You don't have to watch the forum long before coming to realise that the Mercedes EIS (Electronic Ignition Switch) is a major source of problems, especially as the car gets older. It's a complex piece of electronics which has the job of recognizing the right key is inserted before it can be turned and used to start the car and in checking its identity with the shift lever controller and the engine management ECU to make sure all three belong to the same car. It also releases the mechanical steering column lock. The challenge is that since all three of these components are coded, you can't simply swap parts over with known good ones.
I had an unusual fault in that the car would start perfectly on Keyless Go but rarely if ever on the key. I could insert the key and turn it but when I tried to start the car, there was almost always just a "grunt" from the engine. Various places had a go at fixing it - PSC Automotive in Cheltenham were hopeless, they replaced the starter motor claiming it was burned out. It wasn't. The Mercedes dealer wanted me to spend £1100 replacing the shift lever "as a first step" with no guarantee this would fix the problem. The SL Shop in Stratford were all style over substance and got nowhere except a large bill.
I had already gone to some lengths to investigate the problem and my findings were completely ignored by the Mercedes dealer "expert" who had only ever been trained to believe what the diagnostics were telling him which was nothing.
First, some basic facts. One, the battery in the key plays no role whatever in starting the car. You can start the car without a battery even in the key. Once the key is inserted, it is surrounded by an induction coil which - like cordless charging on a phone - powers that part of the key needed to send its identity through the infra-red window at the end. Once recogised and verified, the barrel unlocks and you can turn the key. Microswitches then activate the switched battery voltage.
When it comes to actually starting the car, the CAN BUS is used to get the three units to talk to each other and when all is OK, the engine ECU grounds a line from the green starter relay. That energises the starter solonoid and then the starter motor.
Take a look at this data logger trace showing a normal start. The blue line is the switched battery voltage and the yellow line is the voltage on the starter relay. The two traces are separated vertically for clarity. The arrowheads on the left side show the level of 0 volts. The horizontal scale is time, 1 second per interval. So in this case, I turn the key at 3 seconds, and to the start position at 5.5 seconds. The relay line is grounded to 0v for nearly a second, the engine cranks and starts and the blue and yellow voltages increase as the alternator comes online.
Compare that with the fault condition. Exactly the same as before except the yellow pulse does not last long enough to get the engine started. The "grunt" is just the starter motor being energised for a fraction of the second.
So the Mercedes dealer diagnosed a faulty shift lever (even though all was well with Keyless Go) and the SL Shop diagnosed a wiring fault between the EIS and the Shift Lever, even though a look at the circuit diagram would have told them there is no such connection. Talk about clutching at straws. £99 diagnostic charge from Mercedes, £298 from the SL Shop. Useless, the pair of them.
I lived with it for a while but decided to finally get it fixed and took the EIS to Autotronics in Leicester who at first did a standard repair covering the most likely faults but which did not solve the problem. They advised me then to heat the EIS in situ with a hair dryer. Bingo! Heat it up, the problem goes away, let it cool down, the problems returns, completely repeatable. The EIS has been back to them a second time and they have replaced a microprocessor and the EIS is working perfectly. Cost, £288, a quarter of what Mercedes wanted to charge to go down a blind alley and less than the SL Shop charged before they washed their hands of it.
Other posts here talk about cold EIS starting problems and one in particular refers to another company whose recommendation was just to go to Mercedes for a replacement because replacing the microprocessor damages other components due to heat. The point is, surface mount technology and flexible printed circuit boards are how things are these days and if you set yourself up to be an automotive electronics repair company, you'd better have the skills and equipment to handle surface mount. In practice, you ditch the traditional soldering iron and solder for a hot air gun and then use the right materials (solder paste and flux) to prepare the board to receive the new component. You also need to ability to source replacement parts. Autotronics came up trumps.
So Autotronics are the heroes of this little saga, I wish I had known about and gone straight to them. Very efficient admin and tracking, on the ball engineers. Highly recommended and I would use them again without hesitation. www.autotronics.co.uk
I had an unusual fault in that the car would start perfectly on Keyless Go but rarely if ever on the key. I could insert the key and turn it but when I tried to start the car, there was almost always just a "grunt" from the engine. Various places had a go at fixing it - PSC Automotive in Cheltenham were hopeless, they replaced the starter motor claiming it was burned out. It wasn't. The Mercedes dealer wanted me to spend £1100 replacing the shift lever "as a first step" with no guarantee this would fix the problem. The SL Shop in Stratford were all style over substance and got nowhere except a large bill.
I had already gone to some lengths to investigate the problem and my findings were completely ignored by the Mercedes dealer "expert" who had only ever been trained to believe what the diagnostics were telling him which was nothing.
First, some basic facts. One, the battery in the key plays no role whatever in starting the car. You can start the car without a battery even in the key. Once the key is inserted, it is surrounded by an induction coil which - like cordless charging on a phone - powers that part of the key needed to send its identity through the infra-red window at the end. Once recogised and verified, the barrel unlocks and you can turn the key. Microswitches then activate the switched battery voltage.
When it comes to actually starting the car, the CAN BUS is used to get the three units to talk to each other and when all is OK, the engine ECU grounds a line from the green starter relay. That energises the starter solonoid and then the starter motor.
Take a look at this data logger trace showing a normal start. The blue line is the switched battery voltage and the yellow line is the voltage on the starter relay. The two traces are separated vertically for clarity. The arrowheads on the left side show the level of 0 volts. The horizontal scale is time, 1 second per interval. So in this case, I turn the key at 3 seconds, and to the start position at 5.5 seconds. The relay line is grounded to 0v for nearly a second, the engine cranks and starts and the blue and yellow voltages increase as the alternator comes online.
Compare that with the fault condition. Exactly the same as before except the yellow pulse does not last long enough to get the engine started. The "grunt" is just the starter motor being energised for a fraction of the second.
So the Mercedes dealer diagnosed a faulty shift lever (even though all was well with Keyless Go) and the SL Shop diagnosed a wiring fault between the EIS and the Shift Lever, even though a look at the circuit diagram would have told them there is no such connection. Talk about clutching at straws. £99 diagnostic charge from Mercedes, £298 from the SL Shop. Useless, the pair of them.
I lived with it for a while but decided to finally get it fixed and took the EIS to Autotronics in Leicester who at first did a standard repair covering the most likely faults but which did not solve the problem. They advised me then to heat the EIS in situ with a hair dryer. Bingo! Heat it up, the problem goes away, let it cool down, the problems returns, completely repeatable. The EIS has been back to them a second time and they have replaced a microprocessor and the EIS is working perfectly. Cost, £288, a quarter of what Mercedes wanted to charge to go down a blind alley and less than the SL Shop charged before they washed their hands of it.
Other posts here talk about cold EIS starting problems and one in particular refers to another company whose recommendation was just to go to Mercedes for a replacement because replacing the microprocessor damages other components due to heat. The point is, surface mount technology and flexible printed circuit boards are how things are these days and if you set yourself up to be an automotive electronics repair company, you'd better have the skills and equipment to handle surface mount. In practice, you ditch the traditional soldering iron and solder for a hot air gun and then use the right materials (solder paste and flux) to prepare the board to receive the new component. You also need to ability to source replacement parts. Autotronics came up trumps.
So Autotronics are the heroes of this little saga, I wish I had known about and gone straight to them. Very efficient admin and tracking, on the ball engineers. Highly recommended and I would use them again without hesitation. www.autotronics.co.uk
