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.


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.