Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
Hi, what charging voltage did you have? What makes you think you have charging problems if no warning is displayed? Does the battery go flat? Is car not starting?
Best regards, Paul
Hi Paul, I am an auto electrician, but not experianced in the system Merc uses. I believe they use the ECU to control the alternator charging. I have checked voltage across the battery while the engine is running. Result battery voltage or below. I checked the main power feed on the alternator and the two small wires that plug into the alternator. All had voltage readings. As I said no fault comes up on the dash.
Hi - so are you seeing ~12V at the battery with the engine running? Could you post the VIN number of the vehicle please.
Paul, Yes. I also took the alternator to have it bench tested. The company could not test it. My main concern is there could be a fault elsewhere- perhaps the ECU. I don't want to buy an alternator unless I am certain it is faulty. VIN No: WDB2102062X082750. thanks- Martin
Hi Martin, this vehicle does not have the later LIN bus alternator management & is a straight forward charging system. As you are probably aware accurate diagnosis should be carried out using a scope & checking the amp output of the alternator. If the waveform on a scope is good then it is likely that just the regulator has failed. If the wave pattern is poor then the alternator should be replaced/overhauled.
I would also ensure the main cable has good continuity to the battery & all earth connections are good.
Paul, I the company I took the alternator to could not check it because it is not a conventional alternator. I have checked all cables including the earths- it earths through the body of the alternator and the engine earth is good. Surely a fault would come up as a warning on the dash. There is no warning light as on a conventional system. Martin.
Hi Martin, the system is definately not a LIN bus system if that chassis no is correct. The 611 engine in this vehicle only ever had conventional alternators. I am looking at the wiring diagram & also have a chart up which showsit is not linked to engine management! If you beleive it is then diagnosis will be able to be carried out only via Mercedes STAR diagnosis but from the information you have given me this is certainly not the case & no diagnosis will be listed on STAR!
Paul, does your information show that this alternator is water cooled?
The warning system is not by a simple bulb on the dash but a warning is usually shown up on the dash with a message in the multi function display.
Yes that is correct. No fault is showing on the display. Also the output is 190amps
Hi Martin, yes, it does list it as a water cooled alternator from 1999 models onwards
amp output sounds good - looking further into it it does not look like regulator replacement is possible (from Mercedes) on the later water cooled models. Let me just check the parts system.
Just checked - Mercedes do not offer any regulator as a seperate part for this alternator.
Paul, the two smaller wires- does it show on your wiring diagram that one should have a 12volt ignition supply? The other would normally be the earth via the diode for the warning light circuit. Ref 190amps- that is the output stated on the label of the alternator.
Hi Martin, sorry - I thought you meant you had tested it at that output. The blue wire goes direct to the cluster which (should) put the warning on the display. The blue/yellow wire goes to vehicles with heater booster modules to only allow this to operate when the engine is running.