"lowbeam headlight failure" electrical conundrum

Esjay · Post by **Esjay** » 27 Aug 2018, 08:46

I recently adopted this 2006 v70 and I'm working some of the minor problems that came with the car. The message center is texting: "bulb failure low beam headlight" upon startup. But the low beam lights are working, in fact all the light bulbs are working fine. I noticed that the previous owner had wired the low beam headlight bulbs separately from the wire harness that controls the entire headlight assembly. There is a separate wire connected directly to the + post on the drivers side fuse box under the hood, it goes to a separate fuse cartridge and then on to a small (micro) relay. Two wires exit the relay and go to each low beam headlight. I can only assume that there was a problem with the original wire(s) supplying power to the low beams and this is the reason for the separate wiring. I have taken both bulbs out and made sure that they are seated properly. Not really sure what to do next. The electrical diagrams do not show a "bulb failure" circuit, with a wire that controls the bulb failure message. Any help would be appreciated. I know its not a critical problem, but it is aggravating to have to keep clearing the message at startup.

Georgeandkira · Post by **Georgeandkira** » 27 Aug 2018, 17:30

Has the "bulb out" warning been there the entire time you've owned this car? just askin'
Since you're handy and have an electric meter, try removing both bulbs and measuring the OHMs of each.
They say they need to match or be real close. How close I do not know.

Also, people have posted about "shunts" being involved in the measuring of the difference in resistance. One post featured a guy drilling a hole through one without explanation.

In your case something went awry as evidenced by the additional wiring.

You may indeed live with the subtle annoyance of clearing the darned warning for the rest of your life.

Post by **mrbrian200** » 27 Aug 2018, 21:48

So the PO bypassed the +B from the CEM that normally supplies the headlamps. Are you able to determine what controls the micro-relays?

There's no telling why they did it without a bit of prodding. My haunch would be that the feed from the CEM is bad or there's a broken wire they were unable to trace. The first step would be to test the factory wire that normally would supply the headlamp to confirm no voltage there.

The CEM current senses the headlamps to verify they are operating. With your headlamps being supplied by a bypass circuit, not connected to the CEM, it would see an open circuit which is the same as a burned out bulb.

You might also run your VIN to make sure that the car wasn't originally equipped with HID. It wouldn't be the first time somebody tried to convert HIDs to Halogen in an attempt to save $. In such a case the business with the relay would be to make them work: you'd control the relay from a circuit that has power at KPII, though you'd lose the ability to turn them on with the light switch when the car is off. I don't think there's a way to wire halogens directly into the vehicle harness on a car that was originally equipped with HID.

Esjay · Post by **Esjay** » 28 Aug 2018, 05:28

I appreciate those suggestions and will let you know what I find. The bulb failure message has indeed been on since we bought the car. Both bulbs are new and are the same halogen bulbs, so I assume they have the same resistance. Halogen bulbs are listed as the original equipment, according to the VIN profile for this car. As to what controls the micro relay, I'm not sure what you mean. There is always power to it because it is connected to the + power post, but the lights only come on in KPII. They go out when the light switch is turned to the parking lights only, as it is supposed to in KPII. I will look for the original wire to confirm no voltage and go from there. Thanks!

Post by **mrbrian200** » 28 Aug 2018, 08:59

The relay will use a (low current) control signal from somewhere. The control signal activates the relay to allow the higher current voltage to pass from the fuse box through the relay to the headlamps.

Think of it like a light switch, except the switch is being activated by a wire/voltage rather than your finger. IF there are only two wires attached to the relay, one to the fuse box, the other out to the lamp bulb, that PO didn't know what they were doing/how to wire a relay.