Connecting tachometer with permatune on 1972 911T

[jdb]

Hi, I'm new to this forum, found a lot of tips&tricks which were very helpful in getting together my 1972 2.4T which I restored in the past couple of years.
One thing I couldn't get to work is the tach in combination with the Permatune which was fitted to the car. I found permatune wiring diagrams on this forum which don't seem to match the current wiring on my car. It runs OK but the tach doesn't work (never did in the 15 years I had this car). I'm hesitant to try out different wiring options as I would rather see the tach working than smoke coming out of one of the components...
This is the Permatune 911E unit fitted to the car:

According to the date stamp (11/71), the tach seems to be the original one (12V/24000Imp./230/41/10):

This is the current wiring lay-out:

There's a black/voilet wire in the engine compartment which isn't connected.
Also, the wiring on the back of the tach is loose so I don't really know how to connect properly. There's a black/red double wire and a black double wire that probably connect to the tach.

>>Should I follow the Permatune wiring diagram and re-wire accordingly?
>>how should the tach be connected to the permatune unit? Using white/yellow/green permatune wire? How does this wire connect to the main wiring harness leading to the front of the car?
It would be really great to see this meter come to life for the first time….
Any suggestions would be extremely welcome!
Thanks, Joost

[edmayo]

The Permanent should be direct plug in,,no wiring changes needed.Black/purple is tach lead, needs to connected at dist. and at back of tach.

[304065]

If your permatune is for the 911E there should be four terminals, not six or eight.

Their equivalent for the Bosch unit are:

A= Achtung! 350V high-voltage output to coil positive
B= Batterie 12V from ignition
C= Contact = input from points
D= Ground, this is the braided wire strap to the earth terminal on the firewall and a few other places.

The wiring should not be different from the Bosch to the Permatune, it's intended as a direct plug-in replacement. Does your car still have the original engine compartment wire harness?

As Ed correctly says, black/purple is the tach lead, that goes to the tach C terminal (your later tach from after October, 1970 has a "C" for the points not the /1 of earlier cars and that is a good photo of it)

As you say, if you are unsure, have a qualified expert look at it in person vs. the risk of burning the car up. . . diagnosing electrical problems is difficult remotely and if things are different than stock, pretty much impossible.

[jdb]

Thanks for your feedback. Unfortunately, the wire harness is not 100% original so I don't trust the colors. There's a black/purple wire coming from the engine compartment wire harness but I don't see it coming from under the dash behind the tach. I'll try to find an expert or someone with a similar car and ignition so I can check…thanks again rgrds Joost

[Gearby]

I have a wiring harness that came out of a '72 911E about 30 years ago. That is according to the guy who sold it to me. I purchased the entire harness for a portion of the front trunk harness and now am selling the rest of the harness. PM me if you have any interest. Derek

[savaden]

I have a Permatune in my 912 and I have dealt with the 5 wire version and the three wire version. I think the 911 has a little silver box called a "signal conditioner" that connects between the Permatune and the tach. You can get a modern replacement for this device from Bob Ashlock: http://www.ashlocktech.com/TACH-ADAPT.html. My tach did not work until I used this to supply an adequate pulse to the tach.

SV

[edmayo]

The 72 and up made the change in the tach head, the amplifier was no longer needed.

[304065]

The 912 never came with CDI, it doesn't need it-- there is plenty of time to charge the coil between spark events even at high RPM because there's only four cylinders-- hence the 912 tach only ever expected the 24 volt (according to VDO, in reality like 200 volt) pulse from the coil when the points open.

When Porsche switched from Kettering ignition to CDI in 1969, the tach triggered off of points opening. The pull-up resistor in the Bosch CDI is only 33 ohms, so the maximum current in the points circuit is 420mA. This is enough to switch the CDI, it's an 11 volt square wave from the points, but not enough to create a big pulse up the line.

Whether Porsche didn't want to redesign the tach or whether they had a bunch of tachs ordered from VDO in the old spec we don't know. What we do know is the "intermediate unit" or silver can that says "Einbau Senkrecht Klemmen Unten" ( and you should do just that) had a coil that approximated the high-voltage pulse produced by the original tach. This was used through tach production dates of October, 1970 stamped on the back. You can also tell by the "C" which matches the "C" on the CDI. A "C" tach does not need the intermediate unit: a 69-70 tach does.