In this article I describe what I did to troubleshoot battery issues leading to discovering a common issue with a Mercedes Rear SAM (Signal Acquisition Module) on a 2008 ML550 (W164).
Background
I was getting intermittent electrical issues. At times I would get a fault on the right rear brake lights and turn signal lights. I was troubleshooting the issue by swapping/changing bulbs. I had mixed results until I had some significant battery drain issues as well. In the end, the car completely died. The car could not be opened using the key and when I finally got it opened, using the mechanical - spare key and charged up the battery, the car was complaining of an SRS error and some functions were not working. This occurred right at the peak of the Covid-19 pandemic - so nothing was open to assist with any repairs... Time to roll up my sleeves and see what I can find.
Background - 2021 Dec 23rd – I was driving a long distance and when I attempted to test my high beams at a stop (left lever forward), and all my lights went out? I then started driving and suddenly, my headlights went out while driving down the highway with the headlights set on the "automatic" setting (all my lights went out in a snowstorm to boot!!). I pulled on my “high beam” lever and found that the lights would come on… I fumbled around and was able to manually get the lights to stay on while driving by turning the light switch to the right and I pulled out the switch to activate all the lights. With the lights now back on, I then pulled over at the next gas station, fuelled up, and restarted the car and limped the car back home. The next day I went to start the car and it was totally dead. Thinking it was a bad battery. I attempted to jump start the car and it started but with the following issues:
- No remote key function to unlock doors, open hatch, etc.
- No rear hatch function (from key, door handle or front release)
- Gas gauge not functional
- Turn signals not operational
- Backup camera not functioning
- Lights intermittent on auto (had to limp car home previously using manual lights – turn to right- and pull-out switch)
- ….etc.
Also, the car was showing an SRS fault – Must take into dealer for repair $$$$$:
An "SRS" Supplemental Restraint System Error
After doing some research on the “SRS error”, I discovered that this is usually due to a low battery. However, there is a common issue with Mercedes vehicles where moisture gets into the rear SAM unit causing shorts that drain your battery. Given all my electrical issues I was having - it all pointed to an issue with the rear SAM unit. The rear SAM unit on the ML550 is located in the instrument panel, under the fuse box, near the right rear tire.
Unfortunately, after I parked the car - I could not get into the car with the key remotes – so had to use the emergency key to open the car:
Checking the Battery
So, I charged up the battery – thinking that it may be a bad battery or alternator. I checked the resting voltage and with car running… All indications were that the battery was good??:
- Resting voltage: 12.58 volts
- Running voltage: 13.85 volts
How To View Battery Voltage
Note – this was done using voltmeter. But also checked using hidden menu:
- Show the odometer on the menu
- Turn key to position accessory function (first position don’t start)
- Tap “R” button 3 times quickly:
- Then turn to position 2 to check the voltage under load (it will drop slightly)
- Then turn on the car and you will see if the alternator is working (it should raise to over 13volts)
Checking the Fuses
I then checked all the fuses to see if it was a fuse issue... and I did not find any issues? I checked all the fuses impacting the SAM units (front and rear) and they all appeared ok (fuses under the passenger seat as well as those in the rear).
Checking the Alternator
Firstly, I wanted to be sure that my alternator was functioning correctly so I tested the voltage on the lines two ways: 1 - I checked using the instrument panel and 2 - I used a volt meter. Both these tests test the voltage with the car on and off.
Since the car was at 12.58 volts with the car off and 13.85 volts when running - the electrical drain is not from the alternator as it appears to operating correctly.
Here is the video doing the alternator test using the car's instrument panel.
Note - in this video you can see that the Gas gauge is not functional and the SRS error light is on.
MB Star II - Trouble Codes
Once I had the car running, I could use my MB Star II scanner to check for trouble codes. There were 20 fault codes being thrown.
Taking a look at all the trouble codes being thrown, suspicion was that there was something wrong with the SAM units. I checked the front SAM unit under the passenger seat, checking that it was getting power and it looked fine and then moved to the rear SAM unit. Unfortunately my MB Star II is great scanner but not a programmer so I will not be able to dump the config and program a new unit if required.
Rear SAM Unit – this turned out to be the culprit!! The car could not communicate properly to the SAM (Signal Activation Module) unit in the rear of the car. It is located under the rear fuse box on the right side in the rear hatch:
Here is the fuse box (SAM is located below this):
Trying the Connections on the Rear SAM Unit
Here is what you are looking for below the fuse box (Delphi SAM unit):
My first attempt to resolve my issue with the now suspect rear SAM unit was to simply check all the connections on the rear SAM unit. I inspected the unit but since the unit is a "closed" unit and there were no obvious signs of moisture or corrosion. Re-seating the connectors did result in getting the car working again and I thought that I was out of the woods. Here is the video.
