Alternator no charge - bench test regulator?
 

(US '84) My alternator suddenly stopped charging on my way home this evening. For reference it is a bosch 90A, 10 months old with 5k miles on it. Recently I had an issue with poor/no charge at idle with the blower and headlights on, otherwise it was working fine.

Here is what I found so far:
-alt belt was tight - no indication of slippage
- charge lamp works when key is on
- ~12v at battery (was driven home without alt so most likely why it is not 12.6)
- battery voltage @ alternator b+ connection
- ~10.5v at blue wire when disconnected with key on and all pod connections made (as per Wally's alternator circuit diagnostic method)
- removed alternator for inspection, found that the plastic guard over one of the voltage regulators connectors is melted/distorted but the wire is still intact, definitely not normal - alternator overheat? (Paris-Rhone Regulator)

So here's what I am wondering...
- is 10.5V at blue wire close enough to battery voltage for the alt to charge properly?
- since I have the voltage regulator removed, can I test it separately?

If possible I would prefer to buy a regulator rather than $130 on an entire alt.
Also this is my DD so I cannot afford too much down-time.

usually the brushes are worn but with a new alternator this should not happen.
However if the rebuild used a less than best Bosch regulator than this may be the issue.
I suggest to replace the regulator with a bosch unit,
also remove the engine ground cable on the right side of the block to chassis ,
if it has oil on it wash it in brake cleaner then clean the connecting points,
do the same for the battery ground,
and the 14 pin connector above the hot post as well as the hot post connections

Good suggestions. The current regulator is a Paris-Rhone according to the faded label. There is plenty of life left in the brushes. Slip rings look good.

Can I test the regulator to make sure it is bad? Where can I get a new bosch one (Pelican Parts has Valeo brand $65)

In the meantime I will clean engine & battery grounds + 14 pin connector as suggested. The hot post was cleaned and ox-gard applied fairly recently.

whoa if you have a PR regulator then you have a PR alternator,
please post a picture of the part you have,
front and back

>> Charge lamp works when key is on.

Does the lamp go out once the engine is running? Excitation comes through an internal diode once the alternator starts charging on its own. So if the light is out there's adequate voltage being generated to at least supply the field current.



>> 10.5 V at Blue wire disconnected

Depending on your meter, should be battery voltage when disconnected, key on. Modern meters should show full battery voltage. This is the excitation source for initial alternator light-off on start-up. Relates to question above too.


Stan brings up a good point about the brushes. With the regulator removed, brushes should be obviously long enough to reach the slip rings on the armature.

Cheapo "rebuilds" often just include cleaning and a new regulator/brush set/holder assy. The slip rings on the armature are a bit of work to replace, so cheapo rebuilders often try to skip this step if they are anything less than trashed. You can see the slip rings through the opening for the VR. If they are significantly worn or burned, you will miss out on alternator capacity. Perversely, a worn set of rings and bruhes is treated to higher excitation current, burning these parts even more quickly.

Here's some pics. On a second inspection there was an odd pattern on one of the slip rings (pictured).

Lamp does NOT go out with engine running - all warning lights stay lit, voltmeter reads 11 volts (usually reads 14 with engine running).

My meter is a fairly modern DVOM. AFAIK it is accurate. Will 10.5v be to low for the alt to charge properly?

Replacing any part on a Paris-Rhone alternator is a waste of time and money. On mine, after replacing the regulator, some diodes blew, after replacing them a stator winding burnt out, all in the space of a couple of years. Save yourself a lot of trouble and replace it with a different brand one now. Smiffy

According to the receipt the alternator has a lifetime warranty, is it worth taking it back?

the slip rings look good you could try swapping out the PR regulator,
OTOH you should also test the diodes before you swap in a new regulator.
If you put in a new regulator with a blown diode the alt will only make 13,4v not enough to keep the battery charged
Also if you have a warranty with the new alternator see if you can exchange it for another one.
Then make sure all of the other elex connections are good or the new part may also fail

You can eliminate the low excitation voltage as a possible problem by momentarily adding battery voltage directly to the field terminal with engine running. If it suddenly starts charging you can go after the problem with the wiring/etc im the car. If no change then the alternator is the problem.

Ah, thanks for the tip, I will definitely give that a try tonight. By field terminal you mean where the blue wire connects?

Do I need to remove the diodes to test them?

Yes on where the blue wire connects.

After the on-car test with full field, waste no more time diagnosing a lifetime-warranty part. Your disassembly if discovered will void the warranty. Take it back to the seller and let thgem test/replace if needed.

Alternator troubleshooting links:

http://www.nichols.nu/tip692.htm

http://www.nichols.nu/cat02.htm

Ok I just did the full-field test. With a multimeter reading 11.7v with the engine running, I fed power to the field connector on the alternator, voltage increased to 11.9v and rpms dropped slightly as if there was a load on the engine. Also did a voltage drop test across engine ground cable, battery ground strap, and alternator B+ to engine bay hot post - no voltage drop on either of the three. So is it safe to say the alternator is dead?

get a new alternaotr and check the hot post connection for the exciter circut before you run the new alt no need to fry it from a shorted wire