Faint alternator light glow: don't get it

[Greg D.]

I fried the alternator on my 72T and it was repaired recently, new "correct" ground cables for the batteries installed, and the twin batteries are new as well. First time around, this rebuilding wasn't extinguishing the bright glow of the alternator light (key *out* of ignition), so much to my mechanic's annoyment, the alternator was sent back again for double checking and further repair... I'm only mentioning this as a baseline and to point out the alternator was REALLY checked the second time, given the original cock up.. So I would not start there...

It now works fine, light is off, BUT... I can now see the alternator light faintly glowing when it's darker, and it's pretty visible in the dark. I've searched and read about this, old car, old wiring, corroded connectors etc... I get it, but what gets me is that it doesn't follow that pattern. It'll stay completely dark and then start glowing a tiny bit.. Then it'll go away after 5 minutes, or sometimes not come on at all... Nothing changes, same electrical draw (nothing on), same RPM, the glow is absolutely not RPM dependant, and adding draw like headlights doesn't chage anything.... So I suspected a bad ground or an old dirty connection at the bulb, but then I took some readings that I also don't get:

- Ignition off: 12.7 V at the battery, indicating a good charge after 2 weeks with this rebuilt alternator, so presumably I'm getting a charge and then some.
- At 1500 RPM (hand throttle): 13.3 V - seems fair
- at 3200 RPM: again hand throttle- no help at home, 14.8V !!!

Is that normal? That 14.8 seems high to me... Is my bad ground initial diagnostic turning into "you got a crappy Voltage regulator" issue ? It's also new, but I guess that could happen, right ? Am I out of specs on a 72 motorola alternator, freshly rebuilt ?
Thanks !

[edmayo]

Greg, 14.8 v is too high. Basically the alternator light is just a light bulb in the circuit between the alternator and the battery. If current flows in one direction such as from battery to alternator when key is on but engine not running the bulb will light. When the engine runs and alt. is charging the alt. sends an opposing voltage to the battery, the two voltages cancel (voltage is electrical pressure) therefore no current flow and the light goes out. Since you have such high charging voltage there is just enough current flow in one direction to glow the bulb. (plus that will overcharge a battery at 14.8) If you have the Motorola regulator with the steel cover you can take that off and reset the point tension to lower the charging rate. Our local rebuilder will do that if we give him the regulator with the alternator.

[304065]

Greg, what make is the VR?

The light goes OUT when the voltage on either side of the bulb holder is balanced. So when you have +12.5V (I kind of doubt your 12.7 reading, sounds like surface charge to me) on the battery and +14V on the trio diodes, by the time the voltage gets back to the bulb holder through the blue wire, it's pretty much equal. When it's equal the lamp goes out, or goes so dim you can't see it.

Now, if you have a VR that is cranking 14.8V at speed, that is too high. Something is telling the VR to pretty much send everything it's got at the field. The extra voltage is enough to start charging the battery-- through the bulb! Hence, it glows.

So what make is your VR? And what make is the battery? And what particular type? One of those damn modern AGM batteries, right?

[304065]

Greg, 14.8 v is too high. Basically the alternator light is just a light bulb in the circuit between the alternator and the battery. If current flows in one direction such as from battery to alternator when key is on but engine not running the bulb will light. When the engine runs and alt. is charging the alt. sends an opposing voltage to the battery, the two voltages cancel (voltage is electrical pressure) therefore no current flow and the light goes out. Since you have such high charging voltage there is just enough current flow in one direction to glow the bulb. (plus that will overcharge a battery at 14.8) If you have the Motorola regulator with the steel cover you can take that off and reset the point tension to lower the charging rate. Our local rebuilder will do that if we give him the regulator with the alternator.

Ed, damn fine explanation! You should be the PCA tech advisor! . . .

[davep]

The "idiot" light is a great diagnostic tool if you learn to read it. Looks like you paid it proper attention. Besides that, it meters the correct current into the field coils to get the alternator started, so you have to be certain to have the correct bulb in that position.

[edmayo]

Ed, damn fine explanation! You should be the PCA tech advisor! . . .

Great idea John, I hear the one they have now is a bum

[Greg D.]

Thank you for your reply and for confirming the voltage seems high. Unfortunately the VR is not a Motorola, it's a metal case Marchal 14V, so I guess a new one is in order... Off to pelican :-)
I saw a few options with vastly different prices, I'll look for one that looks the same for fitment purposes...unless I hear otherwise. Thanks again!


Edit: batteries are correct, recommended per this board and cost a small fortune :-( they are also new, I forget the brand, German, perfect fitment, from lauderdale batteries. That's what started the whole mess when I replaced them and accidentally reversed polarity one one of them and fried the alternator, not my proudest monent. In my defence, who the hell uses red ground cables and black positive? The previous owner that's who. I since replaced the alternator, twice, and installed correct metal strap ground cables on both batteries :-)

[edmayo]

Motorola and Marchal are basically interchangeable, so that metal cover should be removable for point access. There is a later Motorola/Marchal that has a riveted on plastic cover, that's the one I'm not sure about since I've not tried to adjust one of those. Be sure to check your charging voltage even on a new regulator. You have the Sonnenschein batteries, great battery, I've used them for 20 years now, but 14.8 WILL blow their internal vent seal.

[Greg D.]

Thank you all, and Ed in particular ;-) I hope I haven't had time to damage the batteries, I've put very few miles on the car since the new alternator courtesy of a dying starter (might be related to the fried alternator). Maybe 10 miles tops...

Just a few follow up questions:
- seems like I shouldn't try and adjust the Marchal myself, I found some instructions on the web but for a different model. Would you prefer (a pro) adjusting the old unit vs. putting a new one in ? Originality of vital electrical components hidden behind a panel is no big worry of mine. I will keep the old Marchal regardless for a rainy day...
- I take it a new one does not need any adjustment, correct... What max output should I expect ? 14.2V or so ?
- I don't plan to listen to the radio much if at all - should I remove the radio attenuation thingie to remove another potential faulty item? I read sometimes those add another layer of problems...

Thanks again !

[porschegal]

you can go two types...the bosch and the hella.. bosh i think we sell for 99.99 and the hella ones are 27.85 ... the bosch ones are more plug and play. hellas do the same job but looks different

2 cents
Marge