Reset Gen II Volvo LCM
12/1/2012
RandyA (Randy)has come up with a method to reset the calibration of a Gen II Volvo LCM (Light Control Module) to deal with changing light loads.
Surprise, surprise!
Just like early OBD1 ECM's used in 80's cars the Lighting Control Module in the mid-generation Volvo's appears to have a "learning mode."
It works something like this: Change the number or type of lamps used in the exterior lighting fixtures and you immediately get an error advising you to check the lighting at the next stop. But, after approximately 8 key on-off cycles and clearing the error each time and it disappears. At first I thought this might be a fluke - so I did some lamp swapping to see the results.
I installed a set of HID low beams in place of the H11 - immediately got an error. Then, after about 8 switch cycles the error disappeared. Hummm..... interesting.
So, I replaced the HID lamps with the original H11 and guess what? Yep - an error in the LCM. After 8 switch cycles, the error disappeared.
Put the HID back in with the ballast. Back to the error. Run about 8 switch cycles and the error disappears.
The only conclusion I can reach is somehow the LCM has a learning mode (not unusual) but is a little hard headed and takes several switch cycles to finally make a permanent impression.
There are some things the LCM will not learn - like a H11 in one socket and a HID ballast in the other. Both lamp sides need to be the same resistance.
When swapping the rear back-up, or running lights or plugging in your trailer to the light circuit, the added load changes for the running lights (number of bulbs) l get an error - but, once the LCM sees this new load several times it gets over it. We are OK on stop and turn signals as they are usually feed through relays and the trail part I not seen by the LCM.
BTW - when replacing or adding LED lights the LCM sees them as an open or non existent light. Thus, it reports an error that will not go away if incandescent laps are totally replaced with LED's. Leave one incandescent somewhere in the circuit like in the front parking light and the LCM learns to be happy with the LED's added.
I have found nothing in any manuals that even makes a suggestion this can happen.
This is a 2004 LCM - don't know if earlier LCM's do the same.
Remember resistance of lamp 1 x lamp 2 resistance/ lamp 1 resistance + lamp 2 resistance = total resistance. If one 1156 lamp is 200 ohms and another 1156 lamp is added the circuit resistance changes to 100 ohms. The LCM has to be able to adjust or compensate for such a change. It learns!