'79 911 Electrical mystery
Thread Starter
6th Gear
Joined: Jan 2010
Posts: 6
Likes: 0
From: Miami, FL
'79 911 Electrical mystery
Every once in a while as I'm driving, my fasten seat belt light comes one and then the tachometer goes haywire. Then it goes away just as quickly as it came on and everything is back to normal. Can someone help?
Rennlist Member
Joined: Oct 2004
Posts: 20,607
Likes: 16
From: Bakersfield, CA, for now...
Do you get a smell of rotten eggs in the car?
Has your alternator been updated to the model that uses an internal voltage regulator? If it has, the shaft that protrudes from the center of the cooling fan will have a receiver end for an allen wrench, and the pulley nut will be 24mm wrench size (vs 22mm for the original alternator).
I'm thinking that you probably have a failing alt/reg combo...
Has your alternator been updated to the model that uses an internal voltage regulator? If it has, the shaft that protrudes from the center of the cooling fan will have a receiver end for an allen wrench, and the pulley nut will be 24mm wrench size (vs 22mm for the original alternator).
I'm thinking that you probably have a failing alt/reg combo...
Race Car
Joined: Nov 2004
Posts: 3,993
Likes: 3
From: Blue Ridge Mountains NC.
Welcome jdepazos. You came to the right place for answers. In case you didn't know Peter Z is one of the soothsayers amongst the ranks in the Porsche community. You just received advice that would have cost an hour of labor at your local shop.
My first instinct upon reading your post was a possible ignition switch fouling up. Do you have a big lump of keys on your keyring?
Also check your battery terminals for good connection and the ground cable from the battery negative post to the body terminal just behind the front bumper in the trunk area. All should be clean and solid.
If you get a chance, post some pictures of your car. AND...join our community with a membership($18)-it will pay for itself in a week, if not already.
My first instinct upon reading your post was a possible ignition switch fouling up. Do you have a big lump of keys on your keyring?
Also check your battery terminals for good connection and the ground cable from the battery negative post to the body terminal just behind the front bumper in the trunk area. All should be clean and solid.
If you get a chance, post some pictures of your car. AND...join our community with a membership($18)-it will pay for itself in a week, if not already.
Thread Starter
6th Gear
Joined: Jan 2010
Posts: 6
Likes: 0
From: Miami, FL
Thanks for the advice. No rotten egg smell, but I have had some problems in the past every once in a while when I would get home from work, stopped at the mail box, turned the car off, and then the car would not start, would not even crank. I would come back in about 1/2 hour and it would start up. I just recently disconnected a Prestige alarm system, which I've read has had this problem and I thouth that this might take care of the tachometer problem as well, but not so. So maybe ignition switch sounds like a possible solution.
Addict
Joined: Jan 2007
Posts: 37,754
Likes: 15
From: Alta Loma Alone
Thanks for the advice. No rotten egg smell, but I have had some problems in the past every once in a while when I would get home from work, stopped at the mail box, turned the car off, and then the car would not start, would not even crank. I would come back in about 1/2 hour and it would start up. I just recently disconnected a Prestige alarm system, which I've read has had this problem and I thouth that this might take care of the tachometer problem as well, but not so. So maybe ignition switch sounds like a possible solution.
https://rennlist.com/forums/911-foru...hlight=starter
Drifting
Joined: Sep 2001
Posts: 2,467
Likes: 16
From: redmond wa
Those are two of the primary symptoms of a too high voltage level. That could result from:
A) The wiring resistance between the alternator output and the battery terminals. Loose or corroded negative side body-ground connections, Starter "bridge"/splice point connection, or battery post connections.
B) Failing VR.
C) Failing alternator
In your case I'm voting that you need to remove the battery post terminals, clean and burnish both posts and connectors, reinstall, tighten, and apply a good coating of axle grease.
A) The wiring resistance between the alternator output and the battery terminals. Loose or corroded negative side body-ground connections, Starter "bridge"/splice point connection, or battery post connections.
B) Failing VR.
C) Failing alternator
In your case I'm voting that you need to remove the battery post terminals, clean and burnish both posts and connectors, reinstall, tighten, and apply a good coating of axle grease.
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Race Car
Joined: May 2003
Posts: 4,248
Likes: 103
From: SoCal
I agree with Ed, i.e. Too many darts being 'thrown'.
Actually, a common problem with the Paris-Rhone (Valeo) alternators,
i.e. overcharging the result of a intermittent regulator. Great location
Porsche chose for an electronic regulator, close to where all the engine
heat is, right? And just to save one wire over the early 911SC with the
external regulator. Probably amounted to about a $.50 cost savings in
production.
Actually, a common problem with the Paris-Rhone (Valeo) alternators,
i.e. overcharging the result of a intermittent regulator. Great location
Porsche chose for an electronic regulator, close to where all the engine
heat is, right? And just to save one wire over the early 911SC with the
external regulator. Probably amounted to about a $.50 cost savings in
production.
Pro
Joined: May 2005
Posts: 535
Likes: 0
From: Derwood, MD
Intermittent problems can be real fun to solve and sometimes they just go away and you never know what solved it.
How to start.
1. Forget one time failures, look for repeatable problems.
2. Get a schematic or current flow diagram from a Service Manual (try to find an agreeable mechanic who will make you a copy, offer a 6 pack in payment.)
3. Don't start swapping stuff, it is an expensive way to trouble shoot.
4. Check all grounds and clean them. If you take something apart, put it back before you take something else apart.
5. Buy a good multimeter. A fast digital one. Harbor Fright has one for $10, wait a week and it will be $3.99.
6. Get a friend who understands electricity.
7. There isn't much in common between the tach and the seat belt light except grounds.
8. Learn to isolate and test.
9, Be patient
10. Buy a Bentley.
11. Remember these cars are 31 years old full of wire designed to last 5 years.
12. Take notes as you go. Memory is the second thing to go. I forget what is first.
13. You haven't provided enough information for anybody to help you, at this stage.
14. Make sure you opted for 'towing' on your insurance policy!
How to start.
1. Forget one time failures, look for repeatable problems.
2. Get a schematic or current flow diagram from a Service Manual (try to find an agreeable mechanic who will make you a copy, offer a 6 pack in payment.)
3. Don't start swapping stuff, it is an expensive way to trouble shoot.
4. Check all grounds and clean them. If you take something apart, put it back before you take something else apart.
5. Buy a good multimeter. A fast digital one. Harbor Fright has one for $10, wait a week and it will be $3.99.
6. Get a friend who understands electricity.
7. There isn't much in common between the tach and the seat belt light except grounds.
8. Learn to isolate and test.
9, Be patient
10. Buy a Bentley.
11. Remember these cars are 31 years old full of wire designed to last 5 years.
12. Take notes as you go. Memory is the second thing to go. I forget what is first.
13. You haven't provided enough information for anybody to help you, at this stage.
14. Make sure you opted for 'towing' on your insurance policy!
Race Car
Joined: May 2003
Posts: 4,248
Likes: 103
From: SoCal
"Forget one time failures, look for repeatable problems."
Until your engine or electrical system fails because you ignored a warning, right?
"There isn't much in common between the tach and the seat belt light except grounds."
Just the common power source, i.e. The battery and alternator!
Until your engine or electrical system fails because you ignored a warning, right?
"There isn't much in common between the tach and the seat belt light except grounds."
Just the common power source, i.e. The battery and alternator!