Electrical Question - PeachParts Mercedes-Benz Forum

jake12tech · #1 10-31-2016, 05:33 PM

Was changing the burned-out bulb in my backseat, disconnected the connections and put in these cheap chinese LED from eBay. Got zero illumination, perfect contact. Thought it was the bulb, so tried a good working incandescent bulb, and about 5 other ones brand new in the box and no illumination. Tested the positive wire and ground wire and got 6-7 volts. Thought I fried something with the LEDs, but still has power. Even putting in the old bulbs gets me no illumination? Can anyone point me in the right direction? Contact is good, voltage at the wires, not sure where else to look.

This is on my 99 e300 btw.

jay_bob · #2 10-31-2016, 06:18 PM

Well like most things in the W210 it is not just a straight circuit.

The 12 V comes from device F4 (fuse box next to the battery under the seat), fuse 3, 15 A, circuit 30 (unswitched battery). Out of the fuse onto a 1.0 mm2 red-gray wire to splice Z7, which breaks the wire out into 2 0.75 mm2 red-gray wires. One wire to E18/1 (trunk light) and the other to E15/3 (rear dome lamp).

The A37 (PSE Pump) controls the rear light. A37:X1 pin 1 is the ground return from the dome lamp, on a 0.75 mm2 brown-red wire.

Since the PSE pump does the "slow fade" feature as well as the master control of the light over CAN bus from the light switch above the rear view mirror, it's a good bet there are electronics in this line and chances are the LED may have damaged the circuitry.

Make sure the well where the PSE lives is dry. This is a common place for water to accumulate and short out the circuit board in the base of the PSE pump.

The PSE pump not only provides the vacuum and pressure for the locks, it also provides pressure if you have ortho seats, and also provides a supplemental vacuum for the climate control and emission controls on gas cars (that just rely on manifold vacuum and don't have a vacuum pump like a diesel).

Also the PSE is the rear electrical nerve center for the back end of the car. It handles the interior lights, receives the switch contacts from the door and trunk switches, controls the power switching for the rear defroster, and the burglar alarm.

If you end up trying to find a PSE in the yard, try to get one from a car with a similar feature set as yours, as it has to be programmed for the features found in your car.

jake12tech · #3 10-31-2016, 08:04 PM

That's terrible news. Same thing happened in the trunk as well when I tried swapping the lights. The front dome lights and courtesy lights all work as well though. I guess I'll just live with it.

I don't understand how so many others have success with these bulbs on youtube, and others, but my little bulb shorted it? It just doesn't seem to be.

jay_bob · #4 10-31-2016, 08:18 PM

Trunk light is likewise connected to the PSE. A37:X1 pin 13. 0.75 mm2 brown/yellow. The switch in the edge of the trunk lid is direct wired to the lamp, and switches between the brown/yellow lead and ground. The PSE connection is how the system knows that the trunk is open, this sends a CAN bus message to the lower field control panel to turn on the LED in the trunk switch.

Just curious, did you confirm fuse #3 in the under seat fuse box? And be sure you have it plugged in correctly, it is very easy to miss the contacts and plug the fuse in wrong so that it is not making contact. BTDT.

Mxfrank · #5 10-31-2016, 08:54 PM

Are these festoon bulbs (tubular)? LED's are usually polarized. Did you try reversing the bulb? They may not play well with dimmer circuits, either.

jake12tech · #6 10-31-2016, 09:21 PM

Quote:

Originally Posted by Mxfrank

Are these festoon bulbs (tubular)? LED's are usually polarized. Did you try reversing the bulb? They may not play well with dimmer circuits, either.

Yes, I did and yes they are festoons. The old bulb won't illuminate anything now either.

Jay-Bob, I did not confirm the fuse. What should it read in on a multimeter to be a healthy fuse? I did pull it and plug it back in which I can imagine was useless and a waste of taking out the seat.

BillGrissom · #7 11-02-2016, 03:25 PM

Sounds like a dimmer is limiting the fixture to 6 V. It should go to 12 V at max adjustment. I don't know the W210. In my W123, the dimmer is just a rheostat in the cluster and a shot of Contact Cleaner often gets more current from it. Many LED's don't light at all until a certain voltage, and then you get almost max output. So, they don't work well with a dimming function. Doesn't bother me because too much dash illumination is usually never a problem in a W123.

Ditto, on "flip LED around and try again". All I have used are polarity sensitive. Doubt one could have damaged anything in your car. But, for those putting them in the dash, be very careful and measure resistance with it installed in the bulb holder. My M-B holders have unique fingers. Some LED's leads cause a short across the bulb holder, which melted a PCB trace in one of my clusters (repaired).