3.6 swap into '69, No Tach signal

[rennch]

Hi,

My 3.6 has been successfully transplanted into the car (See: Installing 95 993 3.6 into 69 T/RSR) but I have no tach signal yet.

I have a Patrick Motorsports Harness, and it has a yellow wire that replaces the black and purple wire on the back of the tach. Simple enough.

However, the wiring for this car was such a mess to begin with, because of the year, that we had to make a ton of custom plugs, etc.

My question is...where does the tach signal come from? Is it strictly fed from the DME, or does the DME get a signal from the engine compartment somewhere? If the latter, where's the signal come from? I can create a jumper wire to figure it out if I know where to connect it.

Thanks!

Michael

[jpnovak]

tach signal should be pin6 on the 55pin ECU connector. Make a jumper and see if the tach works. Then back track the signal through the replacement harness.

btw, the early tachs used a 12V square wave input signal. The later tachs used an ~8V signal that crosses 0V. Not sure your early tach is compatible with the later DME signal.

[arrivederci]

I used a timmins wiring kit and my tach didn't work either. As I was debugging, I found that within his harness is an amplifier for the tach signal coming off the DME. I was getting voltage in but nothing out. So I ran a jumper wire around it and voila...the tach worked! He offered to test my harness because that config has worked in 100+ cars. I chose to run it as-is because I didn't want to rip the harness out of the car. I have no idea what was/is wrong, but that fixed it for me.

[Bill Verburg]

The tach signal is .5 bk/vi
pin 6 @DME
pin 4 @X4/2 14pin connector under the drivers seat
pin 14 @diagnostic socket
pin 6 @tach unless there in an OBC display in which case it is pin 21

[h.wong]

the signal comes from the DME... you need to have a compatible tach..
from 1984 Carrera up to 993 motronic. good luck!

[rennch]

There's wiring for an early or late tach on the Patrick Motorsports harness...but it doesn't specify how early. Your thought is that it has to be at least 1984 eh?


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[rattlsnak]

I know you might be trying to keep the stock look, but Autometer makes a tach that slides right into the hole and works and looks great.. Pro or Sport Comp will work.

here is the Sport Comp.. 5" TACH, 10,000 RPM, IN- DASH, ELEC

[jbell959]

Give Bob Ashlock a call. I bet his tach-adapt will do the trick so you can keep the early tach.

[arrivederci]

Originally Posted by emptyo

There's wiring for an early or late tach on the Patrick Motorsports harness...but it doesn't specify how early. Your thought is that it has to be at least 1984 eh?


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Mine is working fine with a 72 tach. Redone by NHS, but still 72 and I didn't ask for anything special other than a proper redline on the face.

[uwanna]

Originally Posted by emptyo

There's wiring for an early or late tach on the Patrick Motorsports harness...but it doesn't specify how early. Your thought is that it has to be at least 1984 eh?

My '80sc has a 964 3.6 transplant. I left in the stock '80SC tach, hooked up the bl/purple wire to it (from the wires that run from 14 pin plug near the DME to front of car). Been workin 23yrs! Just make sure it hooks to the tach wire connection on the gauge. If hooked to the "upshift" light connection, it can destroy your DME. YIKES!!

[rennch]

Where does the DME get the tach signal?


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[safe]

Subscribing to this since I have a 3.6 conversion going on in my 69...

When I converted my 77 to a modern EFI with the same signal (I guess) as the 3.6 DME I just put a zener diode in series with the tach signal, but I think the the 69 and 77 tachos are different, as implied above.
If an early tacho uses a square wave 12v, just connecting the DME signal should work?

[Bill Verburg]

Originally Posted by emptyo

Where does the DME get the tach signal?


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the fly wheel via pin 49 on the DME

[h.wong]

C/D tach for cars with C/D's ... Motronic tach ..for motronic engine...
84 carrera up to 993..

make sure you are not jumping or boosting the battery when trying the tach..
you might damage the tach with too much amps.. ask how i know..$$$ after
fixing tach... good luck!

[uwanna]

Originally Posted by h.wong

C/D tach for cars with C/D's ... Motronic tach ..for motronic engine...
84 carrera up to 993..

make sure you are not jumping or boosting the battery when trying the tach..
you might damage the tach with too much amps.. ask how i know..$$$ after
fixing tach... good luck!

I guess my eyes must be deceiving me, but the stock '80SC tach (orig CDI) in my
'80SC/964 3.6 transplant sure has done a great RPM reporting job for the last 23yrs!
LOL
Just hooked up the bl/purple wire from the DME to the tach input. Don't think you absulutely need an '84-993 Motronic tach, at least I didn't

[rennch]

Still no tach signal for me. I'm thinking I might need a later model than my 69.

[jbell959]

Originally Posted by emptyo

Still no tach signal for me. I'm thinking I might need a later model than my 69.

Did you check with Bob Ashlock? His Tach-Adapt might help

[rennch]

I wonder if the MSD Tach Adapter would work?

[safe]

I've bought Bob's tachadapt for my 69. Haven't tried it yet, had to pull the engine again...
Good value for money if it does work, as he says it will.

[rennch]

I bought Bob's Tach Adapt, and it works great! I have Tach again!


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