W210 Auto dimming RVM

[RFdesigner]

Having just got my RVM working I thought I'd post a how I did it.

Firstly you will get near to electronics, to minimise static discharge wear cotton clothes (i.e. jeans) and chuck your trainers/slippers in the shoe cupboard. Cotton socks or leather shoes will suffice. (rubber is a great insulator and man made fibres really generate charge, so leave that synthetic fleece behind)

A multimeter is very useful, cheap ones are available from Maplin, though I wouldn't count on their accuracy too much they're plenty good enough for debugging.

1. Remove back cover of mirror, push up and towards windscreen from base of mirror, back panel will pop out, carefully unhitch top clips and remove.

2. Slide out LED from bottom of removed mirror cover.

3. Pull mirror off ball joint, very firmly but gently try not to tug cable loom too hard.

4. Un-clip 9 way connector from circuit board and feed through ball joint on mirror.

5. Now you can test the connector hanging down inside the car:

Pin 1: power to LED from board
Pin 2 ~12V from ignition.
Pin 4 Ground out from board to side mirrors
Pin 5 Output signal to side mirrors
Pin 6 LMP, 800 ohms to GND (pin7) when interior lights are on, open circuit when they're off.
Pin 7 Ground from car
Pin 8 (on mine this had no wire) reverse signal,
pin 9 Ground out to LED

You should check there is 12V between Pin 2 and 7 when the key is turned to position 2.

Also check that pin6 behaves correctly, 800 ohms to pin7 when interior lights are on (I would suggest that anything less than about 2000 ohms ought to be sufficient looking at the circuit).

6. If this is working then you can check the glass. If you have a bench power supply then great. You can feed a smallish current into the top of the mirror and back to the supply from the bottom of the mirror, on mine at about 200mA and about 3V the glass went nice and dark. Please use a current limit, just in case.

7. You will see on the board there is a black chip with 20 pins, this is the microprocessor, it needs 5V and on mine it was seeing about 0.5V. In my case it was a short on board, which I suspect was there since manufacture. There is a 180ohm 'fat' resistor (R6 IIRC) which should have 12V at one end and 5V at the other, on the back of the board there is a very small surface mount zenner diode connected to this resistor to regulate this voltage. (personally I am appalled at this for a power supply... cheapo design by MB.. they should have used a regulator) anyway check the voltage at both ends of this resistor, with the board powered of course. (the glass bottom is ground)

The other easy to check points are the two light dependant resistors which should vary between about 1k and 35+k as you shine a torch at them or cover them up.

In the case of light dependant resistors, resistors or zenner diodes replacements of some sort should be available from Maplin. (no connection, in fact I use RS but IIRC they only sell to trade)

And as per Haynes:

Reassembly is reversal of above.

From this it is possible to deduce that if the LED in the bottom of the mirror doesn't work then you probably don't have power, time to check the fuse box.

Hope this is of use to someone.

Derek

 

[television]

it was a short on board, which I suspect was there since manufacture. There is a 180ohm 'fat' resistor (R6 IIRC) which should have 12V at one end and 5V at the other, on the back of the board there is a very small surface mount zenner diode connected to this resistor to regulate this voltage. (personally I am appalled at this for a power supply... cheapo design by MB.. they should have used a regulator)

Derek

Its common practice to have an R in series with a zennor on a known power supply, but the fault was with the PCB and not the design.

Nice to know though how it works,it will help the next one.

Malcolm

 

[RFdesigner]

Its common practice to have an R in series with a zennor on a known power supply, but the fault was with the PCB and not the design.

Nice to know though how it works,it will help the next one.

Malcolm

I know, I've seen it done all too often, it's fine where you want a couple of milliamps, but by the size of the resistor there was a lot more current draining away there, and that means heat.. which in turn means reduced life expectancy. (assuming that the silicon get over 100C, which the zenner may well be getting (~2W resistor, and tiny surface mount zenner), I'm inclined to replace it with a fatter zenner just in case.

Derek

PS, I noticed you seem to have rather a lot of electronics experience... what sort of areas?.. I'm working with ultra low power RF, (1V 3mA max supply for a single chip Digital Transceiver plus 8051 processor and environmental sensors, plus sleep modes with supply currents down into single figure uA., beats designing yet another mobile phone)