Tachometer Compatibility Question

[lsf911]

I have a 68 911 that has tachometer 901.741.302.04 (dated 7/67) which stopped working, totally dead. Recently, I acquired another 901.741.302.04 (dated 3/71). Both redline at 7K, look identical except the 3/71 tach doesn't have the mounting posts.

When I installed the newly acquired tach in the car, at first it appeared to work correctly. From 0-3000 rpms it seems to be very accurate. However, it stops at 3000rpm. No matter how much higher I rev the engine the needle stops on 3000!

Should this tach be compatible with my 68? It has the exact same part number.

Is the tach faulty? Anyone else ever see a tach cap out at 3K? Oh, almost forgot, the bezel shows signs of a repair in its past.

Thanks in advance for any help.

[blucille]

I believe this will get you going

TACH-ADAPT

[lsf911]

So what you are saying is even though the tachs have the same part number they are not compatible and I need to use a tach-adapt to fix it. Correct?

It is still a 6 cylinder to 6 cylinder Tach. The ignition is not modified. How will the tach adapt help in this situation?

[lsf911]

So what you are saying is even though the tachs have the same part number they are not compatible and I need to use a tach-adapt to fix it. Correct?

[Trackrash]

Your original tach was designed to work with a points coil ignition system. Sometime during '69 Porsche went to the CDI ignition. I don't think your '71 tach is backwardly compatible, but who knows.

If you can confirm the '71 tach is actually functional in the original car, then the adapt a tach will probably fix the problem.

Also FWIW there were two different types of tachs in '71 as that was a year of change for them.

[lsf911]

Well, I was told the tachometer was tested and functional before I bought it. That's probably all the confirmation I will get on it's condition or whether it worked in the original car.
I do have a tach-adapt and I do not see a mode that will help with this scenario.
Any thoughts on that?

[jbell959]

Originally Posted by lsf911

Well, I was told the tachometer was tested and functional before I bought it. That's probably all the confirmation I will get on it's condition or whether it worked in the original car.
I do have a tach-adapt and I do not see a mode that will help with this scenario.
Any thoughts on that?

Most likely, your 71 tach is looking for a high voltage signal. You would set the tach adapt switches for the x1 setting meaning no change in the signal rate. Then you would remove the jumper, next to the terminal block, to set the output to high voltage. If the tach is working properly this should solve your issue.

Feel free to PM me if you have questions. Unfortunately I'm on the opposite coast, but I'd be happy to test the tach for you.

[lsf911]

Ahhh, okay jbell959 that makes sense. I will try that tomorrow and post the results.

Thank you all for your responses.

[lsf911]

Okay, so I put in the tach-adapt, set as suggested. It made no difference. Still caps out at 3k.
So I decided to remove the new tach and install a spare 912 tach that I have with the tach-adapt and it works fine.
Has to be an issue with the new tach I acquired.

[jbell959]

Afraid you're right. Looks like an issue with your tach.

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