Tach / CDI compatibility Query
 

Hi all,
I have a 76 3.0 Turbo. I believe the car should have a 8 pin CDI but mine has a 6 pin CDI installed. The tachometer has been defective since I purchased the vehicle.
At 4500rpm the needle drops to zero and when I change up to the next gear the needle then bounce to 6-7000rpm before settling to the correct place. I recently removed the gauges and sent them away to be restored and requested the tach be repaired.
The unit was tested by the workshop that was doing the work
- it did not work work when connected to another 6 pin CDI
- it worked perfectly when connected to a 3 pin CDI and revs all the way to 7000rpm without issue
However, it is still misbehaving in my car.
Any advice on what to do next?
Kevin

Hey Kevin... That's a tough one without taking a look inside the tach because a stock 76 3.0 Turbo tach will not work with either a 6-pin or 3-pin CDI. They are designed for the 8-pin CDI which delivers a very high voltage tach signal. If it did work with a 3-pin CDI then it's likely someone modified the tach. Without knowing what was or wasn't done with the tach it's almost impossible to diagnose. The fact that it works with a 3-pin CDI leads me to believe it's been modified. Or it's possible that the shop tested it in a 70/71 car with a 3-pin CDI and a tach ballast in which case it may have worked. The ballast that was installed in those cars was designed to simulate the tach signal from the earlier inductive ignition system so the signal is similar to the 8-pin output.

The 6 pin CDI tach provides a low level, <12V, signal.
The typical 3 pin CDI tach signal is the coil signal (CDI output), >200V and NOT a 12V signal.

You need to determine where the blk/pur tach wire is connected on the CDI and what type of tach input is required.
If the tach requires a CDI output signal, then just connect the tach wire to the coil wire of the 6 pin CDI.

The tach signal on a 3-pin CDI system comes from the points which is also used to trigger the CDI. The output of the CDI goes to the coil. There is a pull-up resistor in the CDI box so you end up with a low-voltage square wave since the input is being pulled to ground by the points. This is why the ballast was required in the 70/71 cars because they were still using the old tach driver design that was looking for a high-voltage trigger.

Agree with what is said above.

The 8 pin unit puts out a positive going 300V pulse to drive the tacho on an early Turbo.

The coil output of a Bosch CDI is a negative going 400V pulse (although it falls to 275V with higher RPM). Although not 'correct', it will drive the early Turbo tacho.

Whether using the coil output to drive your tacho is advisable long term is another mater!

Actually, there're some 911 tachs which use the coil signal! Remember, the CDI output signal goes negative and then positive,
so either polarity can be used to trigger the tach.

Solution: The tach needs to be tested for the compatible input signal. Let's avoid guessing!
Re-read post #3, the above was stated there too. Avoid confusing the OP.

Correct.. All the 911 tachs until sometime in 71 are triggered by the high-voltage coil signal.

Thanks for all the info. Guess it needs to be stripped again and rechecked.

Not on an 8-pin. The dedicated tacho signal is a positive going 300V 'sharks fin' only (opposite polarity to the coil output) so the tacho input circuit may well be unique to the early Turbo.

Most of the high voltage Porsche tachos will trigger on any pulse above 150V.

The one exception I have found is the genuine 962 tacho (made by Contactless). It needs a very high voltage pulse (over 150V) and will not work with less.