My right low-beam is out - I have replaced the bulb with a known good bulb and it still doesn't work. I have been reading many, many threads about this, and looking at wiring diagrams, and I think, unfortunately, that my CEM is bad - but they are expensive and I want to be sure before I dig into that project!

So here is where I'm at.

1- Replaced the bulb with a known good bulb - nothing.

2- Checked the fuse, (no. 17) and even swapped it with no. 16 - nothing (and the left still works)

3- Checked for power are the fuse block - it has power ALL the time (as does 16, so I guess that's normal) - even with the key off, but . . .

4- does that mean the CEM is between the fuse block and the bulb? There must be something in between or the headlights would be on all the time, right? Even when the ignition is off.

5- unplugged and re-seated the connector at the headlight nacelle - hope springs eternal, right? Nothing.

6- Used a test light check for power on the car side of the connector - all the pins have power when they should except the Blue-Yellow wire (which, if the wiring diagram is right, is the low-beam).

7- So, something has gone wrong between the fuse block and the bulb - but is it just a broken wire or is it the CEM?

The big question then is where is the CEM in the power stream? Is it:

battery - fuse - CEM - bulb - ground?

Is there some other test that I could perform to check the CEM?

I should add that when I first started getting the "low beam bulb out" message that the bulb would light momentarily when I started the car, and then go out. I thought this was just a quirk of how the bulb had failed - like the filament was touching when it was cold but would pull apart as the bulb heated up.

Thanks

It is difficult for me to understand how ANYTHING in your car can interfere with lighting your headlight by jumper wires - maybe your problem is a ground issue.

Perhaps you should check for connection between your lightbulb socket to ground - disconnect battery to protect your meter.

BTW - have you tried duplicating your tests on the other headlight?

When you performed your tests, was the other side low beam lit?

I am not trying to be snide, just trying to make suggestions that are straightforward.

When you say you have power, are you testing just for voltage?

Voltage is not enough to power a load, you need enough current as well.

Can you light a bulb with the power, a headlight bulb?

Ling story short, in out '88 745 GLE I found that the wiring to one of the headlights had corroded to one strand of copper, the rest of the wire was black gunk. This happened one foot up inside the wire.

The headlight connector read 12 volts when tested, but could not power the headlight.

The same holds true for a ground.

I like to start simple. In my 544, I carry various wires and connectors so that I can rig up temporary wires without getting into complicated issues on the road.

I would run power directly from the battery to the headlight connector. If the bulb still does not light, I would run a ground as well.

BTW how does the headlight get its ground, a wire, or some mechanical method?

I was using a 12v test light, and it lit that at the fuse - but there was no measurable power at headlight plug. So, somewhere between the fuse block and the end of the wireharness that pugs into the headlight nacelle there is a disruption in power. Could be a wire, could be the CEM - that's the question.

It is a ground wire and I found both ends of the ground, tested it for continuity and voltage and it seems to be good. It grounds the whole headlight assembly, and he high beam works just fine, so there is at least enough ground to run the low beam.

I did try jumping power directly to the headlight assembly to light specific bulbs, but there appears to be either some kind of sensor trickery that makes that a difficult task, or I was unable to sufficiently ground the assembly with a jumper wire.

The wiring harness dives down into the engine and disappears from view almost immediately after it leaves the headlight, so I'm having a hard time tracing it back - so I don't even really know if it goes to from the fuse to the CEM then to the light - but, as I said, there is something between the fuse and the bulb or the bulb would be on all the time, since the fuse seems to be always getting power from the battery - whether the car is on or not. (The fuse is getting power right now, while I type this, and the keys are in my pocket!) - so there is something between the fuse and the headlights, I just don't know if it's the CEM or some other relay. That's what I want to know. I think it might be easier to trace the wire if I knew where it went between the fuse the headlight.

I'm looking at page 81 of this wiring diagram:

http://www.volvowiringdiagrams.com/volvo/XC70%3AXC90%20Wiring%20Diagrams/TP3998202%202007%20V70%20V70R%20XC70%20XC90%20Wiring%20Diagram.pdf

It appears that the power path goes: (Battery) - fuse - Light switch (3/111) - CEM (4/56) - Thru a disconnect (54/23) that I don't know where it is, firewall? - RH Headlight (10/2).

It looks like a single power source ( no LH/RH differentiation) until it goes through the CEM. Coming out of the CEM there is a blue/yellow that passes thru the disconnect and continues as a blue/yellow to the R/H Dipped Beam bulb. For the L/H side, it's a Blue/White out of CEM, passing through the other disconnect, continuing to the L/H bulb as a blue/yellow.

So the problem lies somewhere between the CEM and the bulb, including a mid-course disconnect. So in theory you can probe the R/H blue yellow wire at both the CEM end and the Bulb end and draw a conclusion as to where the break is.

Thanks - I think that's a possibility, too, it's just figuring out where the Blue-yellow wire comes out of the CEM, I guess. I imagine that it comes out of the top of the CEM, closest to the engine, which means that I have to take the wiper assembly off to test it. Do you know if there is a scan tool that a shop might have to check the CEM electronically?

However - that said - That still leaves the mystery of there being constant power at the fuses. Page 18 of the same wiring diagram, fuses 16 and 17. The power to the headlights passes through here, but I don't see these fuses on page 81, so I can't figure out what happens to the power AFTER these fuese. If they are after the CEM (the power is already split by this point), then what keep the lights from being on all the time? From what I see on Page 81, these fuses seem to be after AFTER the switch and CEM because the power splits at the CEM, so what's between these fuses and the bulbs?

I do know where that disconnect is, by the way. There is a pair of wiring harnesses that run to the left and right headlight assemblies. They disconnect just before the assembly, I assume so that the headlight nacelle can be replaced without replacing the entire wiring harness all the way back to the CEM.

The more I think about it, the more I either doubt what I’m seeing on the pg 81 diagram. Or else I’m not understanding how that diagram works. I know from my own prior troubleshooting that there are separate fuses for LH and RH dipped beam headlights.

I just looked at my Haynes book wiring diagram, and if it’s at all accurate then the news is encouraging. It shows a single common power path through the CEM/relay/shunt etc and THEN runs to the engine compartment fuse panel where it FINALLY splits into separate RH and LH paths. So as I recall you said both fuses #17 and #18 are showing uninterrupted power flow. That would eliminate the switch, CEM, relay, shunt, as possible points of failure. As a confidence test you could try jumping straight from the fuse to the bulb to verify that everything upstream of the fuse is OK and that the failure is somewhere between the fuse and the headlight. Did you test for interrupted power at the headlight disconnect or at the bulb itself?
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

I have tried to jump power from the fuse the bulb without a lot of luck, and now that I think about it I tested the ground wire on the car side, but not on the headlight side of the disconnect. I could start there.

I get power to everything BUT the low beam at the car side of the disconnect when there should be power. So if there is something wrong it is with just the low beam - and that could include that common ground, but there is still no power ever getting to the disconnect.

And it still leaves me with the same question - what it between the fuse and the bulb that regulates the power. I've looked at the same diagrams as you (Haynes, too) and yes, it looks like fuses 16 and 17 are downstream from the CEM and switch, so what turns the lights off?

I can also check the fuse with the headlight switch in the most "off" position it has. The parking lights still come on, but the low beams are off. If that shuts off the power to the fuse then I know I can eliminate at least the switch for sure, and I know better where the switch falls in the order.