'97 850 Driver Headlight has me stumped

Post by **djm850** » 09 Jun 2014, 10:14

A couple of months ago I noticed that my driver side headlight was very dim on low beam/daytime running and when I would turn on the highbeams it would go out altogether and throw the 'bulb out' indicator light on the dash. The passenger side headlight functions fine on both low and high. So I checked the bulbs/fuses and they were OK, but noticed the insulation on the wiring inside the headlight housing(s) were deteriorating and assumed they may be shorting out. So I replaced both units yesterday with URO replacements and and it still does it on the left side with the new units!!!
I pulled the headlight relay under the cover of the fuse box and visually it looks brand new. I've ordered a new one, but wonder what else could it be?

Post by **cn90** » 09 Jun 2014, 10:54

Check the connector/wiring where it plugs into the HL assembly.

Post by **djm850** » 09 Jun 2014, 11:03

I unplugged, re-plugged, wiggled, etc. on it many times. I've also inspected it and there is no apparent corrosion. Have not tested it though.

Post by **jblackburn** » 09 Jun 2014, 11:12

May have developed a short or burned up within the insulation, especially if the headlight housing was pulling too much current from the thin-gauge wires.

I've heard of the headlight harnesses burning up before.

Post by **abscate** » 09 Jun 2014, 11:36

Pull the connector and look at both sides where they touch. It is very common for the metal to oxidize to a hard dark coating that will drive you crazy replacing head light bulbs every week, until you figure this out. You have to dremel off this coating to get your lights working again.

Post by **djm850** » 09 Jun 2014, 11:59

As stated, the headlamps are brand new and I did inspect the connector on the harness, the contacts are shiny bright and not corroded. Could a bad relay cause this condition?

Post by **erikv11** » 09 Jun 2014, 12:07

jblackburn wrote:May have developed a short or burned up within the insulation, especially if the headlight housing was pulling too much current from the thin-gauge wires.

I've heard of the headlight harnesses burning up before.

In general yes, but on these cars? I've always thought the headlight wiring was quite beefy, in fact Daniel Stern told me that when I asked him years ago about upgrading the wiring to the 850 headlights. Would be good to know. Now, maybe if someone is running over wattage bulbs yes, that's another story.

To OP: I don't know the wiring diagrams but I would assume the relay works both lights via one switch, so not likely. Others will know ...

Post by **djm850** » 09 Jun 2014, 12:35

I just can't imagine anything else in the system. I guess I'm going to have to take it to an indy and let them diagnose with a meter. I really was hoping for a eureka moment when I plugged up the new headlights.

SpeedyPete · Post by **SpeedyPete** » 09 Jun 2014, 12:56

Hello the head light switch contacts is where i would be looking. After many years of service like all electrical contact points they will become pitted/burned and develop resistance, possibly causing a dim light to no light at all. Check your headlight switch! Good luck! SpeedyPete

Post by **erikv11** » 09 Jun 2014, 13:01

But the switch is completely - one switch controls both lights. Worth looking for sure, but seems unlikely.

Can you check voltage at the connectors for each side under various switching configurations? Probably won't tell you more than the lights already are, but I would check with a meter.

Also djm850, I can send you a working central lighting relay for just the cost of postage, I have a couple extras that I will never use.