Very faint battery light - constant
 

Hi,

My E53 diesel has a really dim/faint battery light on at all times while running. I was concerned it was the alternator so took it to the mechanic and they tested the alternator output, which was a consistent 14v during the test. The light is so dim you can’t see it when driving during the day, it’s more visible at night but dim.

The starter motor was replaced by the mechanic about 2 weeks ago and I thought maybe it was a bad earth from the new starter motor, but the mechanic checked this and it isn’t the problem.

The light is constant, not flashing, and doesn’t get brighter even under load (headlights, fan, air con, etc) just always the same. The alternator had new brushes installed 12 months ago, so I thought maybe the voltage regulator was dying, but the voltage is consistent. Any suggestions?

Ignition switch starting to fail is a distinct possibility.

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Very faint battery light - constant
 

I think on the e53 the alternator circuit is connected to the oil level circuit. The TIS would confirm.

In the old days the alternator drive circuit went though a bulb on the dash to automatically light up on fault.


–awr–

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Use the dash display function and monitor the Voltage during the drive.

Or get a cigarette plug type that shows voltage (the one that displays using black LED light). Just unplug it after a drive.

For starters alternator output is measured in amps. Maybe to check how that light is wired in for a diesel because it might not be "turned on" by the DME and instead be hooked right to the alternator. When did the light first come on ?? Did you buy the vehicle this way?

Post the build date and somebody can look up in the TIS.

Often when I'm seeking e53 solutions I'll search E39 which had a much bigger user base.

Found a good handful of mentions of the same happening and not many solutions.

The only suggestion i saw with exciting was to confirm there's a good ground. Check the DC voltage from the frame of the alternator or block to the chassis. It should be less than 0.5V.

Checking the AC voltage at the alternator is also a worthy test. It should be less than 0.25v if more itt should be looked at with oscilloscope to learn why.


–awr–

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Can you confirm that the charge lamp lights up full brightness when the ignition is first turned on - before the engine is running?

The TIS states: -

________________

Charge indicator lamp
Terminal D+ is replaced by an isolated electronic terminal 61E in alternators with a multifunction controller. The task of this terminal is, on the one hand, to activate the battery charge indicator lamp and, on the other hand, to indicate to the various loads that the alternator is in charge mode.

The battery charge indicator lamp itself is supplied via terminal 15 from the instrument cluster. It comes on when the voltage at terminal 61E is below 1.5 V. The battery charge indicator lamp goes out when the voltage at terminal 61E is above 8 V.

Alternators with a multifunction controller can be recognized by the plug connection of terminal 61E. As a result, the 6 mm threaded stud has been dispensed with.

The indicator lamp lights up when:

Ignition on with engine stationary, alternator does not charge, failure of belt drive, interruption in excitation, controller overvoltage, break in charging cable

The second cable in the plug connection at the alternator is terminal 15 that serves as a reference signal for determining whether the alternator is charging.
________________

The actual circuit layout varies from the earlier pre-facelift (M57) cars to the later LCI (M57TU) cars: -

M57
https://i.ibb.co/ynsMZwCq/Charge-Lamp-M57.jpg

M57TU
https://i.ibb.co/4RJ4ygNB/Charge-Lamp-M57-TU.jpg

Measuring the voltage on terminal 61-1 would perhaps give a clue as to why the lamp is lit

Thanks for all of the replies, answers to specific questions below:

1) yes the charge light lights-up to full brightness at start-up, it then drops to barely visibly (rather than extinguishing) and remains there

2) how long has it been happening - I got the car back from the mechanic on Thursday 14th August, this was when the starter motor was replaced. I noticed the light about 2 days later, but it could have been there since I picked it up, it’s so dim I didn’t notice it until I moved the car at night. During the day I you really have to look to notice it. So less than 2 weeks.

3) no I didn’t buy the car this way, I’ve had the car since 2019 and the light has never come on before.

4) build date appears to be October 2003

5) when I wrote “voltage appears to be consistent” what I should have explained is that the mechanic measured charge rate and got a consistent 14v but didn’t measure from the alternator, as per suggestion. I will ask the mechanic to measure this and will also get a plug-in voltage monitor, that’s a great suggestion, thanks for all of the help!

The mechanic should be checking the amperage output of the alternator. It sounds like there is a good chance that the problem might have something to do with replacing the starter so grounds and connections around the starter and alternator, including the jump terminal(if the diesel has this) connection.

Looks like the diagram Wayne posted :bow: shows the light old school to the alternator so that explains why it can be dim.

I don't think it's the case here but the wrong bulb for the indicator can cause this kind of thing.

Maybe the wires got mixed up ??

If this started happening after the start motor was replaced, then focus on the connections that had to be removed to do this work. I've recently been in there myself and the LCI car (M5TU) routes the alternator output via the main starter terminal, so there's lots of scope for connection problems here. The two large cables on the main terminal of the starter motor seem to need to be connected the correct way around. The line from the alternator goes UNDER the line from the battery/jump start terminal.