Car lights pulsate

My wonderful 98 Honda Accord , with 234,000 miles runs great except that lately all the lights in the car, from the dash instruments to the headlights pulsate. They dim a little and then go back to normal and the speed of this change is dependent on my speed. In idle, pulsation is slower and the lights dim at their lowest. It is safe to drive because the dimming is not that much to affect visibility, worrisome.


You probably need to replace the alternator. You have bad brushes or a bad wire in the field winding or stator coil, causing the alternator to vary voltage as it turns. A replacement alternator will fix this.

It could be the alternator, but that is an expensive item. Before doing that, I would check all the battery connections and the ground straps that go from the engine to the body. Start with the battery terminal, make sure they are clean and tight.

Next, look at the - terminal of the battery. There are two cables from this terminal, the larger one goes to the engine block, the smaller one goes to a ground point next to the battery. Make sure both of these are tight and corrosion free.

At the front of the engine, next to the oil fill cap (4 cylinder model) is a small bolt with a small wire attached. This wire goes to one side of the front motor mount and then jumps to the other side of the motor mound. This is the secondary body to engine ground strap. It must also be clean and tight.

I would recommend undoing all the connections, clean up the surfaces with scotchbrite and reconnect them. See if the problem goes away. If it does not, then have the alternator tested for a ripple voltage in the output.

A new set of brushes for this alternator costs $30 on line. If it were mine, I would put new brushes in it before I even bothered to try to diagnose it, because of the miles and the fact that your symptom sounds like worn-out brushes.

The lights can do what you mention due to bad diodes in the alternator. This causes excessive ripple voltage in the charging current. Have the alternator load tested to verify that. Bad diodes will reduce the total charging capacity of the alternator.

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Alternator overhaul kits used to contain brushes, voltage regulator, and the diode trio. I used to do it about every 70K. Then sometimes replace the front and rear bearings. The last one I tried, some years ago, you couldn’t get apart without unsoldering the or uncrimping the connections. If you screw it up, they won’t take the alternator as a core. So all I do now is replace the whole thing without messing with it but I only buy Delco remanned ones instead of any of the auto parts houses. Had the flickering you describe on a new NAPA unit within six months of installing. So its either the brushes, diod trio, or voltage regulator. I would replace the whole shebang and be done with it.

Hey Y’all. I have had same problem recently after replacing my alternator in my '05 Sonata 2.7L V6. Cougar is right! So here’s what’s up. The issue is known as AC current “ripple” and is the result of one or more of the diodes in the alternator’s rectifier bridge not working properly (or at all). Alternators are known as 3 phase meaning that their stator is comprised of three interweaved windings that overlap each other, using diodes in the windings to rectify/convert the induced AC electrical current to DC. As my replacement alternator is only a month old and was a aftermarket item under $100 price point (bought off eBay) it was manufactured with a minimal spec for quality, specifically crappy soldering at the diodes connections (explained why problem “kinda” goes away once car is warmed up/driven 5-10 miles). Pulled it, disassemble, and check diodes with MT/resolder EVERYTHING (full rebuild kits for this make are hard to find). It’s the issue and the fix, but, if your alternator is old and you should decide to disassemble to repair yours there is good chance you will find melt/corrosion, and probably a brush worn to the knub (battery light coming on in dash/instrument display anyone?). Best just replace whole alternator, but don’t be too cheap…like I was.

You can test this with a basic MT, but if you really want to be sure that AC ripple from a insufficient/bad rectifier bridge is the culprit you will need an automotive oscilloscope tester to be sure ($700-$1000), or just watch the strobe/flicker lightshow all your lighting exibits - tell tale sign. PS. I installed a “smart” power inverter recently. No sir, it doesn’t like it.

Under proper working conditions, with a MT, AC bleed current should be anywhere from 20mV-50mV and up to around 90mV. Anything over 100mV is cause for concern, in which case you may see the reading fluctuate/ripple, the cause of your vehicle lighting flickering/pulsating. Might also result in higher than average DC voltage under charging load (15v + at minimal load), although vehicle ECM voltage regulation, if equiped in firmware, should catch this if alternator doesn’t regulate it down first (13.8v-14.6v). Not good for any incandescent lighting, although most other interior electronics should continue to work ok.

If there’s an auto electric shop nearby, and you can do some DIY (removal and reinstallation), they can diagnose and fix just about any alternator or starter or give an expert opinion on what else to do.

Cost reasonable, in my experience. A remnant of the independent local craft businesses that used to be the norm.