Jumping RPM needle?

[Innov8]

My SC has a very jumpy RPM indicator when the car is warm. Sometimes, the needle will float all the way to the right and bury itself over 7,000 or just go dead, only to bounce back to an accurate read moments later.

I have triede to recreate this but there doesn't seem to be any consistent factors that are related (that I know of).

Advise? The distributor looks OK.

[randywebb]

tuneup and if it still happens start cleaning elec. connectors...

I assume the SC's didn't have points - they also cause this when dirty or worn...

for paranoia value...maybe your very expensive CDI box is about to fail...

[Innov8]

I purchased the car from CA about 2 months ago and the PO told me that the car wanted to run above 3,000 rpm and when I drove it, it did run smoother and have more pull above that...but at the 2,800 to 3,000 mark the engine studders slightly. I thought that the PO had advanced the timing by rotating the distributor a bit (I recall seeing this done by high school hot rodders about 25 years ago).

Engine starts fine, and runs great otherwise.

Does this seem like a tune-up is in order? Probably need to find that local P-mech sooner than I thought. Thanks for your thoughts.

[unclebilly]

VOLTAGE REGULATOR - mine was doing all sorts of weird stuff and then I changed out the voltage reg and it's fine now. The voltage reg was still putting out OK but it was making the electrics noisy.

The voltage reg is a sub $20 part.

[lateapex911]

not to mention checking the grounds!

[SC-targa]

voltage regulator.

[bigchillcar]

your car may or may not have an external voltage regulator..if it does, then this is indeed an easy swap if it's the problem. if you know how to use a voltmeter i'd check to see if your voltage while the car is running is varying by much..do you see your headlights changing brightness at night while experiencing this symptom? if so, i'd be inclined to agree with the guess that it's the vr. old, worn out ignition points definitely cause this..but normally along with loss of power and backfiring..don't remember if your car uses points..
ryan

[randywebb]

yes it could also be the VR -- try a voltmeter - if it fluctuates low then you have a problem, maybe the problem

if it doesn't then there could still be a problem that the meter may be too slow to catch - this is less likely tho

[Nitrometano]

May be the needle is on crack!!

I saw a Porsche SC with the same problem. The CDI died some weeks after that. I replace the CDI with MSD and the bouncing tach needle gone.

[Innov8]

I will swap the VR as soon as I can get one. I don't own or know how to use a volt meter and I figure that would run the same as the cost for the part. I do experience the headlights flaring with the engine RPMs, especially the dash lights brightening when the engine is rev'd...even by just a few rpms. I figured that is was the battery but the vr makes more sense.

My only problem is... I don't now where to look for it!? I definately need a shop manual...which one does everyone recommend...?

And as always thank you to everyone for their insight. This forum makes owning the 911 that much better.
--Patrick

[rotorhead]

Before you swap theVR measure the voltage across the battery terminals with the engine running. It should be around 13-14v it it is much higher then swap the VR (Mine was 20-22v!)

[Rufblackbird]

according to Pelican your VR is internal (for a '82 SC)

http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_electr_pg7.htm#item31

[randywebb]

know anybody you can borrow a meter from??

[Sameer]

I had the same problem. Changed my battery and all ok. I never thought it would be my battery.

[Innov8]

The plot thickens. Today, I picked up a voltmeter and checked the battery. As I was driving home, with the car at the middle of the temp gauge, I noticed that the tech needle would bounce with every tick of the directional as I sat at the stop light. This made me sure that the VR was failing.

When I got home, I let the car sit for a short time before I checked the battery. It read 12.4 while idling at the normal temp (top of the first white bar). So I flipped on the directional and the tech started to bounce up again with every click of the directional. I went back to the voltmeter and the reading would of course change with the clicking directional...from 12.4 to 12.6. To little to have an effect on the tech?

I let car idle up to the middle of the temp gauge again and checked that battrery... it had increased some...up to 16.4 and then bouncing to 16.8 with the directional. This also happened while running the wipers and w/o. The same with the aftermarket radio and headlights so I don't think it is a bad accessory.

The fuses all look good and firm so am I looking at a faining VR or what? Is the volt flucuation of .4 volts enough to distrube the tech needle? The warmer the car the higher the volts...does this indicate the VR is on its way out?

Lastly, thanks for the link to the VR on the Pelican site. I figure the internal VR is on the alternator so I will need to pull the whole package out...right?

Any more advice3 on what to check next would be great as I am not looking to pull the alternator if possible. Thanks.
Patrick

[rotorhead]

I'm guessing your VR is bad.... as its job it to limit the output voltage below 14.8v

The first reading of 12.4 was due to the battery recharging after starting the vehicle. But as the battery came to full charge the system voltage rose to 16.4v. (If you had an ammeter on the battery you would have seen the voltage increase as the current decreased) It would be interesting to see how heigh he voltage would eventually have got. I understand high system voltages can fry a DME.

Changing out the VR was my first job on my vehicle (Identical symptoms) here is a link to a pelican tech article on removing the alternator

Tech Article

[Innov8]

Thanks Steve, I will study the article while I wait for the new VR to arrive.

[randywebb]

Yup

tach needle bounces with every tick of the directional at idle ---> almost certainly not ignition; very likely VR or alt. or batt.

the other piece of info - the high voltage - points right at the VR

[Innov8]

Update.
Finally I replaced the VR. Thanks PP...the install was easy 2 hours complete.

After I checked the voltmeter and it was reading 14.2 and 14.4 at a warm idle. After about 20 minute drive and pushy the tach to 5,500 rpm, the tach needle started its dance again...jumping up to 6,000 RPM and then going dead...then to an accurate reading. All very inconsistent.

When the needle is at the accurate read, I tried the directionals again and it cuased the same small bump in the needle with each tick. When I try the hazard flasher, I don't get the same bump.

When I installed the new VR, the copper collar on the shaft, where the VR brushes rub was pretty wore but I think that the brushes where making good contact. If the next items are the battery or alternator, how do I check each one? The battery is about 2 years old and seems strong. I have no idea on how to evaluate the alternator.

What do you think I should do next? Thank you.

[bigchillcar]

check the connections at both sides of your coil (technically a transformer and not a coil)..faulty connections there can cause an erratic tach. also, does your '82 have ignition points? dirty or worn points will cause it in the worst way..
ryan