Strange one:
When the car is running and lights are on, they flicker.
When I disconnect the battery while running, the flickering stops.
If the alternator was bad, the car would stop running, especially with the lights on.

Thanks for any insights.
--
-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

The automobile is not a lawn mower! Yes, you can do that with many small engines for lawn mowers or snow blowers. But for automobiles (or inboard boats -- see below) you never disconnect the battery with a running engine, because that makes the alternator produce voltage spikes far in excess of what the alternator's diodes (not to mention other systems in the car) can tolerate.

One of the crucial functions of the battery is to absorb the voltage spikes potentially generated by the alternator -- think of the battery as a sponge. Also, the voltage of the battery serves to regulate the alternator's output through a feedback.

Boaters with inboards (i.e., marinized automobile engines), as opposed to outboard motors, have to use a battery switch for safety as well as to select between the house battery and starting battery (if the boat is large enough for two battery banks). But one of the first things they're taught is not to move the switch* while the engine is running -- being without a working alternator on a power boat in the middle of the ocean does not make for a good day on the water.

[ * there are exceptions: at least one manufacturer offers a new battery switch that makes the other battery bank connected before the first bank is disconnected so that the alternator is never isolated from at least one battery bank -- thus protecting the alternator from damage.]

Ken, allow me for a moment to don your professorial cap and explain some about our Bosch alternators.

It is true the practice of disconnecting the battery to "test" the alternator is to be generally discouraged, for the reasons you gave. This applies especially to older mechanical and some early electronic voltage regulator systems, but is still a worthwhile rule to follow with our 2 series cars equipped with Bosch.

The reason the practice is still relevant to our cars is an alternator could have a regulator fault which would apply full field through a shorted pass transistor. In this rather rare event, the battery could be the only thing holding the voltage to a marginally safe value. Of course, in this rare event, one's face over the battery is by far the greater danger compared with the overvoltage destruction of engine management components.

Excepting this shorted regulator event, this alternator does not manufacture "voltage spikes" needing absorption by the battery, nor does it react to fast changes in load current with an overshoot which is what I think you are referring to. The little red wire is not a remote sense for the regulator, but a means of providing pre-excitation current to the rotor winding. The voltage regulation is entirely internal to the EL module. The reference for 14V is built in and compensated for temperature.

I'm saying your rule, to not disconnect the battery on a running vehicle, is worth heeding, but for the benefit of having an easy to remember general rule, and not so much to criticize the judicious exception to it in our cars.


--
Art Benstein near Baltimore

Q: How do you pick out a dead battery from a pile of good ones?
A: It's got no spark!

Ken, I hear what you're saying.
However, if the battery is supposed to sponge-up the voltage spikes,
how do explain the flickering lights when it's connected, and no flickering
when I disconnect the battery?
--
-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

I'd like to hear the end of the story. I'd start by checking the voltage drop across the alternator's bushing-bypass ground wire and move on to an examination of the contacts and brushes associated with the EL regulator/brush pack. Battery float voltage might be interesting. It is not a commonly reported symptom you've described.

On the other hand, if you're not that curious, well just swap the alt and the battery and see what develops. My experience tells me folks focus too sharply on part replacement and miss the importance of the wiring connections between them.
--
Art Benstein near Baltimore

"The greatest thing about the internet, is that you can quote something and totally make up the source." - George Washington.

Art, I replaced the alternator, and everything is good now.
I know I should have gone just for the VG, but this was simplest solution.
--
-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

What trichard said. Disconnecting the battery while running can fry the voltage regulator. Good thing yours is already bad.

Easy fix. Get the Bosch, cause the generic Chinese ones suck.
--
~graymatta '93 240, '92 940, (departed) '85 740 and a world of air-cooled VWs in my past

What trichard said. Disconnecting the battery while running can fry the voltage regulator. Good thing yours is already bad.

Easy fix. Get the Bosch, cause the generic Chinese ones suck.
--
~graymatta '93 240, '92 940, (departed) '85 740 and a world of air-cooled VWs in my past

you have a bad voltage regulator.
never disconnect the battery with the engine running

Interesting...
Isn't the voltage regulator built into the alternator?
--
-Cool Volvo- 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

how is it you state.......past owner of....... 1989 240 5 Spd w/ 310K, 1989 240 5 Spd w/ 231k. Past proud owner of 1966 122, 1968 144, 1970 145, 1972 144, 1980 245, 1982 240 4 Spd OD

and you do not know the voltage regulator is a seperable part of the alternator