R129mine
Active Member
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.
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.
