Electrical Problem

jackvan · 05-05-2003, 01:32 PM

My 1989 911 refuses to start after driving for any period of time. I have taken it to an import repair shop that replaced the battery and said that it had a loose connection. Sure enough the car started but after driving for some time, the headlights became dim and would not restart. The car never stalls but appears to be running off the battery. Also, all electrical components work even when it refuses to start. Any help or suggestions would be appreciated.

Rot 911 · 05-05-2003, 01:37 PM

You first need to check to see if your alternator is charging the battery before you can really check anything else. The one thing you can do is make sure both connections on the battery are clean and secure, the connections on the starter and the transmission ground strap.

bigchillcar · bigchillcar's Garage

Ditto to what KurtV said.

Do you know if your alternator has an internal or the external voltage regulator? This is supposed to regulate at 13.5-14.5, if memory serves me. If you have the external, they're not too expensive to replace and quite easy to remove, should be just forward of the ignition box with two wires. The internal one will require the infamous alternator removal procedure. This job takes me about 4 hours, but a good shop would do it in two. On the other hand, if its the alternator you suspect, if you can get it out it can be bench checked by an alternator repair shop...and maybe repaired.

What is happening during all of this with your generator light? Is it coming on? It should be coming on to alert you that the system voltage is dropping and may be running on battery only.
Ryan

jackvan · 05-05-2003, 01:50 PM

I believe that it is an internal voltage regulator. The generator light does not come on. The only time I am able to tell that the voltage is dropping is when the headlights dim. Otherwise it runs perfectly. That is until I try and start it again.

john walker's workshop · 05-05-2003, 01:57 PM

the gen light needs to come on with the key on, but engine not running. does it?

jackvan · 05-05-2003, 04:09 PM

The generator light does not come on with the key on. Also, I am able to get it started by jump starting it and it will stay running but will not restart.

bigchillcar · bigchillcar's Garage

That light not coming on is a red flag. Something is wrong with the charging system. The other symptoms bare that out as well. The properly operating generator's output will extinguish the light...it not coming on ever is a problem. I'm at work now and if I weren't so busy I could troubleshoot a liitle deeper. Perhaps John will weigh back in...
Ryan

jackvan · 05-06-2003, 06:29 AM

I double-checked the generator light issue when I got home last night and the light does come on with the key on and the engine not running.

304065 · 05-06-2003, 07:04 AM

This electrical stuff is pretty simple once you get into it. These are the steps you should follow:

1. Get yourself a voltmeter. This can be anything from a $20 radio shack special to a $5000 Fluke, it doesn't matter.

2. Put the car on a battery charger overnight. Using the voltmeter, verify that you have just over 12v at the battery terminals.

3. Turn the ignition key on and verify that the low voltage light comes on. The way your charging system works, current flows from the battery through the fuse box to the bulb, then back through a blue wire (called the D+/61 wire) to the 14-pin connector in the left rear of the engine compartment, then into the engine wire harness, then to the back of the alternator. Inside the alternator it flows into the field, generating a magnetic field that is required to get the alternator to "excite" and begin producing current. If current's not flowing through this circuit, the alternator will not begin to charge, and the light will remain on.

4. Start the car. At very low revs, the charging light may remain on. When you rev it up past about 1500, however the light should go out. If it does not, the problem is in the regulator or the alternator diodes. If it does go out, you know the alternator is putting out voltage. With the engine running, carefully measure the voltage at the battery terminals. You should see 14v. Any higher or lower could be a bad regulator, bad alternator diodes, or bad connections.

5. The connections are easiest to diagnose. Start with the battery terminals, then check the ground strap from the trans to the body. If you have a good low voltage light, then the charging circuit is probably ok, but if not, check the connections on the back of the oil pressure gauge, and the 14-pin connector. Tons of material in the archives on this, check any from Early S Man for a wealth of info on how the charging system works. Warren also has some good recommendations on how to improve alternator output at low rpms.

Good luck!