This worked for a bit - but was short lived. After operating for a while the electrical gremlins came back. So time to take a closer look at the rear SAM unit.
Removing the Rear SAM Unit
I decided that I needed to move to the next step - removing the SAM unit and do a closer inspection of the internals to look for signs of moisture and/or corrosion. Here I am removing the Rear SAM unit
Repairing the Rear SAM Unit
With the Rear SAM unit removed, I had to open it and take a closer look. Upon a closer examination, I was able to see that the rear SAM unit had, in fact, indications of water damage along with some corrosion. This was definitely likely the cause for all my electrical issues. Time to try and clean/repair the unit.
With the aide of a magnifying light, some 95% isopropyl alcohol and Q-tips I attempted to clean up the board.
I was relatively successful at removing the majority of the corrosion, however there we indications of some deep scaring from the moisture damage.
Here is a video of the cleanup of the Rear SAM unit.
This cleanup fixed the majority of the issues and the car could now be driven. I drove in this mode for several months with the SAM unit pulled out of the wheel well to try to avoid as much moisture as possible. I still have some issues with my right rear turn signal and have lost my gas tank level gauge but can safely operate the car for now. I suspect that I need to clear faults not only in the rear SAM but also in the front SAM that talks to the rear SAM unit.
2023 Update - Ordered a New SAM Unit
In the start of 2023, after several, several months of operating without a gas gauge and right rear turn signal, I have decided to order a brand new SAM unit from China in an effort to clean up all my remaining issues. I could go to the wreckers and try to source a unit but you have no idea whether they would have water damage or not. So I "bit the bullet" and sourced a new one from China for about $400 (the price has nearly doubled during the world wide chip shortages).
Technically, I might still need to find a way to get the new SAM unit programmed once it arrives??
My new SAM unit has all the connectors - ie includes the trailer hitch option in case I eventually want to add this capability. Here is a generic image of the unit showing all the connectors.
North American cars do not use connectors A & H - hence why my original SAM has a cover for these. Since my unit is generic - it should come with all the logic/connectors. I will just cover the A, J and H connectors with Electrical tape for now to avoid collecting any excess debris/moisture.
My unit had a J connector but it was not used. The GL (or 251 models) also have pop out electrically controlled rear pop-out windows that are controlled by the rear SAM (commonly referred to as code 848 devices).
Jan 24, 2023
Big day today! The new replacement SAM unit arrived and I hunted around and located my MB Star II scanner.
Next steps are to read existing codes and clear them on the existing SAM (front and rear) then see where I am at.
in my research, it may be possible to simple plug in the new unit, clear any codes and go from there - time will tell?!
Here is the instruction sheet that came with the new SAM unit
I am heading towards the right side replacement approach - i.e. losing any other config data as I do not have a MB programmer.
Error Codes - Current
There are 12 codes currently on the Rear SAM unit - some of which a permanent and may be able to be cleared. Some are "current" and stored - so likely cannot be cleared. Some of these codes may be an artifact of occasional ground faults and battery discharge / jump issues as I wait to install the new SAM unit..
There are 3 codes thrown on the Front SAM unit
Read the Datastream
Next step is to read the datastream on the Rear SAM - in the case that when I swap with a barebones SAM unit, I will know what the current settings are for the existing unit as I am not able to dump and load the config with just a scanner. This may be of interest in case I want to see what the new unit defaults are set to. What I found is that the datastream is showing zeros "0" for the fuel tank levels - hence no fuel gauge.
Installing the New SAM Unit - Success!!
The moment of truth has arrived - time to install the new "barebones" SAM unit and see what we get. I "hot swapped" the unit and cleared all DTC's. Right away I could see that my right turn signal issue and right rear brake light issues appear to be gone. But I still have a gas gauge that is showing empty - but the orange light for the gauge appears on startup (something that did not work before). I checked the datastream for the Rear SAM and it is now showing values for the left and right gas tanks. I suspect that the gauge may need time to calculate a range of gas remaining. When I look on the front console for the range I am getting "----" so still hopeful that I don't have to reprogram the SAM unit to recognize the resistor levels for the gas tank!
The car key is still recognized and is able to lock/unlock the car.
Also, I am getting DTC's for open circuits for the rear side windows. This is due to the fact that I cannot program the SAM unit and it has the logic for a 251 or GL model that also has electronic rear side windows. This unit was not supposed to support these (code 848) but appears to have the logic enabled by default?? My ML550 does not have this option so I will need to just ignore these DTC's for now.
