I've now had time to do some extensive testing of the two configuration settings in question (
post #1) with no bulbs fitted, with incandescent bulbs fitted, and with LED (non CAN Bus version) bulbs fitted. For each of these 3 bulb type scenarios, I have tried every permutation of the license plate bulb configuration settings to find out whether the Bulb Check Warning Light illuminates or not.
What I believe I have concluded from this test is that the "License Plate Light w/o Pulse Width Modulation () [LED Light]" setting has nothing to do with a reduced voltage to the bulbs to try and increase their life at all. I now believe that this "" setting is actually to do with
warm bulb diagnosis (sometimes also known as hot bulb dignosis) instead whereby a high frequency signal is likely passed through the bulb while it is switched on to check that there is circuit continuity through it. The results in the tables below seem to back up this theory coupled with the fact that I wasn't seeing a ~12v voltage on the bulb supply when was activated on the license bulb plate supply (i.e. the "without " setting was switched off). It's unfortunate that one setting is the reverse of the other (one of them ticked means "inactive" while the other ticked means "active") as it makes it harder to get your head around interpreting the results. I've therefore also included a second "translated" table which makes it easier for your brain to interpret the results from a visual perspective
- Cold Bulb Diagnosis
This is only performed when the ignition is first switched on. This checks each of the bulb circuits for continuity/resistance and will illuminate the Bulb Check Warning Lamp if there are found to be anomalies in any of the bulb circuits. Once the cold diagnosis test completes, it does not continue to check the bulbs so if something happens to a bulb in any of the circuits after the initial cold diagnosis checks were performed, the Bulb Check Warning Lamp will not illuminate. That will only happen if the lights on that particular circuit are switched on (which invokes the warm bulb diagnosis test), or if the ignition is cycled off and back on again.
- Warm Bulb Diagnosis
This is performed on a particular circuit whenever the light switch for that circuit is switched on. The diagnosis continues to operate for the duration that the lights remain switched on for on that circuit, so that if any bulbs blow while in use, the Bulb Check Warning Lamp will immediately illuminate as soon as that happens.
The tables below pretty much speak for themselves but this is a brief summary:-
- With no bulbs fitted at all, you always get a "bulb out" warning whether under an "ignition on" or "light switch on" condition.
- The settings and "bulb out" warnings for incandescent bulbs seem to be consistent with what you would expect.
- The settings for LED bulbs seem to be mostly logical except for 12 in the Table A ( 10 in Table B). With cold diagnosis switched on I would expect to see a "bulb out" warning appear but it is having the opposite effect. Not so with the warm diagnosis though which if switched on always flags up a "bulb out" warning for the LED bulbs (rows 9 & 10 in Table A, rows 11 & 12 in Table B). So, for LED bulbs you at least need to switch off the warm diagnosis (i.e. switch on the "Without " setting).
Table A:
Table B:
This shows the same results as
Table A but with the "Without " column changed to "Lamp Monitoring (Warm)" and the values within that column inverted, plus the rows swapped around so they remain sequential from a logical bit value perspective. For both columns, a bit value of 0 means "switched off" and a bit value of 1 means "switched on". For each bulb type, you can see a much more obvious "pattern" emerging in
Table B (green no's to the left, red yes's to the right).
@RT: Might be worth double-checking the source of the description for Byte 17, Bit 7 on the PQ35 BCMs. I don't know where the descriptions come from but maybe it's one that has just got "lost in translation" or one that just isn't worded particularly well in the document or system it came from