Bouncing Tach
 

Off and on my tach would start wigging out. I did a search and it seems that the alternator or voltage reg. would be the most likely culprit. Now, is checking the VR as simple as checking the volts reading on the battery and check for a flucuation there, or is there another way. Also, where is the VR? Is it in the Alt. held in by 2 screws as it is on my Mercedes? It is definetly the wrong time of the year to be laying the car up for maintenance.

Assuming you are talking about the 83SC the regulator bolted to the relay pannel in the engine bay. To check measure the voltage across the battery while you ref the engine. It should go up to somewhere around 14.2 volts and stay there as the revs go up. If it keeps raising much more you found your issue.

These tachs have some oil in the shaft bushings of the tach needle that acts like a mechanical dampener. Some tachs "dried out" and are more bouncy than others. They overshoot. I replaced mine once on the 74. Another reason could be a poor connection on the 14-pin connector in the engine bay. It is on the relay panel.

Good luck,
Ingo

I had the same tach problem and decided to swap the regulator first because that meant less down time than an alt rebuild. My DMM at my battery showed crazy readings between 9v and 17v.

On my '82 the reg was attached to the backside of the alt. I bought a new reg from our host: 911-603-913-01-M262.

It was obvious when I pulled the old reg that it was worn beyond being useful. Since the new regulator install, no more tach bouncing...no more super-bright headlights that raise and lower with my RPMs. Also installed a volt meter at the same time...

Good luck!

The 911 was converted to an internal regulator in 1982. Search here under "early_s_man" for the original tech bulletin, it will guide you.

Dancing Tachomoter
 

Before you do anything, check the ground strap from the battery to the body and make sure the battery post connectors are tight. Then check you voltage output under various scenarios, if there is a constant voltage with minor variations, your regulator is fine. Check the battery as well. This is cheaper and less time consuming than pulling the alternator with a built in regulator.

it could also just be under damped

but the grounds, etc. need to be cleaned every few years anyway

I wonder if anyone has successfully fixed the damping action as a DIY as apposed to sending it to NHS? I would love to take this on myself, but would rather not go in blind.

do you still have points? Or is this electronic dizzy.

Six year old thread. And the VDO tach movement does not get "under damped" the damping is in the inertia of the movement. Take one apart if you don't believe me. The mass of the movement does not change over time, I have 45 year old tachometers that work perfectly.

nonetheless, NHS can increase the dampening (not that I said the dampening was caused by anything at all...)


it is good to revive old threads as they have information in them that is potentially useful (similar to restoring old cars?)