R129 - 1996 - Fuse 13 - Multi relay intermittent issue

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R129mine

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Joined
Feb 13, 2007
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676
Location
Buckinghamshire
Car
R129 SL500 from 96
A few months ago once, I noticed the wind screen wiper did not work, and at the same time my indicator only worked when turning left, cigar lighter (would indicate 7volts, I have a volt meter attached to my cigarette lighter) and seat heater would not work and the SRS would flicker dimly. This would usually only happen when I would start the engine and would usually start working after about 15-20 mins and sometimes when trying to start the car immediately again.

As my battery was nearly 7 years old I replaced it but this did not stop the issue (before battery change I would read about 13v while driving and after the change it is now 14-14.2 volts)

I had the car in for a service 2 weeks ago and suggestion was made to replace the multi function relay which I did

All was fine for about 2 or 3 days when the fault reoccured and it has been happening intermittently since, initially it would only occur when starting the engine but this weekend it happened after about 45 mins driving when I came off the motorway (it is not fun having to do a 3/4 round about with a non functioning indicator and having to apologise as you drive along)

I checked the fuses and they are all okay but replaced those related to the multifunction relay anyway. There is no visible corrosion or dampness.

I checked all the fuses and read the voltage
- No13- SRS check light, cigar lighter, heated seats, rear window defroster) measures 7.7 volts
- No12 - wiper control, turn signal control - 7.7 volts

there were 2 other fuses also but cannot remember which ones but voltages was 7.7 (my guess is it was fuse no 25 and M1 (combination relay)

All others are 12 - 13 volts

I have found if I remove fuse no 13 all other circuits work fine again and this issue does not happen, I think the issue is with one of the circuits connected to fuse no 13 (nothing too complicated then :)

While researching, I found this post from Honestjohn which describes my exact issue

Mercedes-Benz Umbraco.Web.PublishedCache.XmlPublishedCache.XmlPublishedProperty (Umbraco.Web.PublishedCache.XmlPublishedCache.XmlPublishedProperty - Umbraco.Web.PublishedCache.XmlPublishedCache.XmlPublishedProperty)

21-5-2017: Report of intermittent electrical problem with 1996 R129. Every few weeks SRS warning light on the dashboard comes on faintly while driving and when this happens ,the offside indicator, the wipers and the soft top motor don't work. Fault always disappears by the time the owner gets it to the garage. Will be a problem with one of the Signal Acquisition Modules (SAMs): http://www.mercedesmedic.com/mercedes-sam-module/ To trace the fault you need a very good MB SMM unit specialist. Googling <MB SAM Unit Specialists> doesn't immediately find any, but will help you understand the problem.

He is talking about the Signal Acquistion Module (SAMs) but I cannot find anything online on the Sam module for a r129, it seems that the later Mercedes modules have these

My questions is 2 fold

1) does a r129 from 1996 have a sams module and if so where is it located?

2) any good tips of further diagnosing the root cause, I can test the resistance on the fuse 13 circuits but a wiring diagram would probably come in handy so I can figure out where the wiring goes, in the past I have had an intermittent open circuit on the SRS circuit which was cleared about 7 years ago, perhaps this is now a reoccurrence but this time worse and the issue is with the SRS control circuit?

I would not mind have a specialist have a look at it but would like to do some of the donkey work before taking it to my local specialist.
 
Unfortunately the info you find in HJ is just as likely to be completely incorrect. I don't believe there's much checking and corroboration going on, which means that a one-off failure suddenly gets portrayed as a scaremongering epidemic.

The R129 never really received CANBUS in the accepted sense. IIRC it was fitted partially towards the end of the life to drive the ASSYST servicing and instrumentation but didn't run the rest of the car.
 
I had a W124 coupe with similar issues, advised to swop out relay N10 I believe it was from memory, it was an 1989 model mind you.

P
 
I had a W124 coupe with similar issues, advised to swop out relay N10 I believe it was from memory, it was an 1989 model mind you.

P
i had the multifunction relay replaced, did not help, i believe the part number is 140820 7126 (hazard / wiper relay) and it used to be the N10 in pre 96 ? models

I have seen pictures of the r129 from "93 with the N10 relay but seems to have been changed to this new relay in later years
 

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Your 140 820 71 26 is for a car intended for towing, as illustrated with the car and caravan stamped on it, did it or does the car have a towbar?

I would have expected the one below if there is no towbar on your car.


N103.jpg



Dec
 
Your 140 820 71 26 is for a car intended for towing, as illustrated with the car and caravan stamped on it, did it or does the car have a towbar?

I would have expected the one below if there is no towbar on your car.

Dec

no towbar but fuse 13 (that one again) does have the ability to handle a trailer coupling, probably just a case of sharing components between the w140 and r129 models

r129 fuse card - Google Search:
 
yes all speeds, also the indicator stopped working even when wind screen wiper not switched on and cigar lighter would not work either, nor heated seats and rear window defroster

Replaced it with a new relay so I have to expect it to be not an issue

It is clear to me it is a circuit on fuse no 13 (so not the wipers nor the indicator lights are at fault) but
  • cigar lighter
  • srs control circuit (that is going to be the difficult one as i don't know where it runs)
  • heated seats (probably not that difficult to get to via the chair)
  • rear window defroster (easily discounted)
will try with an Ohm meter when i get a chance and when the fault presents itself

does anyone have a electrical diagram so i can figure out where the ground/earth is for these circuits?
 
ISSUES RESOLVED!! :D:D

I noticed a drain of my battery when idle (flat battery over Christmas period when I didn't use the car for 5 days)

Did a test with multimeter and found amp draw of about 0.3Amps
When I removed fuse 23 (convenience relay and ADS) it dropped to 0.04Amps (much better)

As the car was going in to the bodyshop for some body work (after accident) I asked the Garage next door to investigate the cause

They found that the "open seat function" was causing the problem with the excessive Amp draw, they also suggested that this caused the problem with the circuits on fuse 13 (I had the multi function relay replaced back in Oct/Nov 2017 but this not solve the issue)

They were really good about it and offset the cost of the needless replacement of the Multi function relay against the hours worked on the fuse 23 issue.

3 weeks later the issue has not returned and have fully functioning wiper/indicator/cigarette lighter and seat warmers and no more flat battery.

I am still baffled about the convenience relay and the open seat function, it supposedly moves the chair back when you open the car, but I did not even know this was a feature in this car so not having it anymore is not a problem
 
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