Headlamp voltage

Q My 2007 BMW X3 3.0S D has 57,000 miles on the clock. There seems to be a large voltage drop on the car’s headlights (high beam, dip and spotlight). I am only getting 10.85V on all three of these bulbs. It doesn’t make a difference if the car is running or not – the voltage is the same. I am getting 12.25V at the jump-start point under the bonnet if the car isn’t running, and 14.35V if it is.

I have run a temporary earth wire from the jump-start point to the negative terminal on the lightbulb and it doesn’t make a difference. I was reluctant to do the same test with the positive lead in case it blew something. I bought a new multimeter to double-check the readings and have replaced the fuse and the headlight bulbs, but there’s no difference. I’ve removed the headlight bulbs and connected them directly to the jump-start point and there is a massive difference in the brightness. I can’t look at the bulbs when they’re connected to the jump-start point, but I can when they’re connected to the headlight plug, which seems to confirm the voltage drop problem.

I only tested the voltage on the passenger side, as I need to remove the windscreen washer bottle on the driver’s side to access the headlight bulbs there. However, comparing the headlights on both sides of the car, the brightness seems to be exactly the same and nowhere near the brightness I was seeing when the bulbs were directly connected to the jump-start point.

I have searched online to find out where the headlight relay is situated so I can remove this and test it and, if access allows, check the voltage to the relay box, but I can’t find out where it is situated. I would have thought the headlights and spotlights would have been on a separate circuit, but as they all seem to have the same voltage drop, my assumption must be wrong. The only strange thing I have found is that the brown wire to the headlights is definitely the negative, although I would expect it to be positive if wired using electrical norms, but maybe this is the way BMW does it.

I would be grateful if you could advise me where to look and what tests I can perform. I wouldn’t mind using non-bmw parts for the repair if necessary.

Mark Emery

A Taking one of your last points first, in automotive wiring convention the brown wire is usually the earth. This is standard practice on many vehicles.

As you are able to operate the bulb directly from the battery, I am assuming that your X3 is fitted with halogen bulbs as opposed to xenon units. As you have tested the operation of the earth system and this appears to be operating correctly, this can be discounted as the problem. Having also checked the voltage at the jump-start point, the battery and charging system appear to be in order.

Your BMW does not have a headlamp relay, but instead uses Can-bus wiring with a light control module – this should be situated under the steering column.

The transistors within the module control the output voltage to the headlamps’ high beam, dip and spot, which is most likely the reason why the lights are suffering from reduced voltage. Before replacing the unit, I would check the connecting plug to ensure that the reduced voltage is not due to corroded terminals or a failing connection to the light control unit.