Tach not working 1977 2.7

[Carter2350]

Hello, I am wondering what major issues cause 911 tach to stop working.

I am new to 911's and am converting this one over to ITB's. I have removed cis and installed new throttle body's with wiring, everything is working except I need to pull a tach signal (from the black/purple wire). However when i crank the car the tach does not bounce.

What are the steps to diagnosing the tach on a 1977 porsche 2.7?

Do I start at the gauge or at the points? From what I understand is the points is responsible for the tach signal. I see one spade connector on the distributor that gets power when cranking the car, is this where my tach signal starts? Thank you so much for any help, this has been frustrating and I don't wanna mess anything up.

[mikesarge]

Hi there-

Does your car start? Does your ecu read your tach signal?

I recently went through this too.

My thread, with lots of helpful pointers, is here: Dead tach, RPM signal to megasquirt ok

[Carter2350]

Originally Posted by mikesarge

Hi there-

Does your car start? Does your ecu read your tach signal?

I recently went through this too.

My thread, with lots of helpful pointers, is here: Dead tach, RPM signal to megasquirt ok

Ecu does not read tac signal when cranking. I think I have an issue with the entire system. I think I do remember the tach working when pulling this car in. Should I be able to read voltage with a multimeter from the black/purple wire when cranking?

[Carter2350]

Originally Posted by mikesarge

Hi there-

Does your car start? Does your ecu read your tach signal?

I recently went through this too.

My thread, with lots of helpful pointers, is here: Dead tach, RPM signal to megasquirt ok

I have not tried to start it, I assume the computer needs the tach signal.

Is it normal for the purple /black wire to be grounded? Not sure why it is grounded that doesn't make since to me

[spuggy]

Originally Posted by Carter2350

Should I be able to read voltage with a multimeter from the black/purple wire when cranking?

A scope would be more useful for that.

Originally Posted by Carter2350

I have not tried to start it, I assume the computer needs the tach signal.

A 1977 2.7 has no computer. You say you have a distributor. Try explaining what else you have fitted to the car - CDI? Some aftermarket ignition? ECU?

[timmy2]

On A ‘77 the tach signal out from the CDI plugs into a “T” connector beside the CDI. The body side of the connector is the wire running to the Tach.(and power to the CDI)
The CDI harness connector (that is part of the original Engine harness), inserts in the body side T connector. It has the red wire switched power in to the CDI and the white wire(s) Tach signal out from the CDI.
Hope that helps.

[Carter2350]

Okay, quick update I am so sorry for the struggles. When I splice the tach wire from the megasquirt into the black/violet wire in the 911 harness I no longer have Spark, the second I disconnect the megasquirt tach wire I have spark again. What should I do to get around this problem. Use an external tach adapter? Seems like the resistance ****s everything up

[Carter2350]

Also, it does go pop when I stop cranking, so the spark does happen with everything hooked up as long as you are not cranking