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Old 11-20-2010, 12:10 AM   #20
Scooby Newbie
Member#: 166381
Join Date: Dec 2007

Anybody ever figure the "headlight dimming/presumptive voltage drop" issue . I noticed this from day one driving the baja home in the dark. I finally decided to work it out (as it is getting dark on the way home now and re-noticed it.) I did nothing but chase my tail.

I checked the battery and changed it as it was only putting out 400 cranking amps and was 5 years old. New battery had no effect.

My bro-in-law checked the alternator on a vat and it checked out fine. Puts out 90+ amps at 2000rpm and 40 at idle. Voltage is fine.

We strung up a grounding strap from the alt to the battery. While I swear the car runs WAY smoother now, there was no effect on the dimming headlights.

I will toss in the observation that it seems that the voltage drop is only really reproducible while under way. If I just sit in my driveway and stare at my lights on the garage door, while turning on and off the a/c and wait for the compressor to kick on, there is only the slightest hint of dimming. However, driving home, following a white work van WAY too closely, my lights dimmed almost to yellow (reflected off the white paint) when the compressor clutch kicked in.

This really doesn't make a whole lot of sense. The lights dont even dim that much when the car is off and running just off the batt, i.e., even if the a/c compressor was going bad and putting a JUGE load on the system, the batt should pick up the slack. I suppose I could be imagining the whole thing, but I dont think so.

I suppose it could be a light system specific issue where there is over much resistance in the line such that when the voltage drops a bit on the rest of the car, there is enough resistance to reduce the current within the headlight circuit to a point the lights dim.

Are the headlights on a switched ground or a common ground? I suppose if the relay is going bad or if the common is corroded, this might happen.

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