Wiring problem from Alternator

[sagacis]

After a search, I found lots of electrical problems reported, but none were quite like my problem. Here's the sitch (long story with summary at end):

81 911SC daily driver, basically stock. A year and a half ago, I had charging problems. Replaced original the voltage regulator with a solid state one from a local vendor, but no dice. Had the alternator rebuilt by a reputable local shop. Put every thing back together, still using the new VR. Everything worked fine for a little over a year.

Suddenly one night driving home, the gauges started bouncing. They settled down after a few miles, but my heart rate didn't. At home, I tested what I could with a multimeter, but didn't find a problem.

Two days later (after driving many miles without a problem), the speedo and tach both dropped to 0 and the headlights started dimming. Rolled into an Autozone parking lot and found the battery dead. It was ~10 years old, so I got a new one. All good.

Two days later, I checked again, and the charging voltage was too low. Pulled the alternator and took back to shop for bench test, where it tested beautifully. Replaced the VR again.

I plugged a multimeter into the light socket for the ashtray so I could read voltage while driving to make sure everything is good. The voltage jumped around more than I felt comfortable with, sitting at 16-ish when driving on the highway. Bought a new VR and plugged it in.

Three days later, noticed it jumping around. "Shoulda bought from Pelican," I told myself and ordered the OEM (same price!).

Installed a real voltage meter and put in the new VR. Everything was fine. For two weeks, while running, the voltage meter reported 13.8v all the time.

Sunday, when I started, the voltage stayed at 12.4v for a while, but after two blocks, it jumped up to 13.8. I wiggled all electrical connections when I got back home.

Monday morning, voltage stayed at 12.4v for about two miles, then jumped up. "Hmm...." Monday evening, voltage stayed about 12.4v for 7 miles, then jumped up.

Tuesday morning (yesterday), voltage stayed at 12.4v for 10 miles. Turned around and came home as the battery was draining.

I fiddled around with wiring for an hour and didn't find anything wrong. Cleaned all connections that I could reach. Wiggled and re-seated many connections. Suddenly, it worked when I started once, so I drove to work.

Last night when leaving work, the voltage stayed low. I fiddled in a parking lot to no avail. When to an auto parts store to ask for more ideas. They load tested the alternator in the car and reported it good, but no regulation. I played with battery connection and VR connection. The next time I started, the voltage was good so I came home.

This morning, I am back at 12.4v when running. I added a second engine ground, but nothing. Resistance between the engine case and the body is at the bottom of my meter's ability.

AAARGH! Summary:

12.4v when running when I start cold, but sometimes 13.8v, either after running for a while or fiddling with connections.

I believe it must be the connection to the VR, but I don't know how to verify this without removing the alternator and replacing the field wires. Is this the next step?

Thanks for any help. I'm going crazy with this.

[304065]

Lots of different potential failure modes here:

1) SC's use the dreaded Paris-Rhone alternator which is known to be problematic. Could be a loose wire in the rotor or stator, could be a weak main diode or one of the "trio." Not much you can do about it without replacing the alternator, preferably with a Bosch or even one of the AASE Austrialian Bosch clones that put out 90 amps. . .

2) Never use anything other than an OEM voltage regulator-- the problem is that the "set-point" or the output reference voltage may be different than OEM. There's no way of knowing, for example, what the set-point of the HUCO you get from the FLAPS is outside of a bench test- so it could be putting out a voltage that's lower than what it takes to charge your battery.

3) Batteries are three to five year replacement items. A 10-year old one probably had half the plates working and was putting a big load on the alternator, which tends to make things overheat. If the diodes overheat the solder that connects them to the stator can melt, see problem #1.

4) Never, never, never drive the car with a dead battery. The alternator is designed to bring the battery from 97% charge to 100% after one or two starts. A battery showing 12.4V is 75% charged. So the alternator has to work overtime to try to bring the battery back to 12.6V-- which is asking a lot of a Bosch, asking too much of a Paris-Rhone. Always charge the battery up to full before you drive unless you are in a genuine emergency.

Recommendation: Get one of the high-output Bosch alternators from Pelican. Big big dollars but it will fix your problems.

[RoninLB]

Originally posted by sagacis


Replaced original the voltage regulator with a solid state one from a local vendor, but no dice.

those auto solid state replacements are a problem waiting to happen. Marine grade is top notch if you can get a proper replacement otherwise go with mechanical regulator.

