Hi All,
I have a 1979 911 SC, The tach is occaisionally erratic, no pattern to it really but it operates normally most of the time then swings wildly up and down for a few minutes then returns to normal. I have also noticed recently that the headlights and dash are starting to cycle dim then bright.
I think the first thing to check would be the various ground connections then check alternator output.
I just bought a Bentley service manual but I can't find any info on the tach in there except for its location and I know where it is.
Anyone out there know of a fix for this?
thanks
Billski

 

I think you are in the right track for with the grounds.
I had a few "gremlins" that have not appeared for awhile.

I replaced the front ground from the battery with a standard NAPA
or equal #4 (?) ground wire. Polish the post area and the attachement as well as the individual "rings" with a small brass brush, scotch-brite or whatever, re-assemble. Then look under the dash and take apart any brown connections that are screwed into the sheet metal and do the same.

Then get under the back end of the car, jack and secure safely with stands,
and replace the chassis ground with the above mentioned wire 8-10" will do.
On the chassis I polished the heck out of it and left the stock strap in place because were it attached to the trans its a bear to get at the bolt, I just picked another nut on the trans and cleaned it up and attached.

Just think , the last time these were touched was probably 20 years ago!

Good luck,
Rich

 

I think you will find that 9 times out of 10 this is an alternator issue. Very common.

 

Thanks for both replies.
It looks like my original thoughts were a good place to start but it is helpful to have confirmation from others.
I will post again when I have found the fault for this.

Cheers Billski
1979 911 SC

 

If you have one of those junk Paris-Rhone internal reg. alternators, then
it's probably overcharging causing the tach to become erratic.

Your car came with an external reg. alt. which was the best Porsche alt.
It's unfortunate that it has been replaced by the Paris-Rhone alt. Also,
if it hasn't been replaced, then you may have a gray external reg. which
may cause overcharging.

Have Fun
Loren
'88 3.2

 

I cleaned up as many different grounds as I could find it made no difference at all to the erratic tach. I have ordered a new regulator from the Porscheshop which should arrive tommorrow and I will install it Sat. I'll let you know if it fixes the problem.
Billski
1979 911 SC

 

Make sure that the regulator is an OEM type and not some junk aftermarket one.
It's not the most fun thing to R/R the alternator or to have to replace carpets
around the battery because of battery acid.

Have Fun
Loren
'99 3.2

 

It should be real Porsche reg or OEM at least at £70 plus vat = approx. $150. I will keep you updated when I get it installed.
Billski
1979 911 SC

 

OOOOuuuchhhh!!! At £70 it should be gold plated, OEM items are about £50 tops!! The Porscheshop strikes again!!

 

Maybe a bit late now, however I had the exact same problem on my '86 Carrera. Pulled the alternator had it rebuilt including new regulator for $110.00 US and that included a 1 year warranty. Installed the rebuilt and all those problems are now gone!

Good luck!

 

I installed the new regulator it was a Bosch item but it made no difference to my symptoms.
I guess the next step is to rebuild the alternator. I will update you once I have arranged that after Christmas.
Till then Merry Christmas All
Billski
1979 911 SC