Electrical System Pulsing?

JIM PEMBERTON

MEMBER
2004 FORD FOCUS
2.0L
4 CYL
2WD
MANUAL
200,000 MILES

The car runs, but the lights, all of them to some degree or another pulse with the engine, and they pulse in the dim direction rather in the bright direction. That is, when the engine isn’t running, the lights have a certain intensity that if used as a baseline become dimmer in a pulse pattern when the engine is running. The coils are all functioning properly, and the alternator charges the battery just fine, though I figure that one of those is causing this behavior unless there’s a burned capacitor somewhere that is supposed to smooth out the power when the engine is running.

1 Reply

AL514

MECHANIC
4,796 POSTS

Hello, most likely this is a battery or alternator issue, I know the voltage reading might be correct, but if there are any failed diodes in the alternator and its putting out any ac voltage across the system it will affect the whole system including the engine computer which is what controls the charging system. The guides below will give you some directions as to testing for ac voltage coming out of the charging system. Monitoring the voltage drop during cranking can give you an idea if the battery is holding up. They only seem to last 3 years or so now.
But if you do find any ac voltage ripple, it can damage the engine computer because they are designed for only dc voltage. If you don't find anything, checking Power and Ground circuits would be next or any dirty connections.

One other thing, when checking the charging system, make sure to also test with the headlights on, and possibly the defrost. This will load the charging system and make it easier to spot faults.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/how-to-use-a-voltmeter