JHowell37

Like most of us, my alternator had been struggling to keep up with the demands placed on it so last weekend I finally replaced the voltage regulator, cleaned all the leaves and **** out, scrubbed the undercoating tar from the case, and put the alternator back in the car. I went for a drive and the lights seemed a bit brighter and the meter in the cluster was sitting right at 14 volts while driving, and around 13 when idling.

Went for a longer drive the following day and noticed that whenever I got to around 3000 RPM, the needle on the volt meter would start bouncing wildly, and all of the lights on the dash would start flicker as rapidly as the needle bounced.

Actions taken so far:
1. Removed pod and cleaned all contacts on the cluster and checked for broken or damaged wires. Everything looked good.
2. Disassembled the 14 pin plug at the jump post, and surprisingly, there were no broken wires, insulation, etc. Everything looked great. There was minimal corrosion. Considering the history of my car I was rather shocked because it was without a post cover for years, and was a pile of **** when I bought it. That plug has fared worse on much nicer cars.
3. Disassembled and cleaned the jump post. I'm going to replace the terminals on the wires there because they're looking pretty rough and I can't always get good contact on one of them.

Observations:
1. Voltage at the jump post is 12.8 with the engine off/key off. Battery is good.
2. Voltage at the jump post with the engine running at idle is 14.3.
3. Voltage at the jump post with the engine running at idle with headlights, fog lights, radar detector, and radio turned up, is 14.1. I should be so fortunate right?
4. Doing all of the above with the engine at about 2K-2500 RPM and and the voltage shoots up to 15.5. I closed the throttle immediately when the voltage got that high because I didn't want to damage anything.

So in a nutshell, I'm having voltage spikes that are a result from engine speed.

One thing I noticed when I disassembled the alternator was the slip rings appeared to be in poor shape. I didn't give it much thought until I read some commentary from Earl Gilstrom on the Nichols site, regarding voltage spikes that are a result from worn brushes and slip rings.
http://www.nichols.nu/tip587.htm
The brushes on my alternator are new, but as I said, the slip rings are very worn and are "U" shaped, meaning the brushes aren't getting smooth, consistent contact.

The bearings were still good on the alternator, but I'm not so sure about the diodes. I'm thinking my best option is to have it rebuilt or get one that has already been rebuilt.

If I left something out feel free to let me know.

bernard farquart

There are hand held testers, often available for a free test at a local outlet of your favorite national auto parts chain, that can test your system, and give you a definitive answer on your diode condition. You also can see if the brushes "wear in" to the alternator armature at the contact point, if the brushes are new. Is the battery in fully charged condition? your alternator may want to put out max output if there is a very low, or shorted battery.

JHowell37

My concern is how long it will take for the brushes to wear into the slip ring and if I'll keep having voltage spikes until they do wear in.

The battery is good. As I said, 12.8 volts with the key off.

bernard farquart

I would make sure the diodes are good, this can be done with the alternator in the car, most chain auto parts stores offer the service for free. If the diodes are bad, you will still show voltage output (often high) but it will cook the battery, and can make it build pressure and explode, so I would do it sooner, rather than later.

bernard farquart

I actually sat and spun the alternator while holding 600 grit wet/dry on the contact ring when I rebuilt my alternator for my Benz (same basic design) to clean up the contact. I had some grooves, but assumed the soft carbon brushes would conform pretty quick. I can't say how fast yours will, since I didn't see your alternator, I had no voltage spikes though, so you may have another issue.

Alan

The alternator isn't that smart - it has no idea how the battery looks... it just tries to maintain its set voltage regardless of the current load it is reqired to drive. If the battery is poorly charged the current may be higher but the voltage will not (it will be lower if anything as it struggles to make enough power).

Whenever an alternator voltage surges for a noticeable period it is clear that the alternator/regulator is at fault - nothing else sets the voltage.

Instantaneous spikes (can only see with an oscilloscope) can be due to load variations or damaged diodes but you will not see these on the voltmeter or via lights brightening....

This problem is solely the alternator...

Alan

JHowell37

Alternator replaced with rebuilt unit from Arlington Armature in Springfield, VA. Problem solved.

Garth S

Had an (apparently) identical issue with my ex-alternator: the overvoltage and flashing dash lights was due to one of the six field coil leads to the diode plade being intermittently shorted to the case/ground.

It took more that an hour to sort that one out ..

WallyP

Oddest alternator problem that I ever had was on a Jag XJS V-12. After two years, a close friend who ran an Auto Electrical rebuilding business told me that this was a known problem - the alternator was putting out voltage spikes that were screwing up the ignition system.

According to the Factory Workshop Manual, the same thing can happen on a 928.