Hi guys, my offside main beam isn't working, the bulb is fine, the fuse is fine, but the wire for the main beam is only reading about 5V all the way up to the firewall, indicating that something a little more unusual is amiss.

All other lights on the car work absolutely fine, including the nearside main beam (which is getting 12V or there abouts)

Could this be some kind of relay or control unit fault? Is this a common issue? Easy fix?

I need to get it sorted very soon as my MOT is due in under a month, and I wanted to get it done sooner rather than later.

Thanks in advance!

Edited 16 May, 201511 yr by acme15

check the earth strap on that side isn't corroded or loose

Earth strap?

Any idea where that would be?

Edited 16 May, 201511 yr by acme15

could be from the alt / engine mount to chassis is its anything like a 2f

You should see a couple earth straps in the engine bay, basically just a cable bolted directly to the body.

A quick test for this is get a jump lead and with one side on a negative battery terminal, hook the other side to an earth point closest to the light in question, or the earth it should go to if you can find it. If the light gets brighter then that is your issue.

Ah yes I see them. I dont think it is the issue though as I previously tested the voltage using the earth for the light which is getting the full 12V and it was still only reading <5V.

Any further ideas all?

If you are sure it isn't an earthing issue then I'd follow the wiring back and see if they are on seperate fuses or relays etc. If they are then just simply swap them over and see if you can switch the low supply from one side to the other.

Also if you can I'd continuity check the wiring to see if you are getting a high resistance value anywhere.

Thanks. They are on separate fuses, both are fine, and I've already swapped those just I'm case. Though if it were the fuse I would expect 0V rather than 5V. I'll test the resistance further, but it is a tad awkward as all the wires are bundled and wrapped in insulation. I can see where the wires come in behind the dash, but I&#39;m not sure where I'd find the relays. Are they above the comfort-control-unit?

Since they are on seperate fuses you should be able to continuity test from the fuse to the bulb connection.

Also with the fuses out check how much power you are getting to the fuses themselves.

Finally a blown fuse does usually just show 0v but I have seen cases where a fuse has melted instead of blown and it has turned itself ito a resistor instead.

Doh! Good idea, taa. Project for tomorrow. :)

The feed for both main beams comes through the column stalk and then splits between the two fuses 21 & 22 then out via a 12 pin connector direct to the headlight bulbs, no relays involved

Thanks Nige, I was wondering if you'd show up in the thread. [emoji14]

Its fixed! I still don't know what the cause was though. I cheated and did some re-wiring. [emoji23]

Wire coming from the working headlight, piggybacked from the connector:

Running along the back of the slam:

Fused:

And going into the other one to give it the power that the gimped wiring can't -

also note the custom radiator fan bracket I made, because I have an incompatible fan and rad (bolt through type fan, non bolt-through type radiator):

And bought a pair of new Neolux bulbs, and a couple of spares...

Wish I'd used black wire but I couldn't find any...

Now I just have to wait for my new Brembo disks and pads to arrive, and I'm MOT ready...

Edited 18 May, 201511 yr by acme15

I should add that this is just to get it through the MOT.

How do I get the fuse box out and such when I do it properly?

How do I get the fuse box out and such when I do it properly?

Removing and installing fuse holder

Removing

Disconnect battery → Chapter.

Carefully lever off the cover on the side of the dash panel.

Remove lower storage compartment on driver side

Remove securing bolts -B-.

Release fuse holder -A- by carefully pressing locking lugs -C- and in doing so take out fuse holder -A- inwards.

Installing

Installation is carried out in the reverse order. When installing observe the following:

Tighten all threaded connections to the specified torques

Thanks, the voltage on the back of the fuse box is the same for both lights. I've been advised that I might need to replace the module which controls the lighting. Which one is that? Is that the Body Control Module?

The main or dip beam supply is not controlled by the body control.

The dip beam supply comes from the ignition switch through the main light switch then splits at the fuses for L & R then straight to the headlights.

The main beam supply comes from the battery live supply (TV2) in the fusebox, to the column stalk, then back to the fusebox where it splits at the fuses for L & R, then straight to the headlights

So if the ground is fine, the bulbs are fine, the fuses are fine, the passenger side feed is fine, the voltage on the back of the fuse box is the same for both feeds, and the resistance from the fuse box to the light is the same for both sides, what could be causing a low voltage to the drivers side? :(

From the fuses the wires goes through the bulkhead disconnect plugs

The connector is "P" in this picture should be a White connector and your wire is pin 4 for Left Main & pin 5 for Right Main.

So you most likely have a poor connection in this connector

Thanks Ill have a look at that. Are those connectors under the dash up behind the steering column? Will I need to remove the dash to get to them? If I have to remove the dash. Is my "temp fix" safe as it is, because one loom/one wire is carrying the current for both main beam lights. Will that overload it/present a safety risk? I used main beam a fair bit last night and felt the wire and it was still cold to the touch.

You should be able to get at the connectors under the bonnet.

Your "Fix" isn't really safe as your running twice the load through the N/S headlights wire & fuse so you are risking a melt down !!

I thought that was the case. I'll be sparing with the main beams for now then.