Volvo 240 Charging System Problem
Looking for some suggestions regarding the charging system problems I’m having. This is with a 1992 240 with 187k miles, running really well except for this intermittent charging problem.

Sometimes while driving the Service Engine, Battery, Parking Brake, Brake Failure and Bulb warning light come on dimly. They don’t see to come on around town, rather when on the highway while going >60 miles an hour and after about 5 miles. Today for instance car started fine, no warning lights. After about 5 miles they came on. The brightness fluctuated a little and changed a little as I experimented changing the electrical load (e.g turning lights off, using tuen indicators, turning of radio, fan etc). After about 10 miles I got off the highway and idled at a traffic light for a minute and the warning lights go off and stared off for the rest of the trip (~ 3 miles but not on the highway).

Here’s what I’ve tried so far:

At idle the battery/charging voltage is typically ~ 13.8V. Is this a little low? Shouldn't it be nearer 14.2 V?

The battery is about 1 year old and seems in good shape (although I haven’t had it load tested)

I’ve checked the exciter wire – seems sound and well attached. Disconnecting it and starting or removing while running doesn’t seem to make any difference in battery/charging voltage. Surely this doesn’t make sense and may be a clue?

Check the voltage drop across the alternator/battery cables – negligible voltage drop so seems they are in OK shape.

Checked all of the fuses. In the past I have had fuse problems (e.g taillights) but when I changed to better quality copper fuses all of those problems went way. So fuses seem good.

Removed the instrument cluster and resoldered all of the joints on the circuit board. A couple looked a little questionable (cracked?) although they weren't the ones that the FAQs etc seem to indicate are typically involved in this type of problem. When I reinstalled the cluster after the resoldering the voltage at idle was 14.3V. At this point I thought I had cured the problem. However, this was at cold start-up some maybe the idle was higher and hence the voltage?? Now when I start the car it seems to typically be back in the 13.8V range so what I thought was the fix wasn't!

I replaced the voltage regulator a couple of years ago. The brushes are not excessively worn although I did mess us the housing they fit in a little when I installed it but I think its OK. Doesn't the regulator stop the voltage going too high so being to low is likely not related?

Does the electrical use at higher speed vary that much from idle to make this only occur at higher speed?. Is that because of the fuel pump?

That’s all I recall as being relevant. I’m rather at a loss. Any diagnostic suggestions or ideas?