Dead Tach on 1981 SC

When I purchased the car, the tachometer did not work. What diagnostics should be done to insure it is the tach and not the CD box. There is 12V+ and ground to the tach. The signal wire has voltage (~4-5Volts if I remember correctly) the varies inversely proportional to RPM. But... The tach needle does not move

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Daniel Tisserand
1981 SC

Daniel,

If you know what an oscilloscope is, and can monitor the input signal with one, that would be a good way to verify that one pulse is being sent to the tach. for each ignition event.

At an idle speed of 1000 rpm, that equals 50 ignition pulses per second. The pulses at the tach. input should go all the way to ground, and have an amplitude (height) of 10 Volts as a minimum, and ideally should reach 12.0 to 13.0 Volts if the system battery/alternator Voltage is 13.5 to 13.8 Volts.

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Warren Hall
1973 911S Targa

Thanks Warren, getting a hold of an scope is the tough part. I will see if I can 'borrow' one from work. Is it supposed to be one pulse per ignition event (i.e. six per three per revolution?)

Daniel,

Exactly ... the best thing to do with a scope would be to compare the tach. pulses to the pulses on the primary side of the coil, to make sure they coincide.

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Warren Hall
1973 911S Targa

My tach just flat went out ( broke ) last
year, I obtained a used one from 20th street
auto parts that was in really good shape for
about $150.00.

Daniel, I have one from an SC if you need one.
Marc

Warren,
I hooked up a Sears Tachometer to the TD input behind the tach and to ground. I got nothing ( well I got a signal that was constant and does not vary with RPM). Does this imply a problem with the CDI box or is this not a valid test?

Thanks,
Daniel

That's not a valid test. You can't use the cheap tach/dwell meters on Porsches CDI.

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Tyson Schmidt
72 911 Cabriolet

Any suggestions for a good tach/dwell meter that will work with Porsche CDI??

You'll have to ask Warren I guess. I have a very expensive timing light that performs those functions but it's probably out of the home mechanics price range.

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Tyson Schmidt
72 911 Cabriolet

Thanks for everyone's input. After seeing the gauge rebuild article here on Pelican Parts, I took the tach apart. The return spring is hosed. The tach will be shipped out for repair on Monday :>

Thanks,
DAniel