SRS light on intermittently

[Greggo]

I have a 1999 CLK320 with 40K (low miles and service). The SRS light is usually on when I drive alone. When there is someone in the passenger seat, the following happens:
1. Passenger is under 60kg approx - SRS light on
2. Passenger is over 80kg approx - SRS light will start flashing after a while (25 minutes or so) and will clear if time is longer. Light will come back on next time I use the car after 10 or 15 minutes.

I have checked the connectors under the passenger seat and all seem clean, and wires in good shape. The MB independent says to bring in and spend $100 to put through a code check. I only want to spend $100 to clear the code after I fix whatever is failing if this is possible to diagnose through these symptoms. I have an SRS light clearing tool that came from China (worked a couple of times but no longer clears).

Questions:

Is it likely the seat sensor pad (I have read a lot about)?
Is it likely the wire connecting the seat sensor pad(I have links to how to attempt replacement)?
Is there a module under the seat that needs replacement?
I have heard about the child passenger sensor. Where/what is that and how easy to diagnose and replace?
What is a seat micro switch sensor? Could that be the culprit?

I appreciate any help with this annoying problem. I want to sell the car and dont want the buyer to experience this same problem

 

[mersum1es]

Most likely sensor wires. Usually brokes in place where they go inside the seat (small hole, hard to detect without seat removing. Sometimes need to try continuity test with multimeter and same time twist the wires in order to find out broken wire). Rarely sensor pad itself is malfunctioning, but possible yes.

Instructions for W210:
Photo DIY - SRS Seat Sensor replacement - Benzworld.org - Mercedes-Benz Discussion Forum

rather easy job, hardest is to dress cushions on again. You may also try 'cheat', if you have 2-wire sensor there can be simple resistor installation to get rid of the light, or here is 'emulators' available which emulates real sensors. Or you can buy a spare part, just plug it to the connector (instead of the sensor inside cushions), and if it helps choose do you want to buy or return it... Note: SOMETIMES the light must be reset with diagnostics.

 

[Alban]

you can get a cheap code reader to solve the problem.

 

[Greggo]

Thanks for the advice. I will try the multimeter test. I am sorry I didn't reply earlier since I don't have myself set up to know there are replies. I really appreciate the info.

By the way, I had a local mechanic use his Snap-On computer to check the status of the SRS and it wont communicate with the SRS module. It talks to all the other computers. Just an FYI if someone has additional ideas.

I don't really want to pay another tech $100 to diagnose if the computer is shot...thanks again..will update once I can try the twist and continuity test.

g

 

[mersum1es]

SRS module not responding while reading codes? Is the SRS light on continuously now? If so, check the fuses! (Hope it was STAR diagnostic, some universal diagnostic devices just will not work with MB properly...)

Usually you'll see broken wire immediately after you remove the seat and cushions. You can also try to install a jump wire from sensor to plug, and then test by connecting all wires and powering up.

 

[Greggo]

So far I have tested continuity and have moved cables around to see if they have weak spots(from the junction box to the sensor):

• seat belt sensor - OK
• passenger seat sensor - OK
  On the passenger seat sensor, the resistance is 67 and 97ohms between the single post and each of the posts that are paired. There is no change when the seat is occupied (and the sensor is unplugged).
  
  I will try checking the cables from junction box to ECU brain (under seat???) next.
  
  Also, does anyone know what is supposed to happen (sound/alarm/flashing SRS) when the passenger does not buckle belt? I was going to try that to see if the belt sensor is truly communicating with the srs brain (whereever that is).
  
  The tester my tech used was a SNAPON brand that worked with all the other systems in the car (engine, brakes) but did not talk to the SRS computer. Tech said the computer must be gone (not MB tech).
  
  I have read elsewhere that I can disconnect negative battery lead for a few minutes and it will reset SRS light until it redetects the faults. Can anyone verify that?
  
  Thanks for all the ideas...

 

[Greggo]

I checked driving around with seatbelts off. No additional alarms or lights went off. This suggests to me that the computer doesn't work at all. Does that seem right? And where is it.
Does anyone think I should get a new one and where to get one?
Or should I go find an indie with a star tester

 

[mersum1es]

Not sure about the seat belt buckle alarm existence, my W210 does not have alarm at all.

Disconnecting battery usually does not reset the light.

The 3-wire sensor is giving PWM pulses, I think you cannot measure occupancy via resistor value.

Did you check all fuses?

 

[Greggo]

I forgot to mention that I checked all the fuses (3 spots) side of dash, under hood, trunk. Checked fuses that relate to srs and airbag and belts and they all are ok.