tachometer issue on 72 911 with a 3.2 conversion

[frank912]

Hi All,

Just finishing my 2 year restoration on my 72 911. The car was finally ready for some test driving this week. Everything worked fine but after a few minutes my tachometer died. My mechanic wired the black purple wire for the tacho directly to the - pole on the coil. He said he had done it multiple times without issues. My tachometer was a 72 dated one.

Because the meter died (means it did not move anymore, not even with 9V on the - and purple/black connection on the tacho. Not sure whether this is correct. Writing it down makes me suspect it should be the 12V and - connection on the tacho. But that's not the issue), I installed a different tacho. This tacho was dated 76 and it worked fine.....for 2 minutes! Then it did not move at all until the engine turned at 4-5K rpm....this made the tacho needle crawl towards 1,5K rpm.

I talked to the mechanic but he does not understand it either. Any ideas what could cause this problem. Engine has just been rebuild completely and everything on the car is 'in new condition'. Not suggesting that there be a fault in this but just trying to clarify the situation. I prefer to solve this instead of running a new wire for the ECU to the tacho (not sure whether this would solve the issue). Just trying to understand what the cause is and to solve it. Also curious whether the 2 tachometers are ok/repairable/or salvaged?

Thanks a lot for helping out.

Regards, Frank
The Netherlands

[blucille]

Try swapping in a later tach. The later ignition (distributor pick up and cdi box) is not compatible with the 72 tach.

A gauge restoration shop can upgrade the tach to work with your ignition.

[Frankr]

MSD makes a little adaptor unit that improves the tach signal so it will work fine. https://shop.advanceautoparts.com/p/...g&gclsrc=aw.ds

[304065]

Frank

Your mechanic has burned up about $500 worth of instruments by not understanding the ignition system.

You don't say what ignition system you are using, but I assume it's the 1972 Bosch CDI system. The original coil sticker has a pictogram showing a meter connected to the coil with a big red X through it. . .

(From our friends at Sierra Madre Collection)

This should have been a signal to him to STOP. But oh well.

Here's the thing-- he's mixing up the way the ignition works between the original Kettering-ignition, where the points are connected to the negative pole on the coil (and so is the tacho wire) and the post-69 BHKZ (CDI) system, where the points are connected to the CDI, and then the CDI's 350V output is connected to the positive pole of the coil, and the negative to ground with a curly brown pigtail wire.

The Bosch CDI connections are :

A= Achtung = 350V output
B= Batterie = +12v from ignition switch
C= contact= wire to points, only carries a tiny 420mA current!
D= ground tab, connects to braided ground wires and engine case/ ground on firewall.

The black/purple wire connects to the "C" pole of the CDI unit at the POINTS. It can connect at the points or wherever else you would like as long as it's in the circuit. There is a 33 ohm pull-up resistor inside the CDI that creates a 420mA current in this wire when the points are closed (by completing the ground). When the points open the CDI fires and sends a whopping 350V pulse to the coil, which gets transformed up to ignition voltage and you get the spark.

What your mechanic did was connect the tach to this 350V pulse, which it isn't designed to handle, so the input resistors fried.

Anyway, get yourself the wiring diagram here : http://members.rennlist.org/911pcars/WiringDiag.htm fix everything up like the Factory intended, send your original tach to North Hollywood Speedometer to be rebuilt and send your mechanic the bill.

Good luck! Post up here if you have any questions I'm happy to help and sad that this happened.

[frank912]

Thanks guys for the extensive replies.

To answer some questions: the is no CDI box in the car anymore. The car has been converted to 3.2 engine incl motronic ECU. There is a tacho signal coming from the ECU but because the tacho wire was originally in the engine bay, the mechanic wired the black/purple wire to the - pole of the coil.

I do not have the CDI so I am wondering whether my set up is not comparable to the kettering?

Guess next time we take the tacho signal from the ecu. Rather open the wiring loom than frying another tachometer.



Regards, frank

[M_deJong]

I used a 3.2 tach with my 3.2 conversion, I think you may need to do the same.

[304065]

Thanks guys for the extensive replies.

To answer some questions: the is no CDI box in the car anymore. The car has been converted to 3.2 engine incl motronic ECU. There is a tacho signal coming from the ECU but because the tacho wire was originally in the engine bay, the mechanic wired the black/purple wire to the - pole of the coil.

I do not have the CDI so I am wondering whether my set up is not comparable to the kettering?

Guess next time we take the tacho signal from the ecu. Rather open the wiring loom than frying another tachometer.



Regards, frank

Frank, ok, that is a major difference, but I think still the same outcome. The tachometer drive output of a Motronic control unit is a +5V square wave. That is supposed to go directly to the TD input of a Carrera tach, which is designed to work with that input level.

The 3,2 has the coil positive connected to a fuse and instead of points to switch the ground it uses the MOSFETS inside. The MOSFET turns on, connects the coil negative to ground, current flows in coil, field builds up, then the MOSFET switches off and the coil field breaks down, inducing a current in the secondary windings, causing a spark to ground.

If you connect the tach to the coil negative, you end up with about 200V EMF pulses every time the coil fires, with plenty of noise and ringing on the line. I cannot imagine that is good for the input resistors on the tach.

If you have access to a function generator you can drive the tach with +5v square wave to test it. That won't work with your 72 tach, as it uses an 11 volt square wave.

So we're back to sending it to NH speedometer, tell them you want a new board that will allow you to use it with motronic, they will fix you up.

[Ratech]

I make the product TACH-ADAPT that is very useful for these situations ... check my site www.AshlockTech.com
It will take the 5V ECU signal and convert it to the signal your '72 tachometer needs. Hopefully your '72 tach has not been blown by the previous experimentation.