Pelican Parts Forums - Symptom/Solution – Bouncing/Floating/Off Tachometer and Random Bulb Burnout.

Symptom: It started with the tach just being off by about a grand, it then began to float a little higher about a week later and It wasn’t until I actually saw my tach flopping back and forth (Starting after 2.5 grand and then flipping over to 4 to 5 grand and back when my foot came off the accelerator) that I figured out there was something really, really wrong. I just figured the tach was busted.

On my way home from work I got out on the highway and increased my speed (sub hypersonic) and my front left turn signal/marker bulb burned out. I stopped for gas and the pump fellow informed me I had a light out. So I stopped by the store and got a new one and installed it in the parking lot. I got back out on the highway and it burned out again.

This might sound a little abstract to follow but for some reason whenever you have two things go wrong at the same time on a 911 there is typically a relation that you never would think of.

Solution:

I read in our group about how the Voltage regulator some times had some strange effects (Please read the posts of our more knowledgeable friends before acting). The group was right it’s pretty easy to determine if a bad voltage regulator is causing you some problems. Just put a volt meter on your battery and have a friend step on the gas. If the voltage goes higher then 16-17 volts (regardless of the heavy foot on the pedal), you have a problem. Mine reached 17.4 volts if you tapped to ¼ throttle and higher. Another referenced way would be to turn on your headlights and see if the throttle affects the brightness. In my case the regulator was bad yet the headlights were not increasing in brightness in the least (weird but true, I think that perhaps my bulbs are so old that you just can’t tell).

As I have an 83SC the Voltage Regulator is internal, $60 and 6 hours later (Newbie/Retarded Monkey with big hands) I had fully replaced it. The tach was fixed instantly and the bulbs were just fine from that point on.

Now I did not contemplate the “why” of the bad voltage regulator at the time, but a month later figured it out when the alternator died. The regulator and alternator were the originals and both had 157 thousand miles on them never having been touched before. The bearings and brushes of the alternator were a little sloppy, but being new to this sort of car I had no idea that there is no such thing as a “little loose” when it comes to Porsche. Lessons learned and credit card abused a little by the tow bill to the nearest Porsche dealer who charged me $800 to replace the entire alternator/regulator unit (they don't sell the alternator without a regulator for my year).

As a side note: Unfortunately I was out of town and toolless when the alternator blew. While I was waiting I spoke with one of the younger mechanic’s to figure out what I had done wrong. He explained that not only was the old unit way over due for a problem, but that the voltage regulator dieing is a-symptomatic of an alternator on its last legs.

So, the long and the short is that the tach and the bulbs were symptoms of the regulator and the voltage regulator going out is a typical symptom of the alternator having problems. So while replacing the VR will fix things in the short term it may not be the complete answer.

Have a great one..

Akira28
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1983 911SC Targa
1984 944 -NA