Alternator Rebuild Question

sundaypunch · 04-02-2004, 07:45 AM

This should have gone in the "admit your stupid mistakes" thread from some time ago. After working on my '69S I forgot to secure the battery. During some spirited driving it slid and came disconnected. After that the alternator light stay on. I verified it was fried as I have under 13V with the car running. My battery is OK (12.5V with the car off). I have the alternator our and need to replace or rebuild it. It is a Motorola unit which I'm sure isn't original (see the pic).

I'm curious whether I need to get new diodes as well as brushes and whether a normal rebuild includes this? Also, am I better off trying to find a correct Bosch unit?

304065 · 04-02-2004, 08:08 AM

Hmm, so the failure mode is that with no battery to take up the load in the charging circuit, the diodes go to maxiumum output and overheat. So I would guess that you need new diodes. You can test for this, of course. I would guess that it's the diodes that are the problem, not necessarily the brushes, because the current flowing through the brushes, even that coming off a single "trio" diode (does that make sense? The early Moto/Marchals had only ONE diode in the output stage that fed current back into the rotor, instead of three, but I still think of it as the "trio") isn't that great.

Loren is your expert on this, however. If it were me, I'd find a good shop and have the whole thing rebuilt.

Get that battery strapped down tight: At my last Club Race of the season at Summit Point, I came around turn 9 and the engine quit. I put the clutch in, pulled off-line and coasted into the pits. Expecting to see a puddle of oil under the motor, I jumped out in despair. Noticing that the starter wouldn't crank, I opened the trunklid and there was my battery, lying on its side, the positive post inches from the side of my fuel cell! It had come loose from the high g's and the positive cable came off, killing the engine (but not, fortunately, the Alternator). I put a new strap in and was able to start the race but I learned an important lesson insofar as I may have become extra crispy.

sundaypunch · 04-02-2004, 08:12 AM

My experience was pretty much the same as yours. Like many of us with early cars I'm missing the correct battery hold-down and have rigged something up to keep it in place.

I did just take a minute to look in my manual and see that the Motorola is probably original equipment. I will take it to a local shop to see what they say.

Thanks

Jdub · 04-02-2004, 08:34 AM

Diodes, brushes, and a turndown of the commutator if there is enough material to do so. Let an auto electric shop you trust handle this. Cost me about $125.00 to have this done at Acme Auto Electric in Seattle, Aurora Avenue. John Walker recommended them and I'd second that.

John

sundaypunch · 04-02-2004, 11:54 AM

Lesson learned: check the simple stuff before you assume the worst.

I took it in to a local alternator shop and it tested out fine (50+ amps). I guess the battery lead coming off didn't fry it. I figured it must be the voltage regulator and when I went to clean the terminals I noticed the little 6mm nut on the D- terminal was loose enough for the connector to spin around. After tightening it everything works perfectly.

Looks like another one for the "admit your stupid mistakes" thread. I can't complain since I did get the fan cleaned up nicely.

As a pointer, the John Walker recommended method for removing the alternator from the housing (another thread) is a beautiful thing

Place it between a couple of 4"x4" blocks of wood spaced far enough apart to clear the alternator but hit the housing. Hit the housing against the wood a couple of times and the alternator pops right out.

RickM · RickM's Garage

Bosch does not list an alternator for a 69S.