E39 M5 alternator keeps going out?

[e39.m5.2000]

Hello,
I have a 2000 BMW M5 and I have gone through several alternators in the past few months. I have been getting Bosch rebuilds from fcpeuro, but the last one I had went out after 4 months. I have looked at the battery connection in the trunk and all connections are tight (esp in terms of the ground). I did have the typical power steering leak on this car and I just fixed it this go around (hoses were leaking very close to the alternator). Is there anything else I should be looking at? I do have a small, rockford fosgate class D amp in the trunk (like 150 watts rms) and am wondering if this is contributing to the repeated alternator failure?

PS battery is brand new and I replaced it after last time alternator failed.

[crazy4trains]

Buy a new one. My luck with rebuilds is not good.

[JimLev]

If this is the first alternator that hasn't had PS fluid leaking on it? See how long it will last. What has FCP had to say about all of your warranty claims?
Check the engine to chassis ground connection on the pass side to make sure it's clean and not corroded. Take it apart to visually inspect it.
A 150 watt amp shouldn't be an issue, I'm running a 400W one along with a 150W sub.

[e39.m5.2000]

thank you everyone thus far for your comments and recommendations!

- - - Updated - - -

FCP thus far has not said anything. Also can you tell me roughly where the ground is located on the passenger side? Appreciate your help!

[JimLev]

Pass side engine mount to the frame rail.
You'll need to access the frame rail bolt from under the car.
You might be able to get to the engine mount connection point from the top. You may have to remove the pass air box.

[e39.m5.2000]

Thank you!!

[e39.m5.2000]

Pass side engine mount to the frame rail.
You'll need to access the frame rail bolt from under the car.
You might be able to get to the engine mount connection point from the top. You may have to remove the pass air box.

ok so I cleaned the heck out of this ground and checked the ground at the battery too. I put in another rebuilt alternator from FCP, made sure brand new battery was fully charged, and when I started the car I still have battery light. what are the odds that this is a bad alternator that just showed up?? could it be something else like a fuse?

[JimLev]

Have you checked the battery voltage when the engine was running at idle and at 1500 RPM?
Post those voltage readings.

[e39.m5.2000]

Have you checked the battery voltage when the engine was running at idle and at 1500 RPM?
Post those voltage readings.

ok 12.4 volts off, 11.96 volts at idle and same at 1500 RPM. all signs point toward bad alternator BUT could there be something else like a ground or fuse? seems super odd that the alternator would be bad upon arrival.

[e39.m5.2000]

so i got mad and took the alternator out. had it tested at advanced auto parts. it failed. i suppose it's possible that it could have come dead on arrival. My battery is fully charged from a battery tender and it's essentially brand new. cables were disconnected from the battery during alternator install. Grounds are clean and tight. Is there any possibility I could have "blown" it somehow?

[computiNATEor]

Nah. Buy a new alternator. Not refurb'd, new. Refurbs are crap; my first 530i went through at least 4 alternators. OE, refurb Bosch, refurb Bosch, and new Bosch. The new one lasted 40,000+ miles (until I was t-boned totaling the car), the refurbs lasted 10,000 miles or less.

[Santaclaus4]

Nah. Buy a new alternator. Not refurb'd, new. Refurbs are crap; my first 530i went through at least 4 alternators. OE, refurb Bosch, refurb Bosch, and new Bosch. The new one lasted 40,000+ miles (until I was t-boned totaling the car), the refurbs lasted 10,000 miles or less.

On my 540i I've been through a couple rebuilt alternators myself. They do seem to last about 10,000 miles. I keep sending them back to FCP.

[RobertDenali]

You should be charging at 14V. Get a new alternator.

[JimLev]

I bought a rebuilt water cooled alt from BMW about 12 years ago. I needed one in a hurry and this was my only option to get the car back on the road quick.
Its still in my 540 and working fine. The charge voltage will always vary, not always 14volts. That's what the regulator is for, to vary the charge depending on the needs of the battery and the what the car is using.

In your case e39.M5, the "new" alternator is bad.

[AquilaBMW]

I'm not sure there are any "New" alternators left at Dingolfing. I think the might all be remanufactured ones unfortunately.

[e39.m5.2000]

Thank you everyone for all your help and advice!!!

