Join Date: Aug 2013
Location: Durham, United Kingdon
Vehicle: SLK55 AMG R171 2008 model year Wolfgang Herrmann
Other Toys: Mini Se7en Race Car, MG Midget RWA, 1962 mini 850 super delux, , 1964 Landy 3500V8, 1936 Austin Ruby
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AMG55 The AA dignosed alternator failure......
Don't know if this is AMG specific but I thought Id recount my 'tale of woe' re my 2008 AMG55.
Bought this car recently but with only 50000 miles it hasn't had a hard life, in fact it had been stored for nearly a year and went nowhere. Picked it up after checking it over.... everything working as it should, a few little rattles here and their nothing I hadn't heard with my previous 280 manual SLK. Drove it hard for a few miles and then a few miles round town. My wife drove it for a couple of days, about 150 miles total, then I got a call that it had broken down. The AA recovered the car back to my house where the AA 'technician' diagnosed an alternator failure, with a dead battery so low even the remote locking wont work.... ohhh woe. So I decided to check it out for myself.
After charging the battery overnight then testing its capacity it looked like the battery was up to the job so installed the battery, synced the windows..... turned the key and hey presto, fires into life. The battery monitor on the dash showed 14.2 volts.... dropped a little then picked up. Under full battery load it came down to 13.7 / 13.8, that's with lights on heated seats, airscarf etc etc...... Seems to me that the alternator is doing its thing. Turn off the load and back to 14 volts.... I think the alternators going to be fine, but just to confirm I rigged up a cheapo digital multimeter over the battery. With the engine off I had over 13 volts... starting it up the engine spins up as it should voltage drops to 11 point something under cranking, fires up and back to 14.2 volts. Change the multimeter from DC range to AC... and I see 29 volts AC. Now this MIGHT point to a blown diode pack, but those cheap multimeters that cost £10 from screwfix don't measure the same way my £300 fluke does at work so to double check my diagnosis I did the same test on my workhorse car which is a 250 CDI AMG Blueteck.... now this car is a known quantity, its done 120000 miles and is serviced religiously. The tests confirmed EXACTLY the same figures on AC... so now Im not concerned about the alternator... but what is the problem???
Knowing that the car had broken down I didn't really want to do a lot of miles to find it was going to break down again so I just drove around the block where I live.... The neighbours must have thought I was nuts. Round and round I went, and Im thinking hmmmm what caused it to stop. Then a few hundred yards from home it just died on me. Parked / pushed it up and checked the battery voltage on the dash... nothing to be concerned about, tried to start and the engine spins over as it should but no throaty roar into life :-(
So I walk to the house for a cuppa and a regroup and some tools in that order..... back to the car with multimeter and a few tools in hand. Battery voltage on the meter the same as the dash display, over 13 volts. Key in and the old girl fires straight up. Quickly back to the house in case she dies again, and sat on the drive sure enough she fails again and wont start.
Well this is starting to show some tell tails.....
Battery fully charged, spins over nicely but no go...... when the engine is hot, after a cool down she fires up again until it get hot then same failure mode.
This is classic crank sensor fail mode..... I think being stored for a long time then used and heat cycled didn't help the sensors life. Changing the crank sensor is easy if you have thin hands as down the back of that engine its a bit tight, with bigger hands you WILL end up with a few cuts off the heat shield at the back.
Looking from the front of the car the sensor is located in the back right near the bulkhead. In the gap between the cylinders and the bulkhead, down at the bottom of that gap and facing up at you from about the 1 or 2 o'clock position looking at the front of the engine. It might be covered with a glassfibre tube. Pull this back and squeeze the mating part of the 2 pin connector and remove, you wont be able to see with your hand down their so its all done by feel, better done with a cold engine as well...
Under the sensor housing, so you cant even see it, is the single torx head bolt holding the sensor in place. Its size E8 and accessible with a 1/4 drive ratchet on a long extension bar with a UJ at the end connected to a short extension bar connected to the torx socket.... You still wont be able to see anything so all by feel, loosen the nut and unscrew a few threads then wiggle the sensor so its free to pull out but still secured by the nut, pull the sensor up to the nut head then undo a few more turns, pull the sensor up and undo a few more turns until the nut is free and the sensor pulls out of its location. The reason to do it this way is so you don't drop the bolt down the back of the engine by undoing it fully, its retained by the sensor which is longer than the bolt. Remove the sensor and replace..... I use these same sensors on race cars, but on these cars the ignition module has a light that shows green when the sensor is picking up pulses and shows red when they are missing. Its a classic inductive sensor failure mode of working when they are cold and failing under heat. I had a couple in the spares box so replacement is the reverse of removal, ensure the plug is seated and locked and the glass fibre cover in place. Turn the key and back to normal.... Leave it running till the fan kicks in, turn it off, fire up again then off round the block for a few circuits while checking battery voltage and oil and water temp.... Then of for a few miles running under load and round town.... All back to normal.
So to recap.
Diagnosing a failed crank sensor ask yourself a few questions
Is the battery fully charged?
Is the battery voltage showing over 13 volts under full electrical load when running and returns to 14 volts plus when you turn off the loads?
Does the engine start from cold then die when up to temperature and fail to restart when hot?
When failed does the battery spin the motor as it should but fails to fire?
After a failure does the engine restart from cold with a charged battery?
If the answer to all of the above is YES.... then change the crank sensor, its about a 30 min job if you have the tools and their isn't many of them needed. From a usual parts outlet the sensors are around £40-£50 but they can be found for less.
Also, unless you are 100% sure, don't take the word of anyone re a diagnosis of a fault. You cant just say ..... oh its a failed alternator without knowing a lot more about the situation..... The battery was flat because my wife had tried to start the engine whilst it was at the side of the road.... as it was still hot it didn't start.... but keeping up trying to start a big motor it doesn't take long to flatten even a BIG battery. If she had waited till the engine cooled it would have restarted only to fail again when hot.
Anyway, I hope this tale helps someone else. As a side note to this tale I asked a Mercedes tech that I know about the sensor, and he confirmed that the sensor is susceptible to failure on these cars... not a regular thing and not something to worry unduly about but worth bearing in mind if you get into the same situation as me.