[rick-l]

Your current problem (12.4 volts while running) is the alternator is not charging the battery.

Could be the regulator
Could be the slip rings or field winding in the alternator

What does the light do when this is happening?

[sagacis]

What does the light do when this is happening?

Nothing. It stays off.

Could be a loose wire in the rotor or stator, could be a weak main diode or one of the "trio."

Bench testing and inspection of the alternator showed no such problem. Also, when it is working, everything is fine, so I'm not getting a weak charge or fluctuating charge.

Never, never, never drive the car with a dead battery.

I only did that once, to get back to my garage. I was willing to replace the battery when home if necessary. When measured at the battery, it measures 12.6v, but when running, I'm seeing 12.4v at the dash.

The on/off nature of the problem indicates wiring failure, but I'm not sure where to check. I don't want to spend a bunch of time replacing a wiring harness unless I know that will fix the problem. If I knew, confidently, where the problem is, I am entirely ready to fix it. I don't want to throw a new alternator on it if the one I have is working perfectly in every test.

"Rule out what works independently , and what remains, however unlikely, must the the problem" is how I am now attacking this.

[rick-l]

The light staying off means the diode trio in the alternator is providing all the field current required. This would also include the case where no current is required as in an open winding or bad regulator.

Your alternator seems to work and then doesn't like it was turned on/off with a switch. The only way I know to do this is by removing the current from the field winding. You are on the right track by looking at the regulator field windings.

My only personal experience with this is when I put a new regulator in my 88. The slip rings had deep grooves worn in them(might have been the original problem). The new regulator brushes didn't line up with the old grooves. Lasted about a year before I was having similar issues.

If anyone knows (because I have never witnessed this) when they bench test an alternator how do the apply the field current? Is it just a straight 12 volts with a switch or do you select the field current?

Never use the alternator to charge the battery!!! Just kidding, this has been discussed at length and the only argument for this is the alternator system is poorly designed.

[Early_S_Man]

Does the warning lamp come on every time you turn on the ignition switch?

If the alternator warning lamp is OFF and the alternator is turning, then one of the following is true ... 1) brushes are worn down too far, 2) braided wire connection to brush/brush holder is broken, loose, or intermittent, or 3) Blue #61/D+ wire has broken, corroded, or loose connection between warning lamp holder and brush holder Faston tab.

My suspicion is worn brushes or loose wire at individual brush.

[sagacis]

Today I removed my alternator again and checked all the wiring. I could not detect a problem with the multimeter, so I started looking at the alternator.

Here's what I found. I took off the brushes and examined the connection there. You can see in these pictures how it looks.

The brush assembly:


Disassembled. Notice the solder connecting the tab to the brushes:


So here's my new question: How is the tab supposed to be connected. Just pressed against it? There are matching holes, should I insert a pop-rivet? Melt solder? I don't know what the right answer is, but the connection here is intermittent. Now I understand why it worked after warming up (melted the solder a little and the connection improved). Here's a better picture of the tab going through the cover.

[lonewolf]

check to make sure you have a good ground between the regulator panel and the ground stud behind accumulator
there is a braided ground strap there .
if that is faulty nothing works right.
911s run best at 14-14.5 volts not 13.8

[sagacis]

Here's an update.

I soldered the tab on again. I hope it doesn't melt or break off again. Once I put her back together, I took a spin around the block (and down the road, etc.). Everything was fine for the four miles I drove, but we are having ice and freezing rain now, so I will know tomorrow if it stays fixed.

Anyway, I still don't know how the DF tab is supposed to remain in contact with one of the brushes.

The other brush is soldered to ground, which looks original. Thanks for all the help. I still am suspicious of the grounds, both at the tranny and behind the accumulator.

[PAT4]

What's the GOOD voltage when the engine is running up to 2.000 rpm ?

I read 14.4 V or may be under, around 13.8 V as written upper by sagacis ?

[sagacis]

As long as it's below 15, you are probably fine. If the regulator goes out, the voltage rises dramatically. I installed a voltage meter to keep an eye on it. Since that repair, I've had nary a problem.

[PAT4]

Could a low voltage (around 13.6 - 13.7 V) when engine running an alarm about an altenator failure ?

see here : Voltage regulation