[e39.m5.2000]

One thing I wanted to do was check the voltage on the square connector that goes into the alternator to make sure it was functioning okay. The closest thing I found was for an e39 (image attached) that showed how to check voltage across this connector. Although this is a different car, the voltage readings I got across both wires in the connector did NOT match what is described in the image, but rather is essentially battery voltage across both wires when the key is turned to "ignition on." Can anyone verify that these voltages I am showing on my car are correct? alternator_operational_check.gif

[StephenVA]

Without spending way too much time on starting & charging systems testing techniques, here are the steps you need to do to determine do you have a battery issue, a charging system issue, or a primary wiring issue.

You need to read battery voltage before starting any test on a electrical system as it will influence the results: Starting, Charging, electrical tests of any kind. (Note: battery must be fully charged, See chart)

Step one: Get out a digital volt meter, Read battery voltage (Key off). Read voltage (Key on) at the following points Battery, under hood jump point, and at the alternator. You only need to determine if you have 12 volts at the alternator with the key on (BIG FAT WIRE). A simple hand held digital wave form tester will get you in the ball park when it comes to batteries. (Further discussion on these testers is for another engineering forum). I have a hand held Chrysler tester that was put out in 1980 to help low skilled techs determine charging system functionality. It has three lights: Battery voltage good (12.6v), Alternator good (13.5v), Fault (anything else). That’s it. I had this in my bag of tricks when I sold auto testing equipment.
Second: You need to read the voltage at the BATTERY at idle, 1500, and 2500 RPM to determine voltage regulator function. Better still would be a Volt Amp tester (VAT-40 Image below) to induce a correct AMP load to read actual AMP/Volt output from the alternator and battery, as that is the only true testing method.
Three: Remove and clean each and every ground in the trunk, jump point, and at the frame rail for the block (bad grounds = lots of electrical issues)
Four: Failure to get correct output on running test means DEAD alternator (volt regulator, brushes, windings, etc.) Remove and bench test the unit or drag it to your local auto parts store for a second opinion (which will be worth exactly what you pay for it....)
FIVE: Replace your alternator with the correct manufacturer and amp output, See https://www.realoem.com/ for YOUR car details. Bosch and Valeo on E39/E53 models are both OE vendors.

Base line numbers
Battery Voltage should be 12.6+V (Key off) See chart below.
Battery voltage should be 13.5v - 14.5v on running tests (note large fluctuations at running RPMs is a SURE sign that the Volt Regulator is not doing its job. Smack with rubber hammer and retest. Rebuilt units have large variance of quality control from good 85% of the time to approx 50/50%.
NOTE: The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.8 to 14.3 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might be 14.9 to 15.3 volts. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts.

Alternator tips
To quote another poster...
DIY if you do not own a VAT- 40 here is "How to load the alternator" at engine idle for alternator output tests:

Note: Alternator output should be tested with the engine idling under the following four loads after at least 15 seconds of idling (for the alternator to ramp up):
Turn the high beams on
And, turn the rear defroster on
And, turn the fan on (full force, I guess)
And, turn the wipers on (to the regular setting, I guess)

These loads should be left on while the alternator output tests are being performed.


Once access to the alternator is gained (access for the I6 is vastly easier than the V8):

• Check the voltage from the alternator #30 post and ground
  
  • It should be the same as the battery voltage
  
  
• Now disconnect the harness connector to the alternator:
• Check voltage from terminal 15 and ground (field voltage)
• It should be the same as the battery voltage
• More info: Another way to check alternator output is with an oscilloscope. Observing the "ripple voltage" pattern will tell you at a glance whether or not all the alternator windings are functioning. A "good" pattern should look like the top of a picket fence. If any of the humps are missing, it means one or more of the windings is grounded or open, or there's a bad diode. Most battery/charging system testers also have a test function that can detect bad diodes.
• ANOTHER QUICK CHECK FOR BOSCH ALTERNATORS
  
  One way to check the integrity of the alternator and diodes on Bosch alternators is to check the voltage readings at the D+ (blue wire) terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the alternator.

Volt Drop testing (Image below) can show you issues with grounds and wiring problems. Read the text and test a few spots to determine if your car has clean connections.
https://www.engine-light-help.com/voltage-drop.html

Are you having other electrical issues? Alternative issues: Ign Key switch failure, wiring junction box rusted (see water leaks and under passenger seat wiring connectors), etc....

DIY Sites for more info:
https://www.aa1car.com/library/2002/cm10220.htm
https://www.bimmerfest.com/forums/sh...d.php?t=510579 (Everything you wanted to know about BMW starting and charging systems for DIY)
https://www.bimmerfest.com/forums/sh...d.php?t=590845 Ripple discussion

Low budget backup sensor idea included at no charge....

Helpful?