Alternator not charging

[libarra82]

Hi all,

My 1987 944 NA has had charging issues for a while. With the engine running, battery voltage was only around 12.5V. Since the battery cables were in poor condition, I decided to replace them, along with the alternator cable and voltage regulator.

Unfortunately, the problem has gotten worse. I'm now seeing only 10.5V at the battery with the engine running, and the battery warning light does not illuminate on the dash when the ignition is on and the engine is off.

I tested the blue wire going into the cabin just outside the firewall with the ignition on and measured 0V. From what I've read, a failed bulb in the instrument cluster can prevent the alternator from being excited and cause charging issues. I removed the cluster and swapped the battery light bulb with the oil pressure bulb, which I know is good. The battery light still does not come on, while the oil pressure light continues to work, so both bulbs appear to be functioning properly.

I have a new resistor on the way and am hoping that resolves the issue. In the meantime, does anyone have any other troubleshooting suggestions or ideas on what I should check next?

Thanks!

[dr bob]

To prove the alternator itself, you can momentarily jump the battery lead on the alternator to the excitation pin with the engine running. That will excite the alternator to charge, and it should self-excite from there. If this does bring terminal voltage back to normal 13.5-14 Volts, you are chasing an open-circuit problem between the light and resistor in the cluster and the alternator excitation pin. If momentarily jumpering the battery and excitation pins does nothing, it's likely the alternator itself is the problem.

FWIW, most used alternators will carry enough residual magnetism in the armature to self-excite above 3500 RPM or so. Once that charging starts, it will continue to self-excite until you stop the engine.

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The voltage regulator assembly in the Bosch alternators includes the brushes and holders. Two screws hold it in place from the outside, and it can be withdrawn for inspection or replacement with no more disassembly. The brushes ride on a pair of copper slip rings on the armature shaft at the rear. The slip rings suffer when the alternator is asked to do a lot of hard charging, common on cars that are not daily drivers. It is possible to DIY replace those slip rings if they are seriously damaged, but it takes some high-temp high-load soldering equipment well beyond a typical hobbyist's 40W pencil iron. I leave this to a proper rebuilder.

Finding good rebuilt units is getting tougher these days. There was a thread in the last year by a member who's offering rebuild services. I kinda snickered quietly about this, until I started searching for replacement alternators that had been pretty plentiful. Not so much these days, it turned out. Some folks have turned to adapting alternators from more mass-market cars. If you find yourself desperate for a solution, some Nissan and Ford alternators are mentioned. A little searching in the forums here will get you the to the discussions.

[libarra82]

Hi Dr. Bob,

Thanks for the helpful information. I'll give that a try.

I also started looking into replacement alternators in case mine turns out to be bad, and you're right, they're becoming difficult to find. The only reputable option I've found so far is a rebuilt unit from Pelican Parts for about $1,350.

I replaced the voltage regulator at the same time I installed the new wiring, so I'll reinstall the original regulator to rule that out as a possible cause.

Thanks again,

[944er]

The alternator excitation wire can be interrupted by the alarm, if your car is so equipped. I suggest following the instructions on Clark's for jumpering it out.

https://www.clarks-garage.com/shop-manual/elect-23.htm

Note: These instructions are incorrect for 924 and early 944 cars.

[dr bob]

It's certainly worth researching local rebuilders. If you live in a major metro area, there's undoubtedly a decent rebuilder option. At our last home above Los Angeles, I could probably see a few from the driveway. Do resist the lure of some extra-high output rebuilding options though. That seemed like a mandatory feature in that local market for some reason I could never figure out. Anyway, there are undoubtedly places that will still do an original again if you look a bit.

And a casual mention that Greg Brown in Orange County, California was making some slick 'conversions' a few years ago, with a case that's a bolt-up replacement but filled with later 911 guts. It offers some hot-idle charging benefit, has an overrunning-clutch drive, and is much better with high-RPM use without pitching windings. It has square-profile wire in the windings so is more compact and much more mechanically stable than the original alternators for our cars. At the time he started offering them, the extra cost seemed a little steep. But with the dearth of rebuilt options and $1k+ price tags for the ones that are available, Greg's look like a bargain.

----

I've been casually hoarding normal wear parts for my 928 over the decades of ownership. Alternators, water pumps, a new radiator, front spoilers, a starter, fuel pumps, torque tubes and axles, motor and gearbox mounts, a whole now-rebuilt gearbox, plenty more. Any one of those major hoarded items seem to sell at dollars that would easily cover the investment costs of all of them. At that point it should change from 'hoarding' to 'collecting' or 'investing', right?

[rheidtman]

Rebuilt dozens of 115A Bosch units... yeah, no joke as they are ever-increasingly difficult to locate.
You can review here...
https://www.carpokes.com/viewtopic.php?t=4764

Rick
Oak Creek, WI
USAF retired

[libarra82]

It's fixed! Thankfully, the alternator is still good. I'm not exactly sure what the root cause was, but here's what I did. The issue was at the alternator. I unplugged the cables, removed the voltage regulator and plastic bracket, and cleaned the posts, contact surfaces on the back of the alternator, as well as the nuts and washers using DeoxIT. After reinstalling everything, the charging system started working and is now putting out 13.5 volts.

It could have been caused by dirty or corroded connections, or possibly the way I initially installed the voltage regulator. I'm not certain which one fixed it, but it's working properly now.