Limitations — we all have them. Every now and then we should see if the limitations we had in the past still exist today. After all, if we automotive repair professionals live by the constant improvement model then it’s likely we are now better tooled, equipped and trained to perform a task that was previously impossible. Unfortunately, not everyone lives by that model.
In trying to help someone recently, I tested one of my limitations. A shop owner asked me to look at a Mercedes-Benz SUV; however, I told him I didn’t possess the factory tool for the Mercedes and have inexperience with the brand (I have focused my career on Asian and Domestic vehicles).
He said others had spoken so highly of my skills and he’d prefer to have a mobile technician come to his shop than to tow it somewhere where he “didn’t know what might happen to it.” Right now, it’s a mystery what’s wrong. I promised the shop owner I would put forth my best effort, charge by the hour and utilize all the tools at my disposal. But I still may not be able to finish the job. I warned him payment would still be expected even under those circumstances. He asked how soon I could look at it.
The story begins in a different shop, one that’s owned by a regular customer of mine, where I was programming a vehicle after a module installation. “Chuck” (may not be his real name) called to ask advice from my regular customer about how a 2008 MB GL550 Active Suspension fault could be causing A/C problems. My customer, an air conditioning specialist, agreed that if Chuck would bring the vehicle over, he’d look at it briefly at no charge. Within a few minutes a pickup truck appeared in the parking lot towing a trailer upon which was the almost completely assembled vehicle in question. When my customer heard what had transpired to the vehicle in tow he immediately (and intelligently) tossed it in another direction. Follow along – this gets fun.
The vehicle was originally brought to Chuck’s shop with the complaint that the A/C blows warm. Chuck decided it needed a compressor because “when I applied 12 volts with my Power Probe to the A/C compressor connector, the clutch would not engage.” After replacing the compressor he had the same problem, but now, before he could get the SUV’s engine to even crank, he spent hours determining he must first remove the air suspension compressor relay. Note — this phenomenon was not present prior to the A/C compressor replacement (and to this day I’ve not understood how he came to either conclusion). Now that the engine starts, there are numerous lights illuminated on the instrument cluster and many comfort and entertainment systems don’t work. In addition, if the air suspension relay is reinserted, the engine stops running. Are you wondering about Chuck’s limitations yet? What would be your first steps here (besides turning and running as fast as you could)?
When I arrived at Chuck’s shop the next day, I immediately began regretting my decision to help. The shop looked well equipped, just not for repairing modern autos. It appeared better qualified for industrial equipment, farm equipment or lawn care equipment repair. My instincts said to leave after apologizing for my mistake, but I felt I owed Chuck at least an attempt at resolving his dilemma.
After attaching a charger to the completely dead battery, I started with a complete system scan. Well, as complete as three aftermarket scanners would permit anyway. Each one would access some of the systems on board, but each had differing capabilities. In other words, I could get some DTCs, some data, some bi-directional testing available, some module information, etc. from various systems and in most cases each scanner provided similar data, but more often than not, their information differed between each other. I recorded all the information using my phone’s camera when the tools’ abilities could not store it. It was late in the day so I took some voltage readings, visually inspected the A/C and active suspension compressors (and surrounding areas) and then told Chuck that I would do some research to better understand the systems, analyze the data I stored and look at the wiring diagrams of the systems affected before performing any more testing. He called the next morning to see what I had figured out.
DTC P2289 (CAN signal from ckt 15 does not match) was displayed and described similarly by all three scanners. That caught my eye and took a lot of reading to completely understand. I know from experience how CAN BUS codes can wreak havoc on numerous systems in any vehicle so I prioritize these whenever I encounter them. This was one of 103 active and stored DTCs in the 45 modules identified onboard this vehicle. I’m certain there were more, but my scanners most likely weren’t able to identify all the modules or the DTCs. I also identified other history DTCs I felt should be classified as important (like 5557 – No CAN Message Received from Control Unit N51), but knew those could have been induced by improper testing methods or set because of other DTCs and their causes. I put in a few hours analyzing the data, looking for anything the two systems may have had in common before deciding I’d better go back to test some circuits before condemning any component. I suspected the air suspension and the air conditioning systems had a circuit in common but had no idea at this point where the link could be.
I was curious how so many seemingly unrelated codes could be set in so many modules. I started asking more questions when I returned to the shop. Knowing the air conditioning system diagnostic procedure had not been followed according to the manufacturer’s recommendation, nor were the tool manufacturer’s instructions for use properly followed, I wondered what procedure was followed to replace the A/C compressor. I had him reflect on everything he’d done and learned plenty as he went through each step. These details are important so pay attention.
He’d used a floor jack to raise the left front of the vehicle so he could gain access to the area he needed to work in. He realized he also needed to move or loosen other items to get the compressor out and did so. He installed the compressor and reinstalled the other items the following day, as soon as they were delivered. His description seemed simple enough, however there were things he’d said that set off red flags or were keys to what got damaged and how. Did you get the same feelings?
