SBC causing non start

fastroad

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I recently decided I would add a Mercedes to my list of cars owned. I went out and opted for a real beast SL55 Amg.

I got the car home and the next day decided to take it for a real road test and to my surprise the car would not start. I had the RAC come out to check the issue but did not manage to get it started and told me there was a voltage issue.

I had the car taken to an independent, they carried out a few checks with different diagnosis over two day and finally found the SBC unit making a ticking noise and decided to unplug it and the car started.

My questions are would a faulty SBC unit cause the car to not start and Drain the battery?

Can the SBC unit be repaired or is replacement the only choice?
 

clk320x

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There are companies who refurb them but after I thought about it I would just get a replacement from the MB dealer. Not worth risking it on such a critical component.
 

Gbrowncls55

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Just had the SBC unit replaced on my wifes r230 cost about £1300 plus VAT in total as I remember. Cant really see why it would stop the car from starting though. Did it turn over but not fire. Or did it not turn over at all. Given the cost of the SBC unit it would be worth getting it on STAR at an MB dealer first.
 
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fastroad

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All the ignition lights came on as normal but would not turn over at all, the fan would come on full speed but nothing else would happen.

After unplugging the SBC unit car started without an issue. Not sure if it was sending out faulty signals which prevented the car from starting. I have searched the net and not found anything as of yet.
 

Vick N

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There are places that specialise and can rebuild them for a lot less than new.

I bought a spare working item in case.
 

flowrider

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A few companies will rebuild the SBC pump for about £400 making it as good as new.
 

mersum1es

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All the ignition lights came on as normal but would not turn over at all, the fan would come on full speed but nothing else would happen.

After unplugging the SBC unit car started without an issue. Not sure if it was sending out faulty signals which prevented the car from starting. I have searched the net and not found anything as of yet.
Sounds like grounding problem, fan at full blast means no sufficient voltage to ECU, perhaps faulty SBC unit caused some kind of ground loop which messed some other signals?
 

grober

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Normally these SBC systems have multiple level redundancy meaning you should get lots of warnings before total failure. Either these have been ignored /reset/ cancelled or there may be something else wrong with the engine canbus system [ low SAM voltage] for example . These canbus systems on older Mercedes are prone to water ingress so you may have multiple faults. Unfortunately the diagnostic unit may not communicate properly with certain systems in the car unless the engine is running- catch 22--- Did your independant find any stored fault codes?
 
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fastroad

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The Sam unit had recently been replaced on the vehicle, which leads me to believe it previously had a few faults due to water ingress in the boot. I believe there is some stored faults but my independent said let’s get the SBC done and then check what faults it sends out. My independent thinks the faulty SBC system is causing all the current fault codes and voltage issues as with it not being plugged in there is no voltage issue and the car starts and drives fine (minus the braking).
 
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fastroad

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I managed to get down to the problem or should I say multiple problems. What we thought was originally only the SBC system ended up including a faulty ECU and very common siren which had become faulty. After having all three of these issues sorted the car drives like a dream.
 

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