tach problems with 3.2 conversion

[jj4u3]

I know there is a lot of reading about this but I am a novice and I don't understand most of it. I have a 1972 911T and in the middle of a 3.2 conversion and can't get the 1972 tach to work. Do I have to get a latter model tach or is there something specific that my mechanic is missing

[jager911]

When I did my 3.2 conversion back in '03 on my '74, I had the same issue. I used the violet wire and black wire from the 3.2 harness to the tach. I think there are 2 terminals on the tach. I switched to the other terminal to get it to work, I can check tomorrow. I also have the wiring documented for the 14 pin connector if you need help.

[jj4u3]

I would appreciate any help that I can get on the wiring for the tach

[DonDavis]

*EDIT:*Apparently some say an early tach can be used. Not sure if any modifications are needed


An original '72 tach CANNOT be used. It must be a Carrera '84-'89 tach. Use the original power (red/black wire) connected to pin 5 and the original ground (brown wire) connected to pin 6 on the new tach. Then tape up the original black/violet wire and ignore it.
Ok, here's the wiring for the 8 wires that run foward from the DME relay and the "brain".

Yellow- goes to the ignition terminal #50

Black/violet- pin 2 on Tach

Black- .75 mm wire, needs terminal #15 switched 12v

Black- 1.00 mm wire, pin #3 on the Tach, it's the "upshift light" and not needed unless you've also swapped transmission and speedometer from the donor car. Don't worry about it.

Red/green- feeds the fuel pump via fuse panel 25Amp fuse #3

Red- 4.0 mm wire, positive battery terminal

Black- 2.5 mm wire, needs switched 12v via terminal #15

Green/yellow- A/C, not used

Refer to the factory or Bentley schematics to find the "# terminal" locations if needed. I chased this same issue FOREVER! The tach signal comes from pin 21 on the brain. Put an O-scope on there and you should see a varying duty cycle square wave.

Good luck and let us know how you do!
Oh, and next time post pics of your car!!

[RWebb]

"It must be a Carrera '84-'89 tach."

- Not sure what this is based on. My 1973.5 tach works just fine - BUT I use an msd. Maybe he is referring to the Bosch silver CD box.

There are dozens and dozens of threads on how to get things working & what does not work. Do a Search!

IIRC, around 1970-71 and older tachs need to be replaced, but not '72 -- you never know tho, P AG could have used an older tach in a '72 car.

[copbait73]

I agree, it will work. I used the stock early model tach but off the top the top of my head I don't remember how it was wired. I know the stock Porsche box was retained.

[DonDavis]

If an early tach can be used, then go for it. When I did my swap, the engine came with a tach and all I had heard was a later tach was needed.

I've edited my earlier post.

[RWebb]

and please post back to fill in the data on what works with what & how (i.e. msd vs. Bosch, etc.)

[jj4u3]

Thanks for the input guys. I will try the old tach but I have located a newer one just in case I can't make it work
so far no luck with the old one but that may be me