Mercedes Sprinters are just one of many complex European vehicles with a reputation for being finicky. Check engine lights are a perennial problem, and one of the most aggravating things is the fact that you can fix one fault only to go on a test drive and have a new fault appear.
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| Mercedes master tech Danny Ferrari working on a Sprinter at #RobisonService |
Sometimes there is a logical explanation for this behavior. A plugged particulate filter will block airflow through the whole system. So you can have a fault for that, and fix it, only to get a boost fault because the additional problem is only revealed when you have flow to the exhaust.
A data bus fault can hide faults in modules because they can’t communicate. A failed module can hide failures in the module’s circuits. It can also send you on false trails, like the times we get a wheel speed sensor fault in the ABS, and replace the sensor to no avail, only to discover the problem is in the ABS control unit.
Sometimes people ask why shops can’t get diagnoses “right the first time.” The answer often comes down to averages. Any diagnosis starts with a scan for fault codes, and a good tech backs that up with a review of live data. If the data agrees with the fault, the usual thing is to proceed with a new sensor or actuator in the interest of efficiency with the “most common fix.” In many cases, a new sensor is cheaper that further tests to validate or condemn it.
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| Engineering tools like the Fluke Scopemeter are the next step beyond scan tools at #RobisonService |
When a new sensor does not solve the problem, the technician must do tests and decide where to go next. Often that means updating vehicle software. Software in a car can become corrupted by many things, like transients from jump starts, battery failure, or faults in extreme hot or cold.
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| Master tech Robert Toti using the BMW ISTA system to test and update software in a Rolls-Royce at #RobisonService |
If software does not solve the problem, you are down to two things – the control units or the wiring. It is always scary to replace a control unit because the units in modern high end vehicles are coded to the specific vehicle and cannot be returned to stock after being fitted. A new controller is often a $1,000-plus step with no guarantee of resolution.
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| Engine control units are inscrutable black boxes with 100+ wires bringing signals in and out. They are a huge challenge even for trained techs backed by the resources of a Bosch Car Service center or a franchised dealership |
The next place to look is the wiring. In years past, automobile wires carried signals that were either on or off. If a wiring junction became corroded, the circuit stopped working. When your tail lamp went out, you changed the bulb. If the bulb didn’t fix the problem, you used a test lamp to jab the wires and connectors along the path between light switch and lamp to see where the signal was lost. The process was simple and fast.
Today’s cars rely on data buses to transmit instructions from one module to another, and they don’t have traditional 12 volt wiring until the very end. In many new cars, when you turn on the headlamps, the signal from the switch goes to a body controller. It sends signals to the four corners of the car to tell those controllers to turn on lamps. The wiring between the controllers and from the sensors to the controllers is far more delicate than on cars of old. It carries digital data, not simple on/off signals, and cannot be checked with a test light.
When a car’s wire harness is compromised anything can happen. You might ask, what could go wrong with wire? Unfortunately, there are many things that go wrong with wire in a modern car:
- Rodents chew wires, and every indicator suggests this problem is worsening in newer vehicles. Rodent damage to wiring is accompanies by damage to upholstery and trim, and urine and feces create sanitary issues. Remediation costs can be quite high.
- Heat from a short circuit in one wire (perhaps when a blow fuse is replaced with a much bigger one) can melt wires, and when the harness has a spiral wrapping, one hot wire can melt and short any number of other unrelated wires, with totally unpredictable consequences.
- Water can get into the harness, corrode connections, and increase circuit resistances. This can change sensor readings and reduce responses of actuators.
- Insufficiently flexible wire bundles in folding tops and trunks can create difficult to trace failures.
Needless to say, those issues can be very hard to find. When you do find wiring trouble, it can be impossible to know the scope of the damage. We recently had a Mercedes Sprinter that typified this. The owner had experienced intermittent loss of power and a host of seemingly unrelated engine faults. He brought the car to us after becoming frustrated by a seemingly insoluble problem.
We started in the usual way, and replaced one sensor which solved his initial fault, only to reveal a new fault. After a few rounds of new faults we took a look at the engine control unit connector. On first glance, the ECU connector pins looked clean. But the connector pins were smeared with dielectric grease, which the factory did not use, so we knew it had been off for prior test and repair.
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| Can you see the damage? This is not a trick; it shows how difficult wiring problems can be #BoschService |
Looking closer we saw a pin that had been compromised by test prodding with the wrong tool. Sometimes we see that – technicians with wrong tools or training create problems where there were none before. We opened up the connector to repair the pin, and had a surprise:
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| Damaged water seals admitted water. Pin points to the worst spot, but look close - there's more |
The water seal on the connector was damaged, and several pins were visibly corroded. Once we saw that we understood. Corrosion increases the resistance of electrical connections. In a high power circuit like a brake lamp or a power window motor, any corrosion at all produces a failure. But in a sensor circuit like those on the engine, extra resistance does not cause a failure. It causes different sensor readings, and the result of that can be impossible to predict.
When we repaired the visibly damaged wires the car’s many symptoms seemed to resolve. But was the fix real? There was no doubt we fixed a few corroded wire junctions, but there are 96 wires on this one connector, one of two on the engine controller. When water entered the connector ALL the wires have the potential for corrosion damage, either now or in the future.
When I see water damage the first thing I do is notify the owner because water damage to wiring is covered as flood damage, under the comprehensive part of auto insurance. The next question becomes, how widespread is the corrosion? Are other connectors corroded?
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| Look at the size of the wire bundle under the carpet of this Land Rover. All these wires and connectors are vulnerable in deep water. If a harness like this absorbs water, the car is usually a total loss. #RobisonService #BoschService |
A quick check of connectors on the engine revealed traces of corrosion there too. What’s the fix for this? I’m afraid it’s bad news. You can replace the wire harness and any corroded sensors or connectors. Repairs like that usually exceed $10,000 and insurance companies generally choose to total the cars rather than risk repair. If you don’t have insurance, you have to fix what’s wrong today and know that you will have other problems going forward. Next month or next year some fault will occur, and an owner who’s living with corroded wiring will know to start by testing the integrity of the wires.
This particular defeated a number of previous technicians, either because they lacked the training or the tools to run down the wiring problem. At Robison Service we represent Bosch, who is a leading electrical system supplier to Europe and America’s carmakers. As a Bosch Car Service Center we have the tools and training to sort problems like this out.
Need help with a problem like this? We are as close as a phone call or email. robison at robisonservice dot com or 413-785-1665. We handle all aspects of insurance coverage when applicable to wire harness damage, whether rodent damage or water damage or damage from a collision.
Until next time
John Elder Robison
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| J E Robison is an authorized Bosch Car Service shop in Springfield, Massachusetts. |
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