I focused on what I felt were the most important and most relevant DTCs that were most likely to affect the most systems at the same time. That code for circuit 15 had me perplexed until I read up on it. This vehicle’s ignition-fed circuits are not the same as most vehicles I’ve worked on. Instead, the ignition switch inputs voltage to certain modules, which in turn control other circuits that will then receive voltage as determined and monitored by those modules controlling it. Sounds overly complicated? It is.
Here is where I remember how thankful I am that I learned how to interpret and understand wiring diagrams. Looking at the air conditioning and air suspension modules’ diagrams, they both had a terminal identified as 15 (blk/wht). I finally figured out that 15 was a circuit identifier, as was described in the code I received. When tracing that circuit, I found it originates at both the front and rear fuse blocks. The same circuit is shared by almost all the modules that were setting DTCs. On the air suspension diagram, circuit 15 is fed from fuse 53 in the load compartment fuse and relay box (rear fuse block, as you may call it) and the relay that provides power to circuit 15 is controlled by another circuit 15 in the same fuse block. Yes, you read that correctly. The rear fuse block feeds power to the rear fuse block, which then provides power to the rear fuse block (and ultimately to lots of other stuff in the car). Look at the diagram and see for yourself. Oh, and there’s another circuit 15 relay in the underhood fuse block.
When testing at the rear fuse block, one can see in the diagram there is a circuit 15 Relay that feeds numerous circuit 15s when energized: Fuse 51 (Activated Charcoal Filter Shut-Off Valve); Fuse 56; Fuse 54 (Front SAM Control Unit); Fuse 52 (Rear Axle Differential Lock Control Module & R&L Front Reversible Emergency Tensioning Retractor); Fuse 53 (Transfer Case Control Module) & Airmatic Control Unit; and lastly, Fuse 55 (Instrument Cluster & Instrument Lamp Switch). When there’s something not right with this circuit, almost everything in the car is affected. Since numerous modules monitor the health of circuit 15, many modules were setting codes for circuit 15 CAN System faults. Can you see why I felt I must concentrate on determining why this circuit’s faults should be the first to be resolved? Can you imagine how many circuits were affected by the lack of circuit 15 to the front SAM?
The pictures indicate that only some of the fuses that were supposed to be supplied voltage by the circuit 15 relay actually had any voltage. The wiring diagram showed all the circuitry from the relay to the fuses was internally conducted in the rear fuse block. Since only two of the fuses on circuit 15 had power, I knew we needed a new rear fuse block. I attempted to back feed 12 volts to those fuses that were supposed to have 12 volts but was unsuccessful in resolving all the problems by doing so. Some systems came back online but others refused to wake up. Further research indicated the rear fuse block would have to be programmed to the vehicle once it was installed so at this point, I knew I’d reached the end of my capabilities. I realized my limitations on Mercedes vehicles still existed. Chuck would still need to get this vehicle to someone capable of programming a new rear fuse block and configure it if necessary and I knew this may not resolve all the problems the vehicle had. I recommended he take it to an MB specialist friend of mine who said he would look at it. Steve was able to accurately diagnose it, repair it enough to start properly and may have even made some recommendations to Chuck about limitations.
There may have been some boring technical stuff in this case study, but was there a resolution? Was there a conclusion? Yes, eventually. Have you forgotten what procedure the customer followed to replace the A/C compressor?
The active suspension on this vehicle remains alert whether the key is on or not. Remember, Chuck had to wait a day after beginning the repair for the replacement part to come in. What do you suppose happened when the vehicle was left jacked up on a floor jack overnight? You guessed it! The vehicle tried to level itself and in the process damaged the vehicle’s wiring, mostly under the front SAM, including the grounds to the air suspension compressor. So when the components that weren’t completely removed (just loosened to allow access to other components) interfered with the suspension trying to correct itself, the plot thickened. When a component is activated but has no ground, the current will flow through other components to complete the circuit. When the air suspension compressor attempted to do its job, a lot of other circuits were affected and subsequent damage occurred, like that found in the rear fuse block.
What about the first problem created? The one where a Power Probe was used on a computer-controlled circuit? For you readers unaware, this A/C compressor has no clutch! It is a variable-displacement compressor that is ECU-controlled through pulse-width modulation. It’s possible that by powering a computer-controlled circuit he damaged some other components as well. We all should be careful to not power circuits if we do not have a thorough understanding of that circuit. Chuck never even disconnected the engine harness from the compressor before providing 12 volts to the circuit.
Remember, the testing methods we used in the past may not be appropriate for use on vehicles today. It is our responsibility to know how we are supposed to diagnose the systems we are taking into the shop. Are you accepting that responsibility? Do you live by the constant improvement model? One last question: Do you know where the most expensive training facility is located? Answer: The service bay in your shop!
