JSD

After driving for awhile, the tach gets erratic on my 1972 911T (CD Ignition). It will be reading fine, like at 3000RPM, and then the needle will start jumping up and down the scale with no relation to what the engine is actually doing. Sometimes, the needle will actually drop down and rest against the peg.

Diagnosis? Mechanical, electrical ?

Thanks

Louis

I had the same problem on my '87 930. It would be fine and then for no ryme or reason it would go erratic. I replaced the alternator and the problem was solved. While I was removing the alternator I discovered that the ground wire on the cases was somewhat loose. I tightened it, but decided to replace the alternator anyway. Check with Pelicanparts.com. They have a technical section that addresses this issue.You can check the alternator output with a volt meter to tell you if that is your problem.
Good luck.

Matt Smith

Check the black wire with a purple line on it that runs from the side of the distributor body into the rear loom. It may be loose on the spade connector/distributor end where it picks up its pulses.

JSD

Thanks guys, I'll check it out in the AM.
Anybody else have a different opinion?

type930

Jsd-

The direction towards the charging system is or shall i say was correct for my problem which had the same effect on tach. BUT it was the voltage regulator which i belive is external on your car not mounted behing the alternator like my 1988 turbo. So do your self a favor and see what kinda voltage your getting with a volt meter on battery. Test at rest and also idle and say 3,000 rpm. Bounce tach will happen in both cases overcharge and no charge. I have a gut felling you have an over charge issue and your volt meter will show you higher the 14 volts and 3k rpm which means shot voltage regulator. email me if i can help any more.

JSD

Batteries, both of them, (72 911T) tested fine at 13.7 - .8 volts each with engine off and at idle.

I think wiring is the culprit, although I wonder why the tach seems to work fine until after 15-20 minutes of driving. Then it starts jumping around. Alternator light never comes on.

Checking voltage regulator and alt. tomorrow.

Thanks,

kireneslo

Agree. Found connector loose on 72 911-T.