|
|
|
|
|
#1
05-21-2009, 12:24 PM
|
|||
|
|||
|
Rear view mirror electronics smoking
Hi All,
I had smoke coming from my rear view mirror yesterday and found a burnt resistor on one of the circuit boards (see pics) inside. It looks like the mirror is held in place by a metal spring. the plastic mounting point broke and the spring shorted the PCB inside the mirror. So my question is what is the PCB do? It has two sensors one on the front and one on the back, is it for the locking system? Does it control the auto-dim feature on the mirror? Is this a common problem? I had an old mirror assembly with a broken case that had the same resistor burnt. Does anyone have a schematic for this circuit? Is there any way to test it? Thanks on advance for you help. Bob 1997 SL320
__________________
Bob 2002 SL500 SA 
|
|
#3
05-21-2009, 01:14 PM
|
|||
|
|||
|
That should be the auto-dimming circuit
Some cars also have a garage door opener module in the rear view mirror as well, but I think that is a separate module in its own plastic package. It will have buttons that are accessible on the bottom of the mirror if this is the case.
I guess it's sort of the luck of the draw as to whether the spring shorts on the PCB or not. I fixed a couple of these with the broken plastic retainers on the bezel and they did not short out. It's really unnerving driving a car with those springs broken. It's likely that you've got more than the burnt resistor wrong with the board. I seem to recall that the PCB is not detachable from the mirror back. If that's the case, I don't know how you could repair it even if you had a schematic. You could just put it back together with the connector disconnected. No more auto-dimming.
|
|
#4
05-21-2009, 02:23 PM
|
|||
|
|||
|
Thanks daidnik, for the quick reply, I did remove the PCB and the fried 15 Ohm wirewound resistor. It looks like it told most of the current, it still reads 23 ohms. From what I can tell that metal under the mounting bracket and against the the switching transistor is live through the resistor and the bottom edge of the mirror is ground.
I did like that auto-dimming feature.  Bob
__________________
Bob 2002 SL500 SA
|
|
#5
05-21-2009, 05:55 PM
|
|||
|
|||
|
How was the PCB attached to the mirror back?
How did you get it off?
Now that you've got it off, the circuit looks fairly simple. Can you see the traces on the bottom or are they sandwiched in the middle? Since you're so adventurous, you could try to ID the transistor. I would think that that is what likely got toasted by this over-current. It may not JUST switch, but rather provide shades of dimming. I seem to recall that is more how it functions rather than ON/OFF. That resistor must be a feature limiting the current thru the transistor.
|
|
#6
05-21-2009, 06:29 PM
|
|||
|
|||
|
Three plastic clips held it in place and a red and black wire soldered to each side of the mirror assembly. It's a single sided PCB not multilayer and the etches are still intact and usable. The transistor is a DARLINGTON TIP122, I measured the resistance between the collector, emitter and base (in circuit) and nothing was a direct short.
__________________
Bob 2002 SL500 SA
|
|
#7
05-21-2009, 07:56 PM
|
|||
|
|||
|
Maybe not enough smoke was let out
Maybe it still works.
I hate that funky plastic bezel with the retainers for the spring bars. MBZ doesn't sell you that piece of course; only the whole mirror assy. The one with the garage door opener is $900; . . . NO DEALS either. I can't stop inwardly, and sometimes outwardly grumbling about that design.
|
 |
| Bookmarks |
|
|
Linear Mode
