K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: Augv 18, 2005)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

Seat Belt Extender
About one month ago, the driver's side Seat Belt Extender (SBE) tried to reverse itself while it was not extended and as a result, it managed to damage itself. It made such a racket for 20 seconds and after it stopped, it made a loud 'clunk-clunk' sound when the arm extended. It still operated OK if you could live through the noise but who knows how long it would last.

I've had this car for 6+ years and the SBE has never malfunctioned. The way this happened is an interesting story. My wife got into the car on the passenger side as she normally does and started the car by leaning over and turning the key, something she has NEVER done. I was not in the car and did not close the driver's door fully; it was closed but not 'latched'. The 'S' cars have an automatic feature that 'pulls' the door closed tightly when it's triggered.

The normal operation of the SBE requires that you be in the seat with the door CLOSED and the ignition key 'ON'. Then the SBE will extend the seat belt. When I got into the car, sat in the driver's seat and closed the door, the SBE didn't extend, instead it started to retract. Since it was already retracted, it proceeded to 'grind' the gears for about 20 secs. It obviously 'thought' it was extended, thus the damage to the gears. I'm pretty confident that this chain of events caused the problem but I'm not about to TEST that theory now that it's fixed! Yes, I asked her to NEVER do that again!

Yesterday, I took it apart to see what could be done; after getting it apart and removed from the car, you could plainly see broken teeth on the drive gears. A call to the local MB dealer produced a lot of shock. . . replacement is $560 plus tax, about $600!. There was absolutely NOTHING wrong with it except for the damaged gears. So I decided to see if I could find a 'junked' unit and get the gears from it.

140 SBE in normal operation

After calling a few places, I found a SBE from a W124 chassis which I got 'free'. Now you can't beat free! So I took it home and after comparing the two assys, found that the gears were ALMOST identical! Great happiness! The ALMOST will be explained as I go on.

The W140 part number: 140 860 01 82 for the drivers side. The passenger side unit's part number is different.
The W124 part number: 124 860 08 82 for the passenger's side. The driver side unit's part number is different.

Now the good news! The gears from either can be used in the other unit! So I proceeded to take it apart and remove the two needed gears. After inspecting them closely, I found that the small gear was slightly thicker than needed. Since they are made from nylon, it was a simple matter to sand off the excess to make it fit perfectly.

The other problem is that the second gear spindle is a little smaller that the original. There was no solution for this but it didn't retard the operation since the two gears are under tension when assembled.

SBE Removal
The following disassembly instructions refer to the drivers side. In order to repair the SBE, the rear seat side panel must be removed to gain access to it as the SBE assy is mounted to the car's sidewall for stability.

To start, remove the rear driver's side seat bottom. This gains access to the seat back which is held by one bolt (#10). Remove the bolt and lift the rear seat cushion out. Now you can see the complete side panel. On the car's frame at the bottom of the panel, remove the screw that holds it down. Next, with a proper panel puller tool, remove the leather panel cover. It unsnaps easily.

Now you will see two (2) screws near the 'handle' that hold the panel to the side frame; remove these and start to work the panel loose. As is the custom with Mercedes, the panel is held by 'clips' on the top side. So you need to gently lift the panel 'up' and out.

Once that panel is removed you can now see the SBE unit. Remove the three (3) screws that secure the unit to the side wall of the car and unplug the cable from the electronic control box located just below the SBE on the side wall. You should now have the complete unit ready to 'operate' on your work bench.

Bench Repair of SBE
With a little inspection, you will see the damaged gear(s). For my unit the smaller gear as shown below, was 'shaved' bald for four (4) teeth.

Broken Teeth in Drive Gear

Broken Teeth in Drive Gear

After inspection, the main drive gear had two (2) 'chipped' teeth most likely due to the scraping action that occurred when the unit reversed itself. I hadn't planned on replacing the second gear but after seeing the two 'chipped' teeth, I decided that it needed to be replaced since it could catch and cause the same problem to occur.

It's a simple matter to remove the parts from the W124 SBE to use in rebuilding the W140 unit. The picture shows a 'recreated' version. On close inspection, you can see that both gears are from my 140 SBE assy. I put them back into the 124 SBE assy for comparison so you can see how they go in.

W124 SBE used for 'parts' to rebuild the W140 unit

First loosen the spring so that it can be removed. Then loosen the two (2) screws that hold the Limit Switch. Now you can lift out the gear assy in the 'black' housing so that you can remove the two gears. After that, it's a straight forward task to reuse the gears in the 140 unit and restore it to perfect condition.

W140 SBE diagram showing major parts

To replace the gears, you need to remove the 'spring tensioner'. To do this, push with a large screwdriver until the block releases the spring. Then loosen the screw that holds the 'gear assy' (black plastic) and replace the gears. Then just re-assemble the SBE making sure to plug-in the cable to N42, the SBE Controller module.

Update; 8/18/05
Received an email stating that replacement gears are available from George Murphy at Performance Analysis Co. He makes metal replacement gears for these units, a very good ugrade from the plastic ones. Price is about $60. Contact him at 865-482-9175 from 9am to 5pm ET. Click here to email him directly.

SRS Light is ON
Emer Tensioner Retractor showing the 'squib' connection
When I removed the side panel, the R12/1 squib connector to the Emergency Tensioner Retractor (ETR) was jarred loose. This connector, with red colored end, is not like the typical Mercedes connector in that it does not have 'clips' that lock it into place. It just plugs in and it can be easily jarred loose.

When I tested the SBE, the SRS light illuminated and stayed on. After reading the SRS codes (MENU #11), it pointed to the ETR R12/1 squib. Looking at the ETR, the loose connector was spotted. After plugging it in and performing a RESET, the SRS light when out. For the SRS system, you MUST reset the codes since once tripped, only resetting with the proper tool (a scanner) will the SRS light go out.
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