nashvlille_bill

My wife's W208 has a problem with the lighter; it shows 12 V, but loses it when a load is put on the circuit. I assume it is a bad connection, but I'm a little confused by the wiring diagram. It looks like the lighter gets it's power from the instrument panel, although it has it's own fuse (#23). R3 (lower right) is the lighter, and A1 is supposed to be the instrument panel; I assume 13 and 58k are pins in a connector, but I don't know. I would also like to know what mP means (upper left).

I've attached a copy of the MB wiring diagram that I have, and would appreciate any help.



Thanks,

nashville_bill

Serndipity

nashville_bill........Assuming that you're trying to power an external device, such as a GPS, Dashcam, Cellphone, via plugging it's power cord into the cigar lighter socket, using the above electrical schematic diagram is not likely going to be very helpful.

It's confusing because the diagram only shows the connections that other modules make to the A1 instrument cluster, without showing additional specific wiring details associated with the attached modules.

Specially, the R3e1 lighter socket only shows a connection to vehicle ground (pin 2 to W1) and to the A1 cluster (pin 1 to pin 13), as well as the illumination control module N7-1 (pin 1 to pin 10). The circuit (i.e. 15R) that supplies power to the R3 lighter is not shown. Notes: e1 in the lighter module is it's lamp and 58k is an internal circuit within the A1 module. The validity of the shown wiring has to do with the interior CAN-Bus, which makes it possible to eliminate much of the conventional wiring, saving cost and weight, by interlinking control switches and modules, with the position of the key in the electronic ignition switch control module (N73) determining when power is provided to the lighter socket.

Your assumption of a bad connection to the lighter socket being the problem, of showing 12 V, but loses it when a load is put on the circuit, is likely correct.

Specifically, fuse #23 located in fuse box F1, directly supplies power to the cigar lighter socket as well as to the ashtray and glove box illumination lamps. If the lamps are lit, the circuit is supplying power under load and need to troubleshoot the cigar socket.

If you directly measured 12 volts at the cigar socket with a multimeter is not a sufficient test, because the meter will only draw a negligible amount of current. Electrical power is the product of voltage and current (e.g. voltage x current) and added contact resistance occurring in the circuit will prevent the delivery of current (i.e. power).

Have you checked the hot and ground internal contact surfaces within the R3 socket for resistance due to dirt/grime/oxidation and/or fitment contact of the adapter plug & cable?

nashvlille_bill

Serndipity,

Thank you for your quick response to my above question.
Yes, She is trying to power a phone charger through the lighter socket.
I tested the voltage at the connector at the back of the lighter without a load and with the phone charger plugged into the lighter socket.
The phone charger does work in other vehicles.
I haven't checked to see if the lighting is working in that area, but I will.
For a test, I will supply a load by connecting a 10 ohm resistor to the wiring connection, without having the lighter connected to the circuit. (the 15 Amp circuit should handle as little as .8 Ohms - R=E/I or .8 = 12/15) I think this will be a good test to find where the voltage is being dropped.
I don't understand how the power for the lighter can be supplied via the CAN bus, and have a separate fuse.

Thanks again for your help.

nashville_bill

Serndipity

nashville_bill........the CAN-Bus does not supply direct power to the cigar lighter socket. The power is supplied via circuit 15R , which supplies switched battery power to the socket, when the key is in positions 1, 2 & 3.

Note: As I explained in my initial post, the diagram your posted, does not circuit 15R, because it only shows the wiring specific to vehicle modules that are connected to the A! instrument cluster. It is this wiring that, via the CAN_Bus, enables the electronic ignition switch control module (N73) to supply battery power to the lighter socket, when the key is in positions 1, 2 & 3.

Dollars to doughnuts, your not being able to change the phone charger is due to poor electrical contact when the plug is inserted into the socket. This can be caused by:

-dirt/grime/oxidation inside the socket

-inadequate mechanical contact, when the plug is inserted into socket, such that either the +12 volt or ground electrical circuit is not being made.

or

The later is a very common problem with MB cigar lighter sockets, due to size differences between plug manufacturers, especially with the ability of the plug's side contact to make a good connection to the socket's side wall.

Hint: If you look down into your socket, you'll see some tabs which can be moved more inward, which sometimes helps.