another bouncing tach thread

ninesixfour · 03-17-2008, 07:19 AM

A few times lately when I'm just driving down the road the tach will bounce around wildly for maybe 20 seconds then go back to normal. I have not noticed any dimming lights or problems with any other gauges or electrical components. I've done TONS of reading on this subject and realize now it could be the: voltage regulator, alternator going bad, poor ground somewhere, loose wire on tach, faulty tach, CDI box, bad battery, etc...
Here's what I've done so far.

Battery terminals are clean with no leaking.
Battery voltage at rest: 12V
Battery voltage at idle: 13.4V
Battery voltage at 3000rpm: 14.3V

Took the coil wire off the distributor and had someone crank the car while I held the wire near metal to test for spark and had a nice blue spark.

Took the tach out of the dash and didn't see any loose wires.

Checked to make sure the CDI box was plugged in tight.

I hear the CDI box buzzing with the key in the ON position but it doesn't seem as loud as it used to be.

Checked the transmission ground strap. One end is perfectly clean, the other is a little dirty but is tight.

Checked all wires going to the starter and they're tight and clean.

A local Porsche mechanic I talked to thinks it's the CDI box. I talked to an alternator repair guy and he says it's not the alternator but that he could test it if I wanted him to. He also says every Porsche alternator he's tested puts out 13.6 volts and no more and therefore thinks I have an aftermarket voltage regulator and that it sounds like it's working fine. He also thinks it's the CDI box.

So just about everyone here thinks it's the VR, the wrenches I've talked to think it's the CDI. What are my options at this point? Just start replacing things in acsending order of cost? Try and rig something up to the cigarette lighter to monitor the voltage as the tach is bouncing? Take out the alternator and have it tested? I could try and swap out a friend's CDI and go for a drive but there's no guarantee the tach would bounce even if it were my CDI since it doesn't act up very often. Is there any way to test the CDI other than the spark test I already did?

Any and all opinions welcome.

thanks

Gunter · Gunter's Garage

I believe in spare parts and suggest you do what I did when I bought my SC:

Get a spare 6-pin Bosch CDI module, test it on your engine and keep the spare on board.

If there is high Mileage on the Alternator, consider a rebuild anyway.
What is the Mileage?

Installation of Volt meter is good; easily done:

(It's the gauge under the dash next to the 10-disc control)

ninesixfour · 03-17-2008, 08:48 AM

Update: I had my battery tested and the tester said REPLACE. It's putting out 443 CCA and is rated for 650. Could it be that simple?

Gunter · Gunter's Garage

Battery life is around 4-6 years on average but can be shorter if you have a bad one.
How old is your battery?
What's the mileage on the Alternator?
A good shop will charge your battery fully, then put a load on it to test and will tell you if it holds up.
Yes, a bad battery can screw up things but I think you may have something else. ?

Paul Crowther · 03-17-2008, 10:14 AM

Been there. Replace the battery and hope that's it. My battery was new, alternator was old (along with regulator). I put in a new regulator and alternator (from Pelican) and it fixed it.

Paul

ninesixfour · 03-17-2008, 04:06 PM

I'm curious why the battery or voltage regulator would only affect the tach and not the speedo, oil gauges, fuel gauge, etc.

ninesixfour · 04-14-2008, 04:25 AM

Just an update. I replaced my battery and cleaned my grounds and this did not fix the problem. So I had the alternator rebuilt, installed a new voltage regulator, and now the problem is gone. Thanks for everyone's help.