After 15 years and 165,000 miles of faithful service, I figured that it would be a good idea to have a good local outfit rebuild my alternator. Unfortunately, after re-installing it, the battery light won't go out, indicating that it's not charging.
I did a search of the archives and then took some measurements with my cheapie multimeter. Results: 12 volts at battery terminals whether engine is running or not. Ground wire connection on alternator has good continuity. Continuity from big charge wire on alternator is good -- it reads 12V.
Is there anything else I should check before I pull it back off to take to the rebuilder? (I believe that they replaced everything that could be replaced -- brushes, bearings, regulator, etc.)
Thanks in advance for your help!

Also, some Bosch alternators do not ground through their brackets, they must have an intact ground strap. This may not be needed on yours, however, or BC probably would have mentioned it.

>>Is there anything else I should check before I pull it back off to take to the rebuilder? (I believe that they replaced everything that could be replaced -- brushes, bearings, regulator, etc.) <<

Yup, there is.

On the back of the alternator thre is another connection. It is the thin red wire which comes up out of the under-the-engine harness, along with the black wire to the oil pressure sensor.

The battery light being on means that light is getting a ground somewhere, but not the right place. Check that wire connection on the alternator. Do any other warning light remain on? Parking Brake? Brake Failure?

Good Luck,

Bob

>:)

Bob,
Yes, the parking brake and brake failure warning lights which stay on along with the battery light (oil pressure light goes off). What does this indicate?

OK, that nails it.

Here's how it works: IN "Bulb Test" key position, the warning lights that have good bulbs should be lit. Ever wondered what makes them turn off when the engine starts up?

The oil pressure sensor has a switch inside it that grounds the wire when there is no pressure. Engine starts up, pressure builds up, ground opens, light goes out.

The other ones, those that you see staying lit, get their ground side through the alternator brushes. Simply put (there are diodes in the circuit) those lights' ground side all go to that thin red wire.

It is one of the wires in the gray connector on the firewall (all of which are subject to crumbling insulation). From there the red wire runs in the main harness under the intake manifold and under the front of the engine, and connects to the spade lug on the back of the alternator.

So with the engine stopped, the lights ground to that spade lug, into a brush, around the rotor windings (where the current creates a small magnetic field), to the other brush, to the alternator frame, through the ground wire, to the engine block. Closed circuit, lights light up.

Engine starts, alternator starts spinning. That small magnetic field rotates, and creates electricity in the stator. AND, the ground is lost SO the lights go out. It is more complicated, but I cannot easily explain further, and this will do the job.

IF that thin red wire is grounded SOME OTHER WAY, like under the front of the engine where the metal harness holders can chafe into the wiring, THEN the lights stay grounded and lit, and the alternator does not get that small magnetic field and it cannot charge.

You can easily repair the situation. Put in a new red wire from the gray connector to the alternator. Route it along the firewall, down the RF fender and thence to the alternator. Use 16 or 14 gauge, try to use red.

The back of that gray connector can be popped open with care and a small flat tip screwedriver. The wire terminal in the gray connector is unique, clip it off and re-use it by solder/shrink tubing attaching it to your bypass wire.

The alternator end is a standard insulated .250inch female connector attached by either solder/shrink tubing or a crimper.

As long as you are at it, the black wire to the oil pressure sensor can be re-habbed in the same way. Be sure to route the wire and leave slack so the engine can move a bit and you can get in to change the oil filter and motor mounts.

This will do what you need. Given, of course, that I am not there with you looking at the territory. I cannot be responsible for that you do, unfortunately. There is always a possibility that something else is going on that neither of us knows about. Think things through, take things slow.

Good Luck,

Bob

:>)

Thanks, Bob!
Your response was definitely above and beyond the call. Long live Brickboard!

I'm at work now, but will try your new wire recommendation when I get home this afternoon. Yes, I'll take it slow, not difficult at age 56. We have other cars to get me around, but this wagon is still my favorite. While my car is an '87 (a late '87), it's spent its whole life in Calif., and I see no evidence of deteriorating insulation in the wiring harness. The insulation still feels supple, with no cracking.

Last night, I used alligator clips to connect the big and small ring terminals to my multimeter, to see if voltage was being produced by the alternator while running, but got no reading. Was this a valid test?

Also, wouldn't the rebuilder routinely do a little bench test? The shop has been in business for 20 years and was recommended by a car collector friend.

Thanks for your consideration.

-- Phil Loizeaux
'87 240, '90 BMW K75 (3-cyl) MC, '80 Honda XL 185, etc.

Phil, that was a very good explanation by Bob, wasn't it!

The way I have tested for the wire harness shorting to ground, is by disconnecting the idiot lite wire from the back of the Alternator, (RED,and don't let it touch ground), and turn the key back on. If the wire in the harness, ISN'T shorted to ground, the light won't re-light. Easy to troubleshoot the wire this way. Tom

Thanks, Tom. That's such an easy test, I'll do it first.