.....so I'm driving my 2003 E39 540iT wagon down the road, and suddenly the battery light comes on, on the dash. I thought that the alternator stopped charging or the belt was off...... Wrong, the alternator is still sending 13.6 out to the battery, and obviously the belt is not off because the power steering still acts normal.
Any ideas, any suggestions?
The alternator is about to check out...Replace it asap.
Make sure the little connector that plugs into the top front of the alternator is clean and plugged in good.
There is a small fuse (5A??) in that circuit too, make sure it's good. It's above the glove box.
The plug at the top of the alternator was tight, but I took it out and cleaned it well, but that didn't fix the problem; the red battery light is still on, on the instrument cluster.
#4 fuse was blown, but that wasn't the fix either.
The battery keeps a good charge from day to day, and when the motor is running, the voltage is 13.5 to 13.7 which tells me the alternator is putting out properly.?
....anyone else have any ideas?
F4 is for the general module. Any idea why it blew?
The alt fuses I was thinking about was F15 & F18, check them. Maybe F4 again too.
Maybe the voltage regulator is acting up... usually when the red light comes up I replace the alternator, it’s not worth the risk of being stranded.
1995 525i 5-speed - Thread
With the alt putting out 13.6 - 13.7 volts it shouldn't be the alternator.
It IS the alternator or regulator. Its output is intermittent, and most likey often below the 13.6v threshold -when the red battery light come on.
Take a volt meter and test your battery voltage at the battery terminals (AFTER you loosen them, clean them and re-tighten the posts),
Voltage test are at in the key off position, idle, 1500 RPM, and 2500 RPM Hold speeds for 15-30 secs record any variations.
Report back your findings
Engine Off Voltage:______________
IDLE Voltage: ___________
1500 RPM Voltage _______
2500 RPM Voltage _______
Any digital voltmeter will do.
Note: Loose wires, corroded connectors (These things are 15+ years old now) on both the ground side and the positive side will provide bad readings. Clean and put a wrench on all of them. Especially the ones under the hood. When in doubt use the test below:
Volt Drop Test
Last edited by StephenVA; 04-22-2019 at 11:56 AM.
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While your at it remove and clean both ends of the ground strap from the pass side engine mount bracket to the body.
My 540 has never been driven in the snowy salty winter when I lived back in MA, but I still found considerable corrosion on the bolt that holds the ground strap to the engine bracket. It didn't cause me any issues, yours could be worse??
Does the 540 use the same alternator as the 525? I replaced my voltage regulator and saved a ton of money compared to a new alternator.
No it does not. The 540i alternator is water-cooled and sealed up pretty tight. You can still take it apart if you're careful though, and there is a voltage regulator in it but they're much harder to find since very few people rebuild the water-cooled alternators. Most rebuilds of those are done by Bosch.
1995 525i 5-speed - Thread
FYI, according to an employee at Bosh, the lifetime of the water-cooled alternator on the E39 540i is about 16 years, give or take.
So, yours may be due for a replacement.
Wow, what a lot of good input. I'm away from home until next Monday, so I will do the tests then......thanks everyone.
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