strange speedometer issue

[quattrorunner]

My speedometer, when I turn off the engine, jumps to about 50 on the gauge and then settles back to the resting posistion. Why is this? Thanks.

[quattrorunner]

It's a 77 911s

[Bob Kontak]

I reviewed a few similar threads. On guy with 10k posts said to check the voltage on your battery to see if the voltage regulator is healthy. Battery when running should be in the 13.5+ volts range and steady - not bouncing around. One or two hundredths of a volt only variation.

I know that the tach can bounce with a defective V reg but not sure of the speedo. It's an easy test though.

Other comments were to check the connections on the back of the speedo. Pull the connectors off and re-attach to re-fit the connection. Check ground connections under the dash and make sure they are tight.

Assuming your speedo works fine when in operation so the sensor pick-up on the side of the trans should be ok.

[quattrorunner]

I'll check today as my tach bounces too.

[kidrock]

sounds like your voltage regulator is about to take a dump. Watch for voltage spikes so you don't fry your system.

[quattrorunner]

That would be odd as it's only 5 months old though. We'll see. My alternator is high amp so maybe its too much.

[dshepp806]

Better ckeck that voltage right away!

Doyle

[quattrorunner]

Voltage reads between 13.45-13.75 depending on rpm. Is that ok? Seems ok to me but I can't remember.

[kidrock]

those are good numbers. The problem with VR's going bad is that it is usually an intermittent problem. A couple of the signs, along with speedo/tach bouncing, is your radio shutting itself off, or headlights and/or dash lighting getting extremely bright. I use a $15 volt meter that plugs into my cigar lighter. Porsche really should've installed one, instead of that clock.

On the other hand, Bob Kontak has a good suggestion. I think I've read a thread or 2 before about similar issues that were solved by pulling the offending head (speedo or tach) and thoroughly checking the connections. Probably a good place to start, because it's free and simple and makes a good elimination process.

[quattrorunner]

My old audis had a code type message using the tach and or speedometer and it looks exactly like this, do our cars do this? Doubt it I guess, they're way simple which is good. Just wondering

[Bob Kontak]

Originally Posted by quattrorunner

Voltage reads between 13.45-13.75 depending on rpm. Is that ok? Seems ok to me but I can't remember.

Pretty sure this indicates the V reg is on it's way out. Voltage at battery will change with different loads on it (lights, air conditioning) but not rpm.

My v reg before replacement would jump half a volt at the battery at a constant rpm so yours is not that bad yet. Also it was in the mid 13's. Now (new reg) without load it is at 14.05 and will move a hundredth or two up or down regardless of rpm. If I idle it for a while the number will drop some - e.g., to 13.9 but the variance when revved stays minimal - a couple of hundredths. Must have to do with the battery getting some life back in it. I have not driven it in a couple of weeks.

When I start to turn stuff on at idle voltage drops remarkably - maybe 2 tenths with headlights - then add AC it drops 8 tenths more, add the fresh air blower it is at 12.5 but the variance stayed pretty steady with and without engine rpm difference. Although, more variance with more stuff on - maybe 5 hundredths but relatively steady.

Another check is to idle the car at night and look at the headlight reflection off the back of the garage wall. If it changes in intensity V reg needs changed.

[quattrorunner]

In fact my lights dim at idle slightly. so it makes sense. But it's new.

[gtc]

How are the points?

[Flat Six]

Definitely sounds like VR issue.

Originally Posted by gtc

How are the points?

gtc: I understand points can affect bouncing tach, but I can't seem to grasp how they'd affect speedo. Am I missing something? Appreciate any light you can shed on this for me . . .

[Bob Kontak]

Originally Posted by Flat Six

I understand points can affect bouncing tach, but I can't seem to grasp how they'd affect speedo.

+1

I did find a post where someone other than gtc mentioned points could cause the speedo to jump but I don't remember ever reading V reg causing speedo issues.

V reg needs replacement, regardless.

My speedo will from time to time die or freeze then bounce back to life. Must be a poor connection somewhere.

[Bob Kontak]

Originally Posted by quattrorunner

In fact my lights dim at idle slightly. so it makes sense. But it's new.

I am not sure if an alternator problem can provide the same symptoms as a v reg getting tired. Bummer is most shops can't test the 911 alternator because it does not have a pulley on it.

[quattrorunner]

Just to recap. My speedometer is fine till I turn off the car, then it jumps to 90 and slowly drops back to 0....predicably.
Tach is bouncy but only when I stomp on it or drop it back quickly, not all the time.

[gtc]

Sorry, i must have spaced out - i was thinking about the tach when I suggested the points.