My 3.2 Swap Tach Has Never Worked. Please Help Me Diagnose. Almost FREE, No Payments!

[DonDavis]

Thought I'd try some catchy advertising for my thread.

My Tach has never worked since I completed the swap. The Tach gets its signal from the Speed sensor on the flywheel, right? If it were the sensor, my car would run crappy, correct?

Here's a short list of things I've done/checked...

put my ECU in another car, the Tach worked in that car
tried a different Tach in my car, no joy
verified 12v and ground are good at Tach
wired properly and have verified continuity from Tach to ECU

I'm at the point I'm doing things over and over and getting nowhere. This stuff should be simple and well, it's kicking my butt.

Has anyone had Tach problems when doing a swap?

[DonDavis]

Does the ECU create the Tach signal based soley on the Speed sensor?

[Early_S_Man]

Don,

Is the non-cooperating tach a '72 model, or a later vintage?

Have you tried swapping tachometers, specifically, an '84 or later tach ?

The tach signal can be considered a buffered version of the ignition coil drive signal ... not the same as the speed sensor or reference sensor signal.

[78-911SC]

Don I'll look at mine again when home but if I remember I had the same problem and there was a brown wire on the harness next to the ecm that I ran directly to the tach and I was good to go. Make sure you have the connection on the tach right. I'll try and post tonight if you haven't gotten your answer.

[Early_S_Man]

Tachometer pulses per Rev??

[1972_911T]

I have a 3.2 in my 72 the tach is the original 72 unit and works fine . Which tach wire did you use there are 2 on the 3.2 one is black the other is black with a violet stripe and goes to pin 21 on the DME this is the wire you need to use.



This is what I made when I wired in my DME to the front of the car, everything worked first time with no issues.

Edit

This is based on a Euro 3.2 so not 100% sure if the us version is different.

Hope this helps Steve

[78-911SC]

Steve has the correct answer.

[Walt Fricke]

Steve - that is interesting.

When the SCs switched away from points distributors Porsche changed the tach. Earlier tachs wouldn't work with the signal the reluctor system generated. So I bought a new tach when I put an '82 3.0 in my '77.

But it seems that for the 3.2s you have a choice of signals to send to the tach? And one works with the earlier tachs? Is the other one needed to work with later tachs?

Walt Fricke

(who never really understood the difference, other than by supposition - the points signal might be stronger even after filtering its spikes and whatnot? But the later signal was cleaner/lower voltage and wouldn't work with the early tach? Or maybe the signal from the electronic version was dirtier?)

[Shuie]

Wow. My '72 tach worked without issue. are you sure you have the correct wire or that the wire doesn't have a break in it somewhere? Have you tested the wire end-to-end to make sure it hasn't been broken or splice into somethign else in the harness? I seem to remember your harness being a little jacked due to the wiring in its previous host.

Its weird how we all seem to have different issues with the 3.2 swap into a '72. My friggin' turn signals don't work which blows my mind because I never touched them

[DonDavis]

Thanks guys.

I am running a newer Tach and the one I swapped for testing was from an 88, I think. At any rate, it is the correct one for the motor.

I ran into a wiring SNAFU during install that may be relevant.
My 3.2 Swap is running!

The diagram Steve added is not the correct one for my application. My engine is an '89. The wire colors changed for 88-89.

I don't have the Upshift wire connected. Only #2, #5, #6

[78-911SC]

Alright I just got home and checked mine. The one I have going to the tach is a brn wire that goes to pin 1on the big harness. There is also a grn wire connected to pin 1. That may be power. There were two other wires that were for the tach both blk. The first blk wire goes to pin 11 and the other to pin 21. I caped both off. I'm not sure if that helps.

[Shuie]

the green wire is related to the AC

[78-911SC]

Not sure but I think pin 1 would be the coil which would make more sense. That grn wire goes back toward the rear and the coil has a grn and blk wire connection. I'll see if I can find my notes.

[Early_S_Man]

Don,

According to Bentley ... the '88 and '89 models had the tach signal moved to the #4 pin of the ECU, replacing pin #21.

I suggest putting an oscillocope or CMOS logic probe on the 'presumed' tach wire to check for pulse activity when the engine is running ...

[Walt Fricke]

Warren

Do tachs work basically like the '76 and on speedometers? A basically digital (pulsed, at least) signal is converted to a voltage varying with frequency, which drives the meter needle in the more or less normal way?

Walt Fricke

[FenderGuy]

Does this mean if a 3.2 is put into a 80 SC you need to switch the tach?

[Rodsrsr]

the tachs are mached with the motor. A 3.2 must use a 3.2 tach, whereas a 3.0 must use a 3.0 tach. I found this out when I installed a 3.2 in a 76 body with the original tach. It worked, but was way off. I eventually got the correct 3.2 tach and everything works fine.

[Early_S_Man]

Walt,

That is true.

Frank,

Not necessarily ... several Pelicans have done successful swaps and used their old tach, and it happened that it was able to accept and read the Motronic waveform just fine! My feeling is that it is a matter of how the circuit tolerances add up ... sometimes in a good way, other times, not.

[Rick Lee]

PM Marcesq about this. We ran into an issue with his 3.2 swap and John Walker walked us through it. Pretty sure Marc has a website that mentions the workaround. It was so long ago that I don't remember.

[78-911SC]