Repairing the G's Dash Switches

The rocker switches in the Gwagen are quite expensive (last I heard >$75), the contacts will likely last a very long time but eventually the backlight on the switch does go. You could take apart the switch assembly and try to find a 12volt light that will work correctly but that seems actually like more work than just converting the switch to use LEDs which will, as a bonus, use less power and create less heat.

First you will need to buy some parts, specifically you need some LEDs and some resistors. 5mm LEDs fit perfectly in the the lamp housing so that is what you should buy. Most LEDs have a focused forward facing beam which is not really what we want since the LED will be sitting sideways in the housing when we are done, so a better choice is a diffused White LED. The only downside of the diffused LEDs are that they are generally not anywhere near the brightest you can buy, Oh well, the original switches were never very bright either. I sourced these LEDs from a couple different places, one set worked great, the other set did not. The LEDs I purchased from Active Electronics (Knight Lites) were the same rating as the ones from RPE but were still basically forward facing and not bright enough to be useful at all (and a buck more each to boot).

Next we need some resistors, they come in a bunch of odd sounding standard values so your choice will probably be between 470, 499, 560 and 680 Ohms. Strictly speaking the 499Ohm size isn't one of the standard ones but they seem common enough anyway. You should pick up the 1/4 watt size so they will fit easily into the switch housing and 100 should cost about three dollars unless you are shopping at radioshack. See below if you want to know why these are the correct sizes. I have mentioned a few sizes that aren't actually in the calculated range here because, in my experience, nobody ever stocks the size resistor you actually need and this isn't super hyper critical, honest.

Now you have to start taking things apart:

  1. Remove the switch from the dash, you should be able to carefully pry it straight out
  2. Remove the outer shell from the switch, there are metal tabs on each end of the switch housing in the middle, push them in and then slide the switch's terminals up through the housing.
  3. Now things get tricky, you have to pry apart the bottom of the switch and slide the core down and out. This is tricky because the switch has a nice clicky feel due to some teeny little springs and ball bearings that will likely fly out and get lost when you take it apart. See the picture so you know what tiny parts you are now looking for. In writing this up I grabbed one of the seat heater switches which has two roast settings and off, so a single throw dash switch probably has slightly fewer tiny parts.
  4. Now we actually get to replace the bulb. Notice the complicated routing the wire takes around the switch, we need to replicate this later. The bulb is in the top in that little shroud. Alternately you could cut the legs of of the light bulb and solder directly to them which will save a little bit of fiddling.
  5. Now we get to solder, notice the last step eased us into it by not actually doing anything. First thing is first, the long leg of the LED is the positive side (the flat edge of the plastic housing is the negative side for when you have cut the leads and get confused), this end of the LED needs to go to the terminal the light bulb connects to near the plastic pin (leftmost upper pin in the picture above). If you are looking at the switch, you can see that the light bulb's wire scoots around the base of the light bulb housing on the positive side and loops around back to the pin. What we want to do is put the resistor in that nice recess beside the light housing, so, remove the bulb, put the led in its place and bend the LEDs legs around so you know where to solder the resistor. Now you you can either go to the effort of soldering your new wires to the terminals or cheat like I did, I cut off the light bulb's wires and soldered my LED and resistor to them so I could reuse the little wire holding notch on the terminal. If you look closely at the picture below, you will see that you are getting the benefit of hindsight in these instructions.
  6. This would be a good time to test things and make sure it all works. A nine volt battery should light up the LED nicely, assuming you don't have a fancy bench power supply to bang on the exact voltage. I have looked at a couple switches and the position of the brown ground wire seems to be consistant but since the original light bulb didn't care about polarity you may want to double check that the brown wire is in the correct place (or at least that the positive pin is the one you expect) for how we wired it up. I bet you wish I mentioned this before the soldering step. If you are rebuilding a switch doing something more important or whose operation is less obvious than a seat heater, this is also a good time to make a note about which terminals should do what when the switch is fully reasembled and fully test the thing before putting it back into the truck. The last thing you want is the diff lock to engage when you turn on the headlights.
  7. Now if you haven't run to the mercedes dealer or your local bosch guy to buy new switches after the last step, we can get to the trickiest part. Reassembly. You did test things back in the last step right? This is tricky because those little springs and ball bearings have to go back. The housing fortunately only fits on one way (the right one) so if it isn't sliding on nice you probably have things backwards. When you get the housing slid halfway on you will notice that there are holes on either side, crafty germans, you can put the springs and balls in the through the holes so you will only have to do it a few times to get things in. I used the flat end of a couple 1/8" drill bits to hold everything in place as I slid the housing together which definately helps. After getting the springs and balls in place but before sliding the whole shebang completely together, put the two metal wings on either side back on, they did fall off didn't they? The little wings are captive when the switch is completely together but there is room to put them in after the balls and before the final snap.
  8. You are now done, put the switch back in the dash, complain about how little slack there is on the wiring harness, and enjoy.

LED background, Why do I need to mess with resistors anyway?

Leds need a current limiting resistor lest they blow up, why, you ask? Well the short version is that unlike a light bulb that has a pretty high resistance once it is lit, an led has a tiny resistance all the time. The upshoot of this is that the led will gobble up all the current given to it which will kill the thing in pretty short order if you don't slow things down a bit. More and more accurate information can be found here.

Now that you know you need some kind of resistor, the big question is how big a resistor do you need. The answer depends on a tiny bit of math (Ohm's law) and a bunch of magic numbers.

Voltage Car Running (alternator): 13.8V
Voltage Car not Running (battery): 12.6V
Voltage Drop white 5mm LED: 3.6V
Min/Max Current for a bright happy led: 15-20mA

V=IR ( Voltage = V, Current = I, Resistance = R )
(V Supply - V drop ) / I = R
(13.8V - 3.6V) / 0.020A = 520 Ohms
(12.6V - 3.6V) / 0.015A = 600 Ohms

So it looks like we ideally want to keep the resistor around 500-600 Ohms. With 500 giving a brighter light and 600 giving us one that will last the longest. Empirically I found that a 820Ohm resistor will give the LEDs I bought about the same brightness as the original dash lights. If you want to save yourself the math, an online calculator is found here.

Normally LEDs have a rated life of around about 100,000 hours at which point they are (on average) half bright as they were new. So being near the high side of the spec shouldn't be too big a deal.

One note, resistors are rated to carry a max load in watts, this effectively means nothing in this application because the lowest rated resistors you are likely to run into are 1/4W which should be adequate, having said that I have seen lots of recomendations stating that you should not exceed about 60% of the resistor's rated capacity. If you are worried about it buy 1/2W resistors.