I've posted this issue on Volvo Forums too, and am totally stumped. 1990 240, new battery, alternator, battery ground cable. Battery was replaced two weeks ago because it was 7 years old, and didn't seem to hold a charge. New battery was continually dead after a lengthy drive, so the alternator was replaced. I realize I likely only needed brushes/voltage regulator, but since I'd already bought a load of other new parts, I thought I'd splurge. After the new alternator went in, the battery light on the instrument cluster came on...even though though the car was off. Didn't happen with old parts. Light goes off when car is running. Also, even though the car runs fine and can be driven, the battery slowly discharges, and only shows 11.6-ish volts when engine is running. Shiny new alternator, which tested ok at the store I bought it from, isn't charging the battery. I already tried a continuity tester on each of the fuses...nothing. Tried three different alternators, and that battery light won't go out for any of them. I am totally flummoxed by this. Anyone have any ideas???

if it's not a harness issue, and i doubt it is, then it has to the alternator. the diodes are shorted. very common. good luck, chuck.

one question sages- if the alternator diodes are shorted,shot or dead wouldnt a charging check on the alternator at the parts store show no charge? wouldnt your own check with a voltmeter connected to the big power wire at the back of the alternator and to the ground(alternator body) also show no charge or less than 13.5? thanks tons oldduke

not always. seen many an alt work ok with a shorted diode. alot of times the only complaint is the idiot light on when key is off. good luck, chuck.

Install a Voltmeter in the upper left pod next to radio. Definitive solution to all your charging questions as the meter doesnt lie like the auroparts store will as they have to stand behind there defective rebuilt part.

If it isn't a short, your symptom points directly at a failed rectifier diode.

Best,

Cameron
Rose City

if your battery light in ON with the engine off and key out waste no time and remove your alternator off its bracket and slit the large plastic wire insulation which carries the wires under the timing sprocket up to the alternator and look closely for melted wires and shorts.

this is a great area for fires to start from short circuits.

the battery light on with no key in is a hallmark of this.

A few months ago, I had the same thing (several posts on this forum).

My wife's '93's battery was not keeping a good charge overnight, and charging while running was poor. I chalked it up to just an aged battery, but a new battery didn't help. But shortly after that (battery replacement), I noticed the charging idiot light was glowing when I shut off the engine, and I saw smoke curling up from the engine compartment -- lifting the hood revealed an actual flame(!), candle-sized, on the cross-member below the oil sender and the wiring behind the alternator.

After disconnecting the battery and letting things cool, my "autopsy" revealed the wiring harness's three wires (B+ cable, voltage field/sensor, and oil sender wires) all melted/fused into a mangled mess -- it also destroyed the alternator's ground wire as collateral damage. The actual fire was from oil escaping from the oil sender, fried because it was grounding the alternator's output. A real mess.

"and only shows 11.6-ish volts when engine is running." It is not charging, should be 13.5-14.0 volts with the engine running.

Are you sure you have a good ground on the case of the alternator.
Dan

how about hidden corrosion in the battery cables. I'd do a continuity check on the wires and maybe remove the ground wire, play with it and see if it's funky inside and not being a stable ground
Same with the exciter wire (for lack of a better name),and the power in from the alternator.

I'm just digging up this old post because this situation just happened to me and this was the only post that I could find that dealt with the issue. So I am making everyone aware of this obscure topic by calling attention to this old post.

My mechanic's fix was an easy one on this issue - it was just a loose wire connection from the group of wires hooking up to the back of the alternator. He apparently just tightened the nut holding the connection to the alternator. I did not have any rotted or shorted wires as described in the previous posts. Vehicle had been running fine at the time yet the battery light just suddenly started coming on upon the vehicle being turned off. The battery was relatively new, a new rebuilt alternator was just put in during the spring, the typical fuse connection problem up on the wheel well behind the battery had been fixed a year or so ago - so I was stumped as to the cause until I somehow came across this post.

Anyway, an FYI to all in case it happens to someone else. This was a strange one.....

That seems like a wiring and/or grounding problem to me. Perhaps double check how the alternator is hooked up and wire brush or sand all the connectors.
Other than that I am stumped as well.