bouncy tach

Paul Crowther · 09-12-2005, 04:39 PM

I've seen some discussion about this before, somewhere.

Yesterday my electic tach took some bounces from it's original reading of 2500 rpm up to 3500-5000, with a bunch of wild swings. After several events like this over about 10 minutes, it was over. It did it briefly about 15 minutes later. Last year the same thing happened about the same way.

What's up?

Paul

leverpuller · 09-12-2005, 08:04 PM

my tach did this I have a 82 sc what I found was a bad alternator. replaced it and that was it.

sballard · 09-13-2005, 05:24 AM

I am not sure that the Tachs are the same, but my recently acquired '72 occasionally did the same thing. When I pulled the guage out, I found that the ground wire was not connected! Once grounded, it works perfect. Amazing that it worked at all really.

Steve

'72 911T

Porsche_monkey · 09-13-2005, 05:54 AM

I posted about this on my 74. Some people suggested the weak battery/alternator link, but I personally don't understand how that could cause it.

I don't have a weak battery, my charging voltage is right, but I still get the 'floating' occassionally. I'd really like to know what causes it.

MichiganMat · MichiganMat's Garage

Check the middle fuse in the 3-fuse block by the engine. Make sure the contacts are clean and the fuse is not corroded.

RayM · 09-13-2005, 09:28 AM

My brother in law has a 74 and described this same problem to his wrench and was told to clean his points and this cleared up the problem.

Early_S_Man · 09-13-2005, 09:33 AM

SCs don't have points!!!

Check your system Voltage during a run ... if it is only doing the bounce every year or so ... might be hard to track down the problem!

arcsine · 09-13-2005, 09:35 AM

Mine was the voltage regulator/alternator. Went from bouncy erratic to rock solid.

MichiganMat · MichiganMat's Garage

My tach was bouncing for a few weeks time. Its done it before, so I figured it would be no big deal...

Cruising down the highway, 70mph, dusk on a Sunday night. Engine shuts off completely (uh oh). I pull over, pop the boot.

- Fuel pump is humming.
- I've got lights
- She cranks, stutters, but won't fire.
- Bad CDI? Maybe its time for that MSD I've been eye-balling...

I check the front fuses, all good. I check the rear fuses. A bad connection in that middle fuse had roasted the contact on it so badly that it crumbled when I pulled it out.

New fuse, fired right up, and away I went. Tach hasn't bounced a bit since.

scottb · 09-13-2005, 09:52 AM

Conventional wisdom would point to a bad alternator that's in the process of failing. I'd start my investigation there.

Porsche_monkey · 09-13-2005, 10:06 AM

Quote:

Originally posted by scottb
Conventional wisdom would point to a bad alternator that's in the process of failing. I'd start my investigation there.

because .....

rotorhead · 09-13-2005, 01:14 PM

My experience is that it is probably the Voltage Regulator. It is easy to check just measure the battery voltage across the battery terminals with the engine running. If it is much above 13.5v then it is a bad VR.

Give it a few minutes after starting to recharge the battery and then measure the voltage

75Carrera · 75Carrera's Garage

My '86 3.2 tach will simply fall at times to zero and later bounce back with no obvious event like a bump or thump. I've been told a tach that's been stored face down can develop a sticking problem due to the weight of the dial needle.I'll try the various "fixes" mentioned so far.
What does a tach overhaul cost typically? I've not actually shopped for a new one but someting tells me they'd be spendy.

Paul Crowther · 09-13-2005, 04:53 PM

It's amazing that so many different causes can supposedly create the same symptom.....hmmmm.

Is the only common clue the fact that the tech is electronic?

Paul

CBR900RR · 09-13-2005, 05:05 PM

I had the same thing this past spring. Tach would bounce at higher revs, on rough pavement at speed, and on a short gravel road at slow speed. The same week after a 400km run, go out to start the car to bring it inside for the nite, no start. Dead battery !! Turned out to be a loose grounding strap on the tranny. Due to this intermittent grounding/ungrounding, any vibration was causing a poor contact.
Maybe?

on-ramp · 09-13-2005, 05:15 PM

same thing with my 83sc, but always in the summer, never the winter.

75Carrera · 75Carrera's Garage

I've noticed that too. It seems to be affected by heat and/or moisture... So you guys all believe the problem is somewhere other than in the tach? Battery, alternator, wiring, fuzes, weather?