My car just started pulsing, noticeable at night when the lights are on. The dash lights, horn, wipers: headlights will all get pulses of extra energy and glow brighter, the horn honks at a higher pitch, the wipers will go super fast, the dash lights will also glow brighter. This all happens in a varying cycle throughout a drive at night. The car starts, runs, and drives ok except for this kinda un-nerving behavior. I'm afraid the electrical system won't be able to handle the extra current wherever its coming from. Any ideas?
Thanks. Tom 1990 240 Volvo 5 spd. DL Wagon 158,000 miles.

Pulsing lights is the classic symptom of worn out brushes in the voltage regulator. These days there are places that sell just the brushes if you are willing to solder them in. Otherwise, you need a voltage regulator. It comes with new brushes attached. The regulator is held in the back of your alternator with two screws. I think 1990 was about the time they switched to Torx screws to hold the voltage regulator on.

It is easier to access the voltage regulator if the oil filter is removed. Then you can fit a regular-sized screwdriver onto the screws.

At first consideration, it sounds like a bad voltage regulator -- the battery voltage fluctuating could certainly do this, which is what the voltage regulator (embedded in your alternator) is supposed to prevent as well as keep your battery at the proper level to prevent overcharging. It certainly sounds like a serious loss of regulation, however brief, because it takes a lot of increase in voltage to make the wipers go noticeably faster.

I suppose you don't have voltmeter added to your dash instruments, but do you have a handheld DVOM that you could hook up to your fuse panel and watch (while keeping your eye on the road :-) as you drive around? Or, some auto parts places have a voltmeter built into a plug that goes into your cigarette lighter?

Have you checked your battery's electrolyte level. I'd suspect that your battery is intermittently overcharging and you're low on fluid -- this would be the result, not the cause, of the problem, however. But it would support the hypothesis.

However, the fault could be, instead of the voltage regulator, some high amperage accessory that's intermittently drawing a lot of current. You might be confusing low voltage (because of the faulty accessory) and its effects as the "normal" state, whereas the problem is not high voltage but actually low voltage. Possibilities include the rear window defroster, the door locks (do you hear any clicking) which have a relay that's notorious for going bad, an auxiliary electric radiator fan (do you have one in your model year, I'm not familiar with '90s), and even your A/C compressor.

Another possibility is the wiring between battery and alternator, which could be affecting how well the regulator regulates.

Anyway, for whatever reason, it sure sounds like variations in your system voltage. Go from there.