Setback - Not Out of the Woods
Drove to play hockey and when I came out the car would not start. Eventually got a boost and parked the car and charged up the battery for a day. I suspect that all the computer work replacing the SAM along with all the ground faults in the original SAM unit led to weakening the battery.
Once the battery was charged up - drove and so far so good….
Potential Root Cause for Battery Drain
The problem is the SAM unit needs to be programmed.
“If the new SAM has additional options that receiver does not, the rear SAM can drain the battery because it'll keep polling for the missing modules. Towing package is one, since the rear SAM is responsible to shut down modules that it is in control of when you turn off the car. No towing package installed, but the SAM is coded for it? It'll just keep polling for it, and timing out...which can take 30 min to 45 min, which drains battery.”
What this means is that I will have to do "Variant Coding" i.e. turn off modules not in use (such as rear side windows - if enabled) as well as initialize the Gas Gauge.
Need to find someone to program (Variant Coding) the SAM unit or Copy Original Configuration?
Note that the SDS they use must be a "Chinese clone" version to have "Developer Mode" enabled. Official MBZ SDS won't have “Developer Mode” and it won't be able to change adaptations. Also, the Developer Mode screen are usually in German only, so you need a good translator. What folks usually do is print to a PDF, then cut and paste the text into Google Translate. It gets about 80% of it (it's not so good on some technical terms). Basically, I need someone with an Autel, Launch X431 or eCom Box.
Phoned a local shop that specializes in electrical repairs and even they are nervous and will have to get back to me on whether they think that they can do the copy and/or variant coding.
Time to Find the Leak and Plug!
With the SAM unit replacement done (still underway), it will be time to get some silicone and plug any and all areas of water ingress to the area to avoid corroding the new unit. I will need to likely silicon around the right rear light console as well as any other areas where water might be able to get in, trying to keep the wheel well dry. I may need to check all the way back to the sunroof as I have heard of cases where a leak in the roof seal allowed water to travel along the wiring harness all the way to the back of the vehicle.
Additional Info
Here is what the Rear SAM unit Controls - The 251 model is the GL version.
Control
· Exterior lights function, master control unit for the turn signal functions
Model 164.1
Model 251
· Interior lights (IBL)
Model 164.1
Model 251
· Wiper system (WSA)
Model 164.1
Model 251
· Heated rear window (HHS)
Model 164.1
Model 251
· Central locking (CL [ZV])
Model 164.1
Model 251
· Anti-theft alarm system (ATA [EDW])
Model 164.1
Model 251
· Power windows (EFH) (pop-out windows only)
Model 251
Switching of consumers
• Left tail lamp (complete)
• Right tail lamp (complete)
• License plate illumination
• Center high-mounted stop lamp
• Trailer illumination (with code 550)
• Left/right rear door inside handle illumination
• Left/right rear door exit lamp
• Load compartment illumination
• Right rear door central locking motor
• Rear-end door actuation central locking motor
• Fuel filler flap central locking motor
• Rear door locking unit
• Alarm signal horn (USA)
• Alarm signal horn with additional battery via interface (with code 551)
• Lumbar pump (with code 404 or 405)
• Rear-end door ambient lamp (model 251)
Scanning of switches, pickups and sensors
• Circuit 15
• Circuit 54 (stop lamp)
• Fuel tank sensor 1
• Fuel tank sensor 2
• Driver door actuation central locking motor, rotary tumbler
• Front passenger door actuation central locking motor, rotary tumbler
• Left rear door central locking motor, rotary tumbler
• Right rear door central locking motor, rotary tumbler
• Rear door locking unit
• TAG rear-end door lock contact switch
• Signal radio LF radio antenna interference filter
• ATA [EDW] hood switch (with code 551)
• Automatic loading floor switch (model 164 with code 942)
• CL [ZV] radio receiver
• Alarm signal horn with additional battery via interface (with code 551)
Note
In vehicles with code 551 the towing sensor (ATA inclination sensor) for the ATA function is integrated in the rear SAM control unit (N10/8).
In vehicles with trailer hitch with code 550 the trailer recognition control unit is integrated in the rear SAM control unit (N10/8).
Switching the following relays
• Circuit 15R relay
• Circuit 15Rx relay (power outlets)
• Heated rear window relay
• Rear wiper relay
• Trailer circuit 15R relay
• Deluxe seat relay
• Electric loading floor retract relay (model 164 with code 942)
• Electric loading floor extend relay (model 164 with code 942)
• CAN link between rear SAM control unit and cockpit CAN-B voltage distributor (plug G)
• CAN: Overhead control panel control unit (plug G)
The rear SAM control unit (N10/8) is a device on the CAN-B in the overall network.
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