Jumping tach and death

mquinn · 05-21-2003, 07:16 AM

I am quite confused! A few days ago I was driving and noticed the tach start jumping and it started getting worse then the car started missing, then it backfired and stalled out. I landed up towing it home.

It idles fine up to about 2k then it starts jumping about 200 rpm then at 3k it jumps up and down about 2000 rpm then at 4k it bounces from 0 to 8k.

So far I have pulled every plug and cleaned and checked gap. Put a new distributor cap and rotor. Changed the coil to a know good one. Checked the battery (took it to advance auto and they load checked it). Put a volt meter on the battery when it was running (and having the jumping tach issue) and got 13.89 - 14.00 volts, the voltage never really deviated.

I am running the pertronix 2 pickup rather than the standard points and condenser. I have a 1977 911S Targa. I have had this setup for 6-8 mo. w/o any problems. I did have a track weekend 3 days prior to this problem arising.

I have also replaced the pertronix 2 unit (with same) and no change. Pertronix suggested I bipass everything by hooking the + side of the coil to a 12v source and the red wire from the pertronix to the + and black to the - thus bipasing everything (CDI and tach) and it did not help.

I am banging my head - please make the pain subside.

M.

Amac · Amac's Garage

M,
Almost every time I've seen this happen, it's been
a battery or alt. problem. When the voltage gets low
your tach will bounce, the motor will miss, sputter and
die, and your radio will lose the signal.
If you know the battery and alt. are good, it could only
be a wiring problem. Save yourself time and stress, run
it by your local dealership. They can find it fast, went through
this about 3 months ago myself.
Good luck!!

Superman · 05-21-2003, 08:28 AM

Unless you drive a Porsche that is still under the factory warranty (and maybe even if it is), NEVER take it to a dealership. I know there must be exceptions (Laura, for instance), but they are so infrequent as to be insignificant. Yesterday we heard of an instance where, very probably, the dealer welded (poorly) an engine mount, rather than replace it. Without telling the owner. I stay away from dealerships. They are (generally) the worst alternative out there.

mquinn · 05-21-2003, 08:32 AM

Any suggestions for GOOD electrical Porsche mechanics in the Charlotte area? I have to trailer it because it is not drivable.

rstoll · 05-21-2003, 08:38 AM

Could this be a bad ground strap??

Groesbeck Hurricane · 05-21-2003, 08:52 AM

M.,

Take the battery out and go to Sears or somewhere else and ask them to test the battery. This is much cheaper than taking the car to a mechanic.

Is your charge indicator staying above or below the normal range? It could be as simple as replacing the bushings in the alternator (if memory serves me the bushings in your model are replaceable).

I would also check the ground strap (underside near front of tranny) and probably take it off and clean the connections.

Make sure to disconnect your battery before doing anything with the electrical system outside of simple testing.

Superman · 05-21-2003, 08:57 AM

On my SC, the ignition systems is pretty uncomplicated. If it has a good distributor, and a good CD box and a good coil, good plug wires and good spark plugs, and if it is getting sufficient voltage, then it will work fine. In other words, everything related to ignition system management is in the engine compartment. The only thing that comes in there which can impact performance is voltage. Given good components, and proper voltage, when the engine spins, the plugs fire.

I'd be too curious to take it to someone else. I'd ground a wire against an intake runner and run the engine at various speeds to see if the spark changes. If it gets faint and yellow at higher engine speeds, then the spark voltage is changing. If it stays blue and snappy, then I'd look at why ignition timing is not advancing. One thing's for sure, if you have the right voltage of spark at the right time, and you also have a proper fuel/air mixture, then the engine will run like a top. If it doesn't, then one of those things is not happening.

Hope this helps.

briankeithsmith · 05-21-2003, 09:00 AM

MQuinn:

Try Jim at EuroWerks - 704.882.6466
or Terry at The 911 Shop - 704.596.1302

A friend fo mine just had this problem this weekend at the track. I think he determined it was an alternator related problem.

Brian

pookie · 05-21-2003, 09:05 AM

If you need to take it to a mechanic there is a gentlemen named Jim (I cannot think of his last name) who runs a shop called Eurowerks. It is located just off of 485 on Independence. He has done work for me in the past and is very fairly priced. His number is 704-882-6466 or he has a website at eurowerks.com.
Hope it helps
Ryan

AZ1977911 · 05-21-2003, 09:40 AM

Presuming you have points, check to make sure they have not stuck together.

AZ1977911 · 05-21-2003, 10:01 AM

OOPS apologies, I see you have the pertronics.

mquinn · 05-21-2003, 10:48 AM

I have removed the battery and had it tested (for initail voltage, load, post, drain, and all). Is there any way that I can test the alternator w/o messing something up? I can get my hands on a amp meter - but I saw a site that showed puting a 500 ohm resistor in line to the battery and puting the meter on the ground strap - way wierd (or is it true?).

briankeithsmith · 05-21-2003, 10:55 AM

Eurowerk's web site is actually www.eurowerks.us

Brian

Superman · 05-21-2003, 11:41 AM

You can certainly remove the alternator and take it to a test/repair shop. They will have a machine that spins the alternator for testing.

mquinn · 05-21-2003, 11:48 AM

Isn't removing the alternator on a 911 rather involved? Have to see if there is a alternator "wizard" page out here that describe it....

Superman · 05-21-2003, 11:52 AM

No, it's not that tough. Remove the belt. Remove the housing strap. The tough part is reaching around to remove the approximately eight nuts that hold it onto the housing.

RoninLB · 05-21-2003, 12:48 PM

it's rare, but a bad tach can disable the ignition system.. if all else doesn't help, try disconnecting the tach...........Ron

RoninLB · 05-21-2003, 01:00 PM

Quote:

Originally posted by rstoll

Could this be a bad ground strap??

I have run jumpers directly from the battery to the hot and cold wires of the CD box to eliminate those problems when the ignition crashed..it doesn't hurt to run a permanent extra ground, 12ga. wire, from the chassis ground above the box/under the lid to the box case and plug ground...............Ron

Graham Archer · 05-21-2003, 01:07 PM

A loose ground connection to the battery gives the symptoms you describe.

Early_S_Man · 05-21-2003, 01:07 PM

M. ...

I think you should write/phone a formal complaint to Pertronix ... complaining about the IDIOT who told you to bypass the CDI system and hook 12 VOLTS TO THE COIL!!!!!! The IDIOT obviously knows nothing about ther basic operating principles of a CDI system! The 1:100 turns ratio of the coil would only produce about 1200 Volts when 12 Volts is fed to the primary ... certainly pretty useless for troubleshooting an ignition system!

Hopefully you did not overheat your coil by running approximately 25 Amps through the primary winding while attempting this stupid trick from the Pertronix idiot. New Bosch CDI coils are rather expensive.

Take out the Pertronix crap ... put in a set of points and see if your tach & miss problem goes away. Otherwise, a Voltage regulator problem is highly likely!