Knowing when to say “No!” is something that we learn as we progress through the ranks in the motor repair business, but sometimes we still succumb to the urge to take on an interesting job. We recently had a 2003 BMW M3 drop in on us, with two warning lights displayed and the customer complaining that he kept losing his gears. This car is fitted with an SMG transmission and my knowledge was somewhat limited to be honest. I had read some horror stories, where owners had spent literally thousands of pounds before a fix was found. This really ought to be the job for a BMW dealership or independent specialist, but the masochistic side of me drew me into an interesting diagnostic process.
A global diagnostic scan pulled two significant fault codes. The DME (Digital Motor Electronics or Engine Management System to you and me) had the following DTC and definition… “Engine MIL on at request of SMG”. The SMG (Sequential Manual Gearbox) system had the following faults stored… “P1717 Hydraulic Motor On Time too long”.
This BMW SMG transmission is essentially a conventional manual gearbox, with an electrohydraulic control system, that has been developed using technology developed for the BMW/Williams Formula 1 car.
When considering the merits of taking on this kind of diagnosis, it is important to weigh up the customer as much as the vehicle. There is little point in proceeding if the owner is going to baulk at seemingly expensive and time consuming test and repair processes. If they are the sort that want the car by the weekend and have only a limited budget, then it’s time to walk away. The owner of this car was away in Abu Dhabi for several weeks and had expressed that he “Wanted it fixed!” and authorised a generous spend. Although he knew it could cost thousands to fix it, there was still considerable pressure on us to make sure that parts were not fitted unnecessarily.
Fortunately we have access to a tool with dealer level diagnostic capability and also an excellent technical information system, which is essential if an informed diagnosis is to be made. Control unit software versions were checked and found to be subject to newer version releases. This has become a big part of fault diagnosis in recent years. Faults can be logged due to over strict parameters which can be altered by system developers. In this case, both the engine and transmission controllers were brought up to date. Other system tests were carried out and the hydraulic pump relay was replaced (at the suggestion of the guided fault finding system). I had little faith in the relay being to blame for a prolonged pump run fault, but on the other hand, at only £10 (about US$16) for a component that is directly responsible for powering the pump, it didn’t seem unreasonable.
We ran the car for a week without a fault recurring and the owner’s brother in law came to collect. Typically ten minutes later, he returned with the exact same fault scenario. Intermittent faults will make the most professional repairer look inadequate! The car was left with us for more testing.
This time we tested and confirmed the accumulator residual pressure. This is done by an automated test plan performed on the diagnostic tool. The hydraulic system is vented and the residual pressure is displayed. We had 32 bar, where the minimum specified pressure is 29 bar. The pressure generation appeared to be reasonable too, taking approximately 20 seconds to build from 50 bar through to 70 bar (the system cycles between these two values).
It was fortunate that an identical car was available to us that we could take some good data readings from. This approach has often been pivotal when tackling complex and rarely seen faults and this time it would prove to be just as helpful. Initially only basic comparisons were made, i.e. pressure generation and decay times were compared in “live data” using a stop watch. This didn’t really show anything conclusive, so a new approach was required. There is only one way to study the pump functionality in minute detail and that is with the scope. By capturing the hydraulic pump current signal, during its operation from zero through to 80 bar and comparing to the same capture from the known good car, it was hoped that some conclusive information could be found.
Here is a capture taken of the hydraulic pump current draw. Taken over a 20 seconds (total screen time) and with a 20 amp vertical scale.
Figure 1: The faulty car
