71 tachometer don't work with 3.6L engine

[yelcab1]

I'm wrapping up a 3.6L transplant into a 71 chassis. The drivetrain came from Timmins of Instanttorque fame. The last hurdle is the 71 tach is not working with the signal coming out of the wiring harness provided. It was working before the transplant.

The tach connection is quite simple, 12V, GRD, and the tach signal. That's it.

Things i have tried:

1. Connect it straight in as recommended by Timmins, did not work.

2. Took it to Palo Alto Speedo, they asked for the configuration. OK, 964 engine / 71 chassis. Helmut called somebody and suggested a Ashcroft conversion board. Did not work. (Helmut been very nice to me) with signal at 5V or 12V.

So, back on the forum for the real experts. What is wrong? There are 3 signals: Power, Ground, and tach. Here is the tach signal. It's of the right frequency, 0V to 13V (alternator voltage), it should work with the old 71 tach, no?

[tperazzo]

Did you remove the jumper on the Ashlock box?

[yelcab1]

Originally Posted by tperazzo

Did you remove the jumper on the Ashlock box?

Tried it both ways. Did not work.

One last ditch effort would be to buy a used 964 tachometer. But I would be losing the left and right turn signal indicator.

I wonder if the turn indicator (most likely 12V signals) would work with the 964 tachometer which I assume to run off 5V

[jbell959]

A 71 tach requires a high voltage signal provided by a ballast that Porsche installed in the 70/71 cars. It's a silver can mounted on the CD panel.

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[jbell959]

I'm not positive, but I believe it's around 24V that the early tachs require. It's a long story, but basically these tachs were designed for the pre CDI ignition systems. Rather than changing the tachs when they went to CDI, Porsche came up with this "Intermediate Unit" or ballast that takes the CDI tach signal and makes it work with the older tach design. That was corrected in 72.

I don't believe Bob's Tach-Adapt fixes this issue. What Hartmut should have suggested is replacing the internal driver board in the tach with Bob's tach driver. That will fix your problem. Bob's sells them if you're comfortable opening up your tach and rewiring and calibrating it.

I also have a driver board, based on Bob's design. PM me if you'd like for me to do it for you.

[Bob Ashlock]

Hi, Bob Ashlock here ... I sent you email, let's work this problem out.

[al lkosmal]

Bob is the tach guru.


Here is some reference info for you.

[jbell959]

Originally Posted by Bob Ashlock

Hi, Bob Ashlock here ... I sent you email, let's work this problem out.

You're in good hands with Bob!

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