Bouncing tach with engine off?
 

Been reading through the numerous tach threads but none that I have found thus far have a symptom that I have as well. My tach bounces and twitches even when the engine is off but key in the Run position. No weird lights coming on, on the dash, turning on all power accessories does not help. So I do not believe it is the voltage regulator at fault. The car is an 84, and not sure if the alternator is original or has been rebuilt/replaced. I will be hooking up a meter later to confirm if I can see any voltage spikes, but it is the bouncing twitching with the engine off that has me confused. From what I can tell, the tach in the 84 receives its signal directly from the DME. Bad gauge? What wires should I be checking for? The wires behind the gauge are guddentight.

Thanks as always!

Anytime the OVERCHARGED battery is connected to the CDI there is a possibility that the elevated battery voltage will cause the SCR to "self-fire".

I thought the 3.2's do not have the CDI type ignition. I apologize for my misunderstanding if I'm wrong, just learning the ignition components of this engine now.

Also, this problem started after the car has been sitting for about a month and happened right away when started up. So I guess it still can be the voltage regulator? Is there an easy way to identify an alternator as an external or internally regulated one while still mounted on the motor? Want to prepare by ordering a new external or buying a rebuilt one to have ready when I pull it. Thanks again.

Best get some voltage measurements, ASAP....go from there....

My '89's alt has the internal V-reg......those who don't have the internal reg will have the regulator back over on the left side of the engine compartment ( I think that's it...)

If she's dropping unreg'd voltage, best to get it changed out before the DME gets really pissed.

BEST!

Doyle

Looking at getting a new alternator. Our host lists different options for alternators, but then lists just a regulator that the description fits my year. Could I be lucky and just order this? At $80 dollars, much cheaper then a whole new alternator.

Voltage Regulator (Marchal, on Alternator)
Brand: Valeo
Applications:
911/911 Turbo (1982-89) 1
This part may also fit in the following categories:
911SC / Turbo (1978-1983) (from 82) 1
911 Carrera / Turbo (1984-1986) 1
911 Carrera / Turbo (1987-1989) 1
928 (1978-1982) ( 82)

Look for bad connections to the tachometer, dirty connections at the fuse box in the engine compartment and in the front fuse box. Also check the ignition switch for proper function. Don't waste money buying one part after another without a firm diagnosis. Do some testing first. Since the engine is not on when this happens it should be easier to track down since the alternator/voltage regulator are not operating until the engine starts and are therefore unlikely to be the problem.

I did recently clean/sand all fuse and relay connections while chasing down a heater controller relay issue. Going to pull the tach and clean connections there. Not sure how to test the ignition for proper functionality, it seems to do everything it is supposed to. Don't have any issues with the engine cutting out or anything like that. I will be borrowing a proper rms meter to see if I can catch any spikes from the alternator.

Any confirmation if I could use that "internal" voltage regulator to replace mine instead of purchasing a rebuilt/new alternator?

Adam - In King of Prussia, there is an Auto Elctric place on Henderson road. I used them to rebuild my alternator and it was only like $100 to get done. I would give them a try.

Auto Electric and Hydraulic Hose
120 Hansen Access Road
King of Prussia, PA
(610) 265-2072

He also does a killer job on rebuilding fuel lines. Check yours next time you are looking at the top of your engine. If they look old, have them rebuilt ASAP.

I put an analog meter on the car and took readings at the rear fuse box. 12volts with car off, 14 volts with the car running. When the Tach started bouncing around and even when it was pegged into redline, the meter still read 14volts without jumping or anything. Maybe I will take it to a specialist to double check, but its seems that maybe the tach might need to be rebuilt?

If you have a friend with a similar year car you can borrow a tach from him, switch it into place for yours and see if the symptoms are the same or put your tach in his and see if it still misbehaves. Is there a Porsche shop near you that would let you do this? That would confirm your diagnosis. It sounds to me like you are on the right track.

Bouncing tach with key off = fried alternator diodes

Doesn't bounce with the key off, but it bounces/twitches when the engine is off, and the key in on/run.

With the key on/engine off and tach bouncing switch on as much 12 volt supply load as you can, lights blowers, rear window heater, etc.

Tach stops bouncing then fix the CDI.

I thie issue is with the ignition switch. You can turn everything on in the car, work the power windows, but the tach still is all over the place. It is not really bouncing per say but either pegged, or stays at random places. Tonight though, I turned the car off but the engine stayed running. So I think the ignition switch is faulty. Maybe the tach is a symptom of this? Does that make sense?

Also, 3.2 motronic engines don't have a CDI. Right?

Any ideas if the tach issue could be related to faulty ignition switch? If so, would this symptom be a part of the mechanical (engine not stopping with key out) or the electrical part of switch (tach issue). Thanks!

You are having some interesting issues. How about removing the tach and seeing if the engine not stopping fault repeats. I think that having the engine not turning off would be top of my fix lists, did you stall the engine and disconnect the battery to kill it? Or did turning the key on off on off sort it?

Yeah, well the engine not stopping is a new issue unfortunately. I was able to get the engine off by cycling the key a couple times and cringing while the starter engaged a couple times.

Before the engine not turning off. I was testing a DIY heater relay that I built for the autoheat controller int he back. Plugged it in and the heater fans functioned like normal, but I pulled the key and the radio went off, but the dash warning lights were on and I was still able to operate the headlights. odd. I started it up and shut it off and everything went off like normal so I took it out for a quick drive. Ran fine, minus that tach doing it's weird thing, got back, removed key and engine was still running.

On a side note, I hooked up the meter again to battery and rear fuse block and confirmed readings were normal while the tach was not functioning correctly. Still a little above 12v with engine off, little over 14 with engine running. Needle on the meter stayed pretty much steady, no spiking that I saw.

Another reason I think the tach issue maybe related to the ignition switch. The shift light still seems like it works normally even though the rpms on the gauge are way off. I believe the shift light gets its signal/power directly from the DME right?