V70 Headlights Out, odd voltage measurement

krunch · Post by **krunch** » 30 Jul 2012, 02:33

Couple of days ago, I got a "Dipped Beam Failure" alert on my 2001 V70 and both low beam headlights weren't working. After turning the ignition off and then on again, the alert disappeared and the lights were on again, for a couple of days. Then the same alert appeared and lights were out, but this time they did not come back on. Checked the usual suspects: Bulbs, fuses, (both in engine compartment and in the cabin) all were intact. I've read through a couple of threads on this forum with similar "no low beam" symptoms, and one about a "shunt" in the CEM looked like a possible match, but, (and here's where it gets weird), with a multi-meter checked the voltage at the lamp connectors. With a bulb in either or both connectors, there's no voltage. With no bulbs in either connector, I get 12v from both connectors. Not sure what to make of that. Ground fault? Or would the bad shunt produce that? Incidentally what is a "shunt"?

In the dark.

RaymanSean · Post by **RaymanSean** » 30 Jul 2012, 05:15

Somewhere in the circuit you can not pass current. The 12V with no load followed by no voltage with a load is a typical high resistance problem. It could be the shunt, a broken wire, a poor ground etc. With the bulbs plugged in measure the voltage by placing the negative meter lead on a known ground (the intake manifold works) and then use the red lead to measure voltage. In the ideal world, you would see 12v at one of the bulb connectors and 0v at the other. If you see 0v and 0v then your problem is on the positive side, if you see 12v and 12v then you have a grounding problem. You can also check for power at the fuses the same way by leaving the bulbs installed. Chances are that your problem lies in the control side of the Circuit. A shunt just measures the voltage drop across very low resistance resistor to determine if current is flowing through the circuit. While a shunt can develop a high resistance, they are pretty robust devices.

I have not had to dig into the electrical side of my volvo yet. However there should be the equivalent of a relay that controls the headlights. It could be a relay, or it could be a transistor, either way there is a device that takes the low current signal from the headlight switch to switch the high current to the headlights. The relay may be included in the "shunt" in these cars, or it may be found elsewhere. Hopefully someone who knows more about the wiring of the car will speak up and point you in a less general direction.

Post by **jimmy57** » 30 Jul 2012, 08:26

There is a relay and a shunt for low beams. High beams are not monitored so no shunt is used for those.
The shunt is a slightly resistive conductor and the voltage drop across the conductor is used to track the functioning.
The attached copy from the wiring diagrams shows relay CMI7 and shunt CSH4 as the low beam parts.
Both are suspicious. Sometimes the shunts fail or the terminals where the shunt inserts get hot and the terminal looses it tension and the connection gets loose. The relay can fail too.

krunch · Post by **krunch** » 03 Aug 2012, 16:35

Thanks to everyone for the excellent guidance. After determining that it was not a ground problem, (by connecting from the hot terminal of the low beam fuse directly to the lamps and getting them both to go on), it seemed to come down to the shunt or relay in the CEM. Sure enough, after re-reading the very detailed description of how to reach the CEM elsewhere on this forum, I found that the shunt had one blade that had overheated and actually melted the plastic encapsulant. There was plastic residue covering and insulating the blade. I sanded it off the blade, and with a very thin, fine file I cleaned the blade socket on the CEM. Plugged it in and had low beams again. I'll probably spring for a new shunt, just in case there's something in the old one causing the overheating. Anyway, thanks again, This is a very helpful forum!