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  #1  
Old 03-16-2005, 08:26 PM
sixto's Avatar
smoke gets in your eyes
 
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W140 SRS question for JimF

Jim,

Is it possible to pull SRS codes with a DVM or duty cycle meter?

Thanks,
Sixto
95 S420
87 300SDL

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  #2  
Old 03-17-2005, 09:49 AM
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sixto, I'm not JimF, but I saw your question. I hope you don't mind me jumping in. I had an SRS light last year in my 95 S600. Turns out the SRS system in this car is digital. The signal pulses from digital systems are very fast. A DVM will not react fast enough. I borrowed a very fast digital portable scope from work, and tried to take a reading of the output for the SRS module. I could not get a reading. I tried all sorts of time / frequency / amplitude seetings, all with no luck. I finally broke down and bought a scanner. The scanner read the code without problem. Faulty passenger seat occupancy sensor.
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  #3  
Old 03-17-2005, 10:04 AM
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I too am not Jim, but why not just build use an impulse code reader - it's pin #30 on the 38 pin connector.
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  #4  
Old 03-17-2005, 12:49 PM
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'94 S500: only 793 sold!
 
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Hello, Sixto . . .

Quote:
Originally Posted by sixto
Jim,
Is it possible to pull SRS codes with a DVM or duty cycle meter?
Thanks,
Sixto
95 S420
. . I am JimF and just saw your post.

pcmahar is correct, the system is digital and needs a scanner to read out codes. So I take it that your SRS light is on??

Personally, I always said that one SHOULD be able to read the 'pulses' with an oscope but it MUST be one that can be triggered by the 'start' of the event you are trying to measure AND that it is a STORAGE scope. You set it up to start a SINGLE sweep from an external trigger, so you see a single 'flash' on the screen. Then you recall the 'event' and can now count the pulses.

But when I set off my SRS light (during my fix of the SBE, MENU#25), I used an AST Retreiver to read the code (just one, "#3") and then reset the light and code. The SRS system is DIGITAL in my car, so impulse reader won't work nor will a DVM/DC meter.
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  #5  
Old 03-17-2005, 01:17 PM
sixto's Avatar
smoke gets in your eyes
 
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Thanks for the input. Jim, I wish I understood that scanner and scope stuff you described I'll run it by a sparky buddy and see if he has access to the right equipment.

Yes, the SRS light stays on for about 2 minutes after each start.

I was going to check each of the buckle switches and the passenger seat occupancy sensor and hope it's one of those.

Are scanners generic or are there specific ones that work with a 95 S420? Are they $50 items or $250 items?

Thanks,
Sixto
95 S420
87 300SDL
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  #6  
Old 03-17-2005, 02:24 PM
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'94 S500: only 793 sold!
 
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Location: San Diego, CA
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You are . . .

probably correct in the diagnosis; a bad driver's seat belt module. The symptom is that it stays on longer than it should. But to be sure you need to dump the codes stored in the SRS module.

Are scanners generic or are there specific ones that work with a 95 S420? Are they $50 items or $250 items?

I wish they were cheaper but a "Mercedes" type scanner (one that uses the 38 pin connector) is about $350 - $500.

One that uses the OBDII connector in '96 and on cars, can be had for $150 or so. Personally, I would not recommend a OBD type scanner because of the universal 'cryptic' codes they produce. By definition, that have to be crytic so that you can get the same code for ALL cars.

Best bet is to find somebody who has one. My tech has an AST Retreiver w/ the 5724F cartridge ("brain") and it is great. But it's $2200!
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  #7  
Old 03-17-2005, 06:12 PM
Jack
 
Join Date: Sep 2000
Location: Sugarland, TX
Posts: 285
The last time when my SRS light stay on for a few min after startup, the problem was the passenger seat belt buckle. I was lucky, it was replaced for free by MB.

Jack
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  #8  
Old 03-18-2005, 12:35 AM
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Location: fairfield ca
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srs light

if your near american canyon i will read and reset the code for you i have the scanner.you can call between 8 and 5. 707-643-8746.we do this for free for anybody.

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