15 Jul 2024
The voice of the independent garage sector

Making the smart choices

Top Technician 2019 champion Neil had a recent rebound job with a Smart Roadster that reinforced the importance of following a test plan

By Neil Currie

This article is about a recent job I had that was very interesting indeed. I diagnosed the fault, but then the vehicle was taken to the dealership as we didn’t have the equipment required to finish the job. Of course, it ended up back at me with different symptoms.

The vehicle in question was a Smart Roadster; Not your average common car often seen on the road. However, as we do all makes and models we would give it a shot and see what happens.

The vehicle was presented as a no crank with a flashing key symbol displayed on the dashboard. My first worry was that being a Smart, technical information would be in short supply. Luckily though, there was plenty available and the diagnostic scan tools listed the make and model so we were off to a good start. Verifying the customer complaint showed the ignition to come on with the flashing key symbol. However, when attempting to crank the engine nothing happened. Checking the key itself, it showed no output when tested for an RF signal output. Testing the battery within the key showed it to be flat so a new battery was fitted. However even with the key now outputting a signal the car did not respond and it still would not crank.

I then scanned the entire vehicle and not many fault codes were stored. It was noted though that the front signal acquisition module (SAM) did not communicate. This is the Mercedes/Smart name for body control module. This unit is responsible for key data among the many other functions it carries out and would be the reason our car was immobilised. I then decided to access the module to carry out some tests to locate the fault and why we could not communicate. Referring to wiring diagrams, the power and ground wires were tested and found to be okay. When removing the connector to get pin numbers to verify I was on the correct plug to check the CAN BUS wires, there was small signs of corrosion on the pins pointing to a possible water ingress problem.

Testing the CAN BUS signals with a scope showed textbook signals so the next step was to remove the SAM unit and get a better look at it.

Upon removing and inspecting it again, signs of corrosion were visible so this gave me enough evidence to split the plastic covers and inspect the circuit board itself and as we can see in Fig.1, this board had substantial water damage which had corroded the board.

This now explains why we cannot communicate with it and have a no crank complaint. Pricing a new unit up from the dealer showed it to be over £500 and with this car being a 2004 model the cost of the repair posed the question of whether this was a viable repair compared to the value of the vehicle. There was an option to possibly clone the information from the old unit to a second-hand unit but with it being so badly damaged I felt it may not be possible to read the relevant chips on the board and a new unit was the best way to proceed.
After some research, we were unable to program a new unit with our diagnostic equipment and the customer was contacted and advised of costs involved and if they wanted to fix the vehicle it would need to be recovered to the local Mercedes dealer. Having sentimental value, the customer decided he wanted the car to be repaired so the vehicle was taken away to be repaired and I thought that was the end of the job however a few weeks later it was booked back in. It now started and ran, but had a list of new symptoms the customer said were not present before the vehicle developed its previous no crank and flashing key symbol issues.

Reading the job card, the vehicle now had an ABS warning light illuminated, the radio did not work, both electric windows did not operate at all and the heater blower motor was inoperative. The local Mercedes dealer had advised a new ABS module was required at a cost amounting to four figures, and further investigation was required to look into the rest of the faults. However, the customer wanted a second opinion so paid for the work carried out and removed it from the dealership. I decided to pick the ABS fault first and work from there. Connecting and reading faults, we had one code stored. Please refer to Fig.2. This referred to a fault at the ABS valve relay. Checking technical data listed the possible cause as internal module error and suggested checking module power and ground supplies. It looked like the Mercedes dealer was possibly correct then but you never condemn a part without carrying out all possible tests first so I gained access to the ABS module and checked the wiring referring to a diagram. Upon testing the power supplies, I found no 12v power supply at pin 6 and checking against the diagram showed it to be supplied by the front SAM, but this was new? I then decided to have a quick study of the wiring diagrams for the windows and other non-working items for a possible link and interestingly It was noted that all items were fed from the front SAM with some sharing the same connector.

Technical information
With my newly found information, the next step had to be to gain access to the front SAM and the relevant connectors and test from there. Upon removing the unit from its housing and lowering it to gain access to the wiring connectors, I was met with what you can see in Fig.3. Not only one connector not fitted but two different connectors left unplugged from the new SAM unit. Reconnecting both and retesting the customer complaints showed all issues to now be fixed. The customer was delighted and were glad they got a second opinion as following the Mercedes dealership diagnosis would not have fixed this car! Now I am not one to badmouth dealerships and their technicians, but something went wrong here and mistakes were made which could have cost the customer thousands of pounds.

I know I sound like a stuck record with my articles, but I cannot emphasise enough the importance of having a solid test plan, process and technical information to help fix cars coming through your workshop. I had never worked on this model of car before but armed with the information I needed along with a good understanding of system operation I was able to make a diagnosis and repair, and get the car back to the customer to use as intended.