I've had this nice '91 740 Turbo for almost 4 years now. We bought it cheap with the hope that my wife could get at least a year out of it for her new job. I've replaced the heater core on it, put in a new e brake cable, changed the oil regularly and it's been a champ.

A few months ago it started to have battery problems. I changed the battery out and it's been fine until the last few days. Now the battery is draining and we need to jump it in the mornings. I checked the voltage on the battery:

- 12.4 V
- 12.8 V at 2krpm+ with lights on

I'm thinking this might indicate a bad alternator and I should probably just take it to an auto electronic place and let them do their thing. Anything else I should check?

 

there's a pretty good chance its just the alternator brushes, thats an easy fix.

Voltage Regulator Brush Pack

 

Thanks. I'll give it a shot and see what comes of it.

 

btw, 12.4 is a more-than-half drained battery, and 12.8 is way too low for a proper charge. the battery should be more like 12.6-12.8 when its fully charged and resting (take it off the charger for an hour or more to lose the residual 'surface' charge), and nearly 14V when the alternator is running right with a fully charged battery.

the other thing that can mess with your alternator is the "D+" circuit which goes through the dashboard 'alternator' warning light. make SURE that light comes on when you turn the ignition on, and goes out when you start the car. if it doesn't come on, then replace that bulb (remove the drivers side kick panel, and the knee brace (two pieces, my 92 needs a Torx wrench) and the warning bulbs are accessible from behind.

 

Thanks pierce, you are like an encyclopedia of Volvo fixing. No light on the dash. Brushes are a bit worn, but not to the extent in the pictures at the bottom of the first page of this post on another forum: Check/replace your voltage regulator. - e30tech.com Forums

And... clumsy me, I dropped one of the retaining screws down into the abyss (actually both of them plus a tool or two, but I got everything but that one screw back so far)

 

wait. if the light doesn't light up when you turn on the ignition before you start the car, then REPLACE THAT LIGHTBULB. that light is an integral part of the circuit, if there's no current on D+, the alternator can't bootstrap itself.

 

Well, I took the alternator out, had it checked at the local auto electric and it's fine as far as they can tell. Put it back in, turned the key on and got an alternator light that turns off after the car starts. Double checked and I'm still getting a lowish voltage at the battery while the car is running revved up to 2k rpm. I'm not sure if I just didn't look hard enough at the idiot lights (failed the idiot test) or if that D+ connector wasn't making a good contact before. Still seems to be some kind of problem, though. I know we had a problem before where if my son put his USB charging device in the cigarette lighter it would drain the battery over night. I wonder if there's more shorts in the dash somewhere (probably caused by me when I replaced the heater core a couple years back). The radio also used to lose it's preset stations every so often.

 

if the + wiring is good between the alternator B+ to the starter to the battery, then I'd be looking at the grounds. when the engine is running, see if there's a voltage between the alternator's ground lug, and the batteries minus terminal. I'd expect this to be no more than 0.1V

 

Voltage between the alternator ground and battery negative is 0.03V. Between alternator + and - is 14V. I should check between alternator + and battery + - back in a minute with an edit. Back. And with possible good news maybe. While checking voltages, I noticed a bit of an arc at the positive terminal of the alternator. The lead to it has some bare wire that I think is shorting out on the alternator body. Insulating that a bit got me over 13V at the battery. Even with the insulation there's a 0.3V potential difference between alternator + and battery +

 

if the battery is at a low state of charge, that will drag the voltage down from the numbers I think I gave for a fully charged battery. also it might take a bit of an RPM boost to hit the full values of about 13.8V (+/- a bit) with a fully charged battery.

I believe the + lead goes from the alternator to the starter and then to the battery. if you can get to the starter + terminal, it might be worth seeing which side that 0.3V difference is on (alternator to starter or starter to battery).

 

I've got the voltage at the battery at idle up to 13.4V now so I'm pretty happy with how it's going. At least the car is now serviceable for my wife and sons while I'm gone over the next three weeks. Your assistance is greatly appreciated, pierce, pretty sure I couldn't have gotten this far without it. I'll drive it around a bit today and double check the numbers once the battery is more fully charged. If you are ever on Vancouver Island (Courtenay area) I owe you a beer or seven (or excellent home made cappuccino if you prefer).