Calling all Electrical Geniuses: Tachometer problem on a 69T above idle...

[gruen911]

Guys, I have had 3 different Porsche mechanics try to figure this problem out.
My car is a 69T with a 2.4T (rebuilt Supertec S spec motor). The car runs fantastic and all electricals seem to work - except the tachometer...
The problem with the tachometer is that it reads normal at idle between 900-1,000. But as soon as I accelerate, the needle jumps up and reads 2-3 times the actual correct number. I did some reading and found out that the 1969 cars used a ballast resistor which dampened the impulse reading to the tachometer. Knowing this I tried initially to source a ballast resistor. But because these are hard to find, and even harder to find a good one in working condition, I had my original tachometer simply rebuilt with an internal ballast resistor (same as what the 1970 and later cars used for their tachometers). Despite this, the tacho still shows the correct idle, but 2-3 times correct rev number above 2,000 revs????

Strange details: If I keep one foot on the accelerator pedal above at any higher rev reading, and I press the brake pedal, the rev number drops down to the correct number. Similarly, if the engine is revving above idle (2,000 plus rev's), and I put on the right blinker, every time the blinker flashes, the rev number drops down to normal and then bounces back up every time the signal flashes. WTF is this???

The mechanic looking at the car now is RUF trained and puzzled. He believes that the problem may have to do with the car (a 69T) not having originally had a CD box , which has been retrofitted to accommodate the 72T motor. Since the wiring for the tachometer is pretty straightforward (only 3 cables run the early tachometers: 1) ground, 2) wire to battery, 3) wire to either the CD Box or the coil), I suggested that he bypass the wiring currently going to the tachometer and simply run a cable directly from the tacho itself to the coil or CD Box, the battery cable straight to the battery, and the ground directly to the car's metal. He is going to try this tomorrow.

Anyone ever heard of this problem or experienced it themselves? Why would a tachometer read normal at idle, 2-3 times the correct number above idle? The more time this guy spends trouble shooting, the higher my bill goes. ;-(

All ideas and thoughts appreciated. Thanks...

[blucille]

A later tach, from a car with a 6pin CDI (for instance) would likely work just fine. The gauge rebuilders can modify your tach to work in this situation.

[304065]

If your mechanic runs a wire from the negative terminal of the coil to the back of the tach, and you are using a CD box, you will burn up the tach. It's not designed for a +350V pulse.

A few background questions.

1) What is the voltage measured at the battery with the engine off? Engine at idle? Engine at 3000 RPM?

2) What CD box are you using?

There is no such thing as an internal ballast resistor. The "ballast resistor" term is a holdover from the Kettering ignition of the SWB cars and the 911T. What you are describing is the "intermediate unit" which says on the side "EINBAU SENKRECHT KLEMMEN UNTEN" (nothing special, just "install vertically with contacts underneath" as the units would fill up with water otherwise.

If you had your tach modified to work off the points circuit with CDI, that's a good thing.

My expectation is you have a defective voltage regulator but I need the rest of the info to figure it out.

[Jonny Retrofit]

I'd tend to go with the voltage regulator theory. But...

Unlike the later cars, on the '69, the tacho is driven from the coil primary voltage and uses the 'intermediate unit' to condition the signal. Some aftermarket igniton units, employing multispark with a long spark interval (e.g. MSD) can cause the tacho to overread. Usually though, the overread happens between idle and around 3400 RPM, then normal operation resumes above that when MSD reverts to a single spark.

A bad case of points bounce may also affect the tacho but that would generally be more erratic.

[gruen911]

A specialist who rebuilds CD Boxes, tacho's etc, has determined that the problem is the wrong coil. The one in my car was built for a battery (non CD box) system. Since my motor is a 72 911T, I need a coil for a CD box. Hope that solves the problem, and the coil has not damaged the tachometer...