Helloooo, so I have a 1985 Volvo 240 that has not been charging the battery properly. I had a warranty on my alternator and was able to replace it but it hasn’t changed the fact that the battery keeps dying. Also the last alternator wasn’t even a year old. I’m not well versed enough to figure out what else it could be. Anyone have any ideas on what the problem might be?

 

please do these steps exactly in order, and report the results of each question/test.,

1) take a volt meter, measure the DC voltage at the battery before starting the engine, it should be about 12.6V for a fully charged battery thats sat at least an hour after being taken off a charger.

2) turn on the ignition without starting the engine.... does the 'BATT' (or 'ALT') light on the dashboard light up? this light is an integral part of the D+ circuit that 'tickles' or 'bootstraps' the alternator.

3) with the ignition still on, measure the voltage from the small red wire on the back of the alternator to engine block ground (the aluminum rail with the ground screws near the fuel injection), you should see around 5-9V with it connected to the alternator, or 12V with it disconnected. this skinny red wire is the D+ circuit from that dashboard BATT light.

4) now start the engine, does the BATT light go out completely ?

5) with the engine running, measure that voltage at the battery again, it should be at least 1.2V higher than the first reading, eg 13.8V, maybe even 14.4V ... it probably isn't since you're not charging....

6) now, put one probe of your volt meter on the side of the alternator, scratch off any oxidation if needed, and put the other probe on the NEGATIVE terminal of the battery, you should see close to zero volts. if you see most of 12V, your alternator ground is bad.

 

I am having the same problem. I'm interested to hear if anyone knows how to fix it.

 

'how to fix' depends on 'why its not working'. those tests I detail above will pin this down.

in general, there's several failure modes.

1) the alternator itself is completely worn out, this requires a new alternator
2) the alternator brushes are worn out, you can replace the regulator assembly quite cheaply, but if the commutator inside is deeply worn, see 1)
3) the D+ circuit is not working
4) the alternator ground is bad


"rebuilt" alternators are all over the place quality wise. many places just clean them up, slap on new brushes, do a very cursory test and ship them.

 

Hey Pierce, just wondering if I have a related problem? My 940 seems to charge fine - battery is 12.8v stopped an 13.7v when running. But, with the red wire connected and ignition on, shows only 1.7v and disconnected shows 11.7v.

 

does tthe BATT light come on with the ignition and go out when its started? you're seeing 13.7V when its running? sounds good to me.

 

The lights work fine, on and off, just wondering about the red wire voltage being so low?

 

ok, hmm, I've been running around for years with that 5-9V number in my head, I think from my old VWs... I just checked my 1993 Mercedes 300CE, which has a similar alternator circuit, it was about 2V. I no longer have a volvo handy to check.

whats actually /important/ is that there's current, and its not 0V and not close to 12V when connected and ignition on without starting. this D+ current provides the 'bootstrap' magnetic field to the alternator, once the alteernator starts to make juice, it provides its own field current.

 

OK, sounds good. Thanks. While I have you on the phone what do you think about O2 sensor testing? Mine fluctuates nicely between zero and .8 after it's warm. But when I pull a vacuum hose to lean out the mixture it continues to fluctuate and won't stay in the low numbers. Anything to worry about?

 

what vacuum hose leans out the mixture? the one on the fuel pressure regulator ? if anything, pulling that would richen the mixture, as the vacuum causes teh fuel pressure to drop. there's enough dynamic range in the lambda system that it can probably compensate for that via adjusting the injector duty cycle

 

Yup, I used the FPR. Didn't think about what that would do to fuel pressure(richness). Good insight. Thanks

 

To confirm, I pulled a different hose and the voltage stayed between .1 and .2. All is well. But, the reason this thread caught my eye is I thought I might have a charging or spark problem. My car has run fine for quite a while but 2 weeks ago at idle it would miss(or kick) for just a split second randomly. hot or cold. Definitely not a fuel problem, wouldn't sputter or stumble. Just a quick miss and back to normal and never happened above idle. Plugs, wires, cap, cps, etc are all fine. I noticed the 2 leads to the coil we're twisted around the distributor wire and thought it might be arcing? Unraveled them and checked that the O2 sensor and CPS we're connected well and the problem disappeared. Hasn't been back since. And I swear, right after that, my mileage went up at least 15%, for about a week. But since then back to previous mileage. Could the coil be wearing out? I've seen different resistance numbers for the coil on different Volvo sites. Some say the secondary should be 8k others say 10k. And .75 and1.6 for the primary. Mine are 1.1 and 8k. Would that matter?