Just an update. Since my battery was going south I used that as an excuse to get a lightweight battery (Odyssey 680), mount, and kill switch. The new battery did not cure my bouncing tach so now it's on to the alternator and voltage regulator.
Sorry for the poor terminology but the copper piece inside the alternator has 2 little ruts in it from those spring-loaded tabs on the voltage regulator so the alternator is being rebuilt and a new voltage regulator will be installed. I'll report back on the results, probably not until Monday.
While the car was up on jack stands for my turbo tie rod install I also installed some solid transmission mounts as well as cleaned that nasty transmission ground strap. I also found that the ground wire from the top of the engine case had some green stuff on one end so that'll be replaced too.

 

I had my alternator rebuilt and a new voltage regulator installed. The ground from the top of the engine case was also replaced as well as the battery ground during the course of the kill switch install. I got the car back today and went for a drive and there was no problem with the tach. Finally!

 

Subject: Inoperative/Erratic Tach in my 930
My Problem
The tach on my 1985 930 only works intermittently. The symptoms are as follows:
When I start the engine from a cold condition, the tach always reads zero – no fluctuation. After I drive the car for some time and the engine warms up, the tach will sometimes begin to work. When this happens, the readings seem accurate and stable most of the time, but there have been cases when the tach will surge to a reading 2-3000 rpm too high and then will oscillate. Often, it will abruptly return to zero while I am driving. There are no apparent engine abnormalities that accompany these tach symptoms – actually the engine is very strong and very reliable.
In contrast to the cold start situation, when I start the car on a warm summer day when it has been sitting out in the sun for a period of time, the tach will usually work immediately. Symptoms noted above then make themselves apparent. So, obviously, temperature is factor.
For what its worth, when the car is being smog checked and the shop puts a lead on the coil so that rpm can be recorded by their smog equipment, their readings are steady and apparently correct.
Background:
I bought my 930 new in Germany and had it “Federalized” in Houston.
For the first 5 years, it was serviced at Vasek Polak in Hermosa Beach (LA area).
After 3 years, it developed an erratic tach. After many dollars spent and many expert opinions, I still have the problem, 24 years later. At least 2 other (high-quality) independent Porsche shops in LA spent many hours on this problem to no avail. I am hopeful that this forum can provide an answer.
Diagnosis of the Problem
As I see it, the problem is resident in only 2 possible general areas: 1) the tach itself, and, 2) the electrical system that feeds signals to it.
• The Tach itself:
The first thing every expert did was to remove the tach and send it to North Hollywood speedometer. This was done 4 separate times over a 15 year period. I don’t know what was done to it at the shop, but when it returned, the tach worked but only for a short while. One time, it failed before I even left the parking lot at Vasek Polak. I am sure that the source of the problem is NOT the tach itself.
• The Electrical System:
Within the electrical system, the obvious suspects are: 1) the Alternator, 2) the Capacitive Discharge Unit, 3) the Voltage Regulator, 4) the Coil, 5) the Wiring/Connectors.
- I have had the alternator replaced – no effect on the problem.
- I have had the CD unit replaced – no effect on the problem (luckily I kept the original Porsche unit that should be worth a lot because there was obviously nothing wrong with it!).
- To my knowledge, no one has ever said anything about the coil, so I assume that no one has done anything to it either. Is this a good area to look for a solution?
- Any ideas relating to the Voltage Regulator?
- What about Wiring? To me, the fact that heat in the engine compartment is the only thing that makes the tach work (although intermittently), makes me think that a slightly broken wire or connector could be the problem. When the copper heats up, it expands, closing a gap and making a good (at least better) electrical connections.
Any help that anyone can provide would be greatly appreciated. I just hope that no one ridicules me too much for driving a Porsche 930 for 20+ tears with an inoperative tachometer.
Thanks in advance.

 

I have the same issue as the original poster. All components and voltmeter numbers seem to check out.

What is different with mime is that the tach fluctuation occurs also BEFORE the engine is running, when only the ignition is switched on. Does the voltage regulator do anything before the alternator starts to turn?

 

The thyristor, SCR, in the CDI is firing due to the DC-DC converter voltage rising too high above the SCR breakdown point. Once the engine is running the DC-DC voltage never rises too high before the SCR is fired normally.

Turn the headlights on, increase the electric load, without starting the engine and see if the tach is normal.

 

I'm pretty sure that last post was written in Swahili! Way over my head.

So the upshot is - if the tach stops bouncing with the lights on, I need a new CDI?

 

yes.

 

my car - 1985 - 911 cabby with 94K miles - my tachometer was sporadic for about 6 months bouncing all over the place then stopped completely. I tested the dme for signal and it was fine but thought why not just send them the tach as well and have them test it for a diagnostic fee of $35.00 which can be applied to a repair. They bench tested it and it worked for them but they offered to rebuild known parts that go bad over time in our tachs for $85.00 so I said go ahead. And guess what? When I got the tach back and installed it worked perfectly again! I was very happy! Thank you Palo Alto Speedometer!