Main
Tuesday
Jan142014

Mercedes G500 Differential Locker switch pack fix

Overview: This article describes how to do a simple modification on the G500 diff locker switches to eliminate a design problem that causes the lockers to drop out randomly.

The Mercedes G500 diff lockers are controlled by a box on the dashboard. Let's call it the Switch Pack. You will find it in the dashboard above the radio. Here is what it looks like:

This box is a bit too clever for its own good. It has a complicated logic system that forces you to select the lockers in sequence: center. then rear, then front.  So for example, you first press the middle button to select the middle locker. A yellow light comes on to confirm you selected it. When that locker eventually moves into position, which might take a few seconds depending on various things, then a red light comes on to confirm the locker is activated. After you select the center locker, you can then select the rear locker, and another yellow light will come on to confirm you selected it. But power will not be sent to the rear locker until the red light has come confirming the center locker is actuated. And the same thing for the front locker - you can select it after you select the rear locker, and a yellow lamp comes on, but no power is sent to the front locker until the red light for the rear locker comes on. 

None of this is usually a problem. You probably want to activate the lockers in that sequence anyway, and it is probably a good safety idea to prevent you to just going straight for the front locker, since if you did that by mistake you might be surprised by the sudeen lack of steering and have a crash. So it sort of makes sense. 

Unfortunately, it also introduces a failure mechansim of its own. Suppose that the center is locker is working just fine, but the sensor switch is dirty. The red light will never come on (no big deal), but that will cause the Switch Pack to not send power to the other lockers. That's unlikely to happen you might be thinking, but in fact it is exactly what happened to me on one my first trips in my G500, and it caused me to get badly stuck in a salt-flat in the middle of nowhere in Mexico. So I became highly interested in fixing this problem. I came up with a very simple hack to prevent this happening again. Here's what you do:

First, remove the Switch Pack from the dashboard. The proper way to do this is to first remove the wood trim from around the radio. On my 2002 G500, you do this by opening the ashtray and pulling on the wood trim. It is held on by four spring clips. Those clips will probably fall out and drop down behind the radio where you will never find them, so you might want to have four spares handy.

 

Once you have the trim off, you can reach up and push the Switch Pack out from behind. It is only held in by some springy metal tabs. Undo the connector at the back, and bring the Switch Pack somewhere you can work on it.

The Switch Pack can be opened up by pressing on the plastic tabs while pulling the two halves apart. You need six hands to do this. There is probably a factory tool for it. I found it easier just to bust all the tabs, it seemed to go back together just fine without them.

Once you have it apart, pull the circuit boards out and look at the board with the switches mounted on it. You are going to remove two transistors.

If you are a bit of a slacker like me, you can cheat by just unsoldering the center leg of each transistor and lifting it up with a screwdriver...

If you do it like that, it's a bit easier to put it back if you change your mind (you won't though).

Then put it all back together. Now the Switch Pack will send power to each locker as soon as you select it. It doesn't care anymore if the previous locker has engaged or not. So if you have a faulty sensor switch, you can still use all the lockers.

After I came up with this hack (back in 2005) I did a bit more investigating and discovered that my sensor switches were in fact OK. The fault was actually inside the Switch Pack. It was reading the switch incorrectly. It turnes out there is a design error in the Switch Pack that leaves the sensor switch inputs "floating" (ie not tied up to the postive supply via a resistor). Since the input is a CMOS gate with extremely high impedance, that means the input can just float around to any old voltage randomly, which indeed it will do. You will know it is happening because sometimes when you select all the lockers you will hear a random on-off-on-off clattering of the diff locker vacuum solenoids on the other side of the firewall in front of the driver. Accompanied by the lockers dropping out randomly because of the reduction in vacuum this causes. 

So if your lockers are dropping out, and you hear those solenoids clicking on and off for no reason, this mod will fix that for good.

Reader Comments (1)

dr om,

thanks for your article regarding the differential lock switch package. I own a 1999 G500 that is soon going to be a radio expedition vehicle.
Unfortuneatly, it has a faulty A463 545 03 32 switch pack. I was told by the previous owner of the car that coofee was spilled into the diff lock switch unit. The symptoms is that (with no switch actuated and with the ignition off) the front axle yellow locker lights up for request but the vacuum pump doesn't start. I believe the front axle locker will engage though if manifold vacuum appears. I did not test this.
When I press the center diff switch, the vacuum pump starts.

I am well used to SMD work and opened the switch pack for inspection and realized that someone already had cleaned it. I believe that some of the CMOS inputs may be pulled high (or low) caused by the semiconductive residents of the coffee. Do you know if any schematic diagram exists somewhere?

73 de Tord

March 2, 2014 | Unregistered CommenterTord

PostPost a New Comment

Enter your information below to add a new comment.

My response is on my own website »
Author Email (optional):
Author URL (optional):
Post:
 
Some HTML allowed: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <code> <em> <i> <strike> <strong>