but I got this one because it goes to 8000rpm, and since its a rare tach with probably only around a thousand ever made, I dont think I'm gonna just throw it out. Not to mention these tachs are hard to find and chances are if you do find one it will be bad too.

thanks for the info though, I will do a search again. I searched once but didnt find the thread your referring to.

 

It was actually surprisingly easy. It sounds complicated though.

 

easy as it may be it doesnt do me a damn bit of good when I cant find the thread. I found a bunch of threads to fix the 91-92 tachs, but that doesnt do me any good because the PCB's are completely different on the 83 tachs.

It looks like I'm gonna have to work on a fix on my own. I've got 2 extra tachs to play with so I should be able to figure it out.

 

Got out one of my spares and snapped a few pics. Tomorrow I am hoping to do some testing. Seems from the threads I read that on the earlier tachs the problem is usually the capacitors, so I'll look there first. Im pretty sure whatever equipment I need is in my class. I think I'm gonna pull a couple of the resistors out and check them too since the board looks like it got really hot where they are mounted. Heres the pics I took:

 

more

 

We really should get those things stickied. I was looking through the older tach fix long before I found the 90-92 threads.

 

Yeah that would be a huge help. I was surprised that the 91,92 thread wasnt turned into a tech article. That was the first place I looked for info but theres nothing there.

If you would happen to remember the thread and could give me a link I'd be much greatful, I searched for well over an hour twice without luck. All I could find was the 91/92 stuff and a ton of dead end threads.

 

I was doing the search earlier today to help you out, but no luck. The board's search function is not working right (which is how I found that thread in the first place) and the Google search didn't produce the thread I wanted. Once the board search works, I should be able to find it again with little difficulty.