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Discussion Starter #1
Recently, I've noticed that the interior lights in my 01 M5 are fluctuating/pulsating at both idle and elevated RPM. I've also noticed that the taillights are doing the same. Used a voltmeter to measure battery voltage (at idle) at the engine 12V lug and got the following results:

12.8-14.3VDC - The voltage is never stable, always jumping around.

100-300mV AC ripple.

I did have the headlights on, both seat heaters, stereo, etc but it seems strange that the voltage from the alternator is so inconsistent, even with a fairly high load.

Has anyone seen anything like this before? Sounds like a voltage regulator issue to me, but wanted to run it by the collective knowledge base here first.

Thanks!
 

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Discussion Starter #2
As another data point, I just went back out to the garage (car has been sitting for ~45 mins), started it up and measured again and got 14.1-14.2V steady. No matter what I did with electrical load (I turned on/off headlights, HVAC, seat heaters, and stereo), the voltage never changed.

Sounds like something heat related, which is pointing me toward the alternator I think...
 

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Hard to say which is going on bad battery or bad Alt. If your battery is fully charged the alt turns off. A fully charged battery is 12.75 so 12.8 V, on/off is what you might have seen first. Then you started the car and stole a lot of juice, so the battery never got back up to 12.75 during that test, so you never saw the alt turn off.
Go get your battery tested at a parts store, that sells batteries,Napa or similar. They will use an electronic load tester. The battery could be surface charged or the terminals could be dirty, that also includes the terminals where they connect the leads to the body or starter. These tests are free and narrow it down.
 

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That is what it sounds like to me too. I have said that enough times and been wrong, now I know to have them get the battery checked first. On my own car I knew better so I replaced the Alt... turned out to be the battery.
 

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Sounds like the voltage regulator

Assuming battery has steady voltage with engine off, but then fluctuates pretty wildly with engine running sounds like the voltage regulator. I swapped out my alternator at the beach last spring, only to have the rebuilt one fail again within 3 months. When the rebuilt one went out I was seeing voltage all over the place, as high as 16-17v running, but never held steady. After replacing it again (alt was under warranty), the new one held voltage between 13.8-14v with engine running regardless of RPM.

Unfortunately, you still have to pull the alternator to either swap the regulator or get it on a test stand to figure out what exactly is wrong with it.
 

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Discussion Starter #7
Thanks guys. I'll have the charging system tested by a pro soon. I agree that it sounds like a regulator and that part is available separately for $160-$180, but a reman Bosch is $235 with exchange from PartsGeek right now, so I may go that route instead.

The fact that it's not particularly consistent makes it tougher to troubleshoot, but my fear is it'll leave me stranded at some point - probably at the least opportune time imaginable. :) So, depending on the results of the system test, I may just swap it out preventively.
 

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Thanks guys. I'll have the charging system tested by a pro soon. I agree that it sounds like a regulator and that part is available separately for $160-$180, but a reman Bosch is $235 with exchange from PartsGeek right now, so I may go that route instead.

The fact that it's not particularly consistent makes it tougher to troubleshoot, but my fear is it'll leave me stranded at some point - probably at the least opportune time imaginable. :) So, depending on the results of the system test, I may just swap it out preventively.
Do not risk it.. mine went out at 95000 miles ,, it was the voltage regulator ,, i am glad i made home cos car is totally died on me at home!! what is the milage? good luke on fixiing it.

Thank you.
 

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Discussion Starter #9
Do not risk it.. mine went out at 95000 miles ,, it was the voltage regulator ,, i am glad i made home cos car is totally died on me at home!! what is the milage? good luke on fixiing it.

Thank you.

Thanks. The car only has 26K on it, but I'm learning the expensive way that mileage seems to have less to do with common failures than time.
 

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Discussion Starter #10
Battery checked at my local auto parts store and its supposedly good. I'll order an alternator and install as soon as it shows up
 

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Discussion Starter #11
Follow up:

My battery died twice since the last time I posted and I ended up replacing it before I exchanged the alternator.

After putting in a new battery from Advance Auto Parts, it seems to have resolved all my issues. Charging voltage is very steady at 14.1-14.2 and I only see about 130mV of ripple, which seems about average as compared to my other cars.

So, it sure looked like a voltage regulator, but appears to be a bad battery. Go figure.

Thanks for the help and suggestions!
 

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Rather than create a new thread I thought I'd just add onto this one.

Does anyone know what would cause steady 14v @ the jumpstart post (on the engine) but back in the boot flux between 12.00-12.8v at the battery? Battery was "tested" by the store it was purchased at and it came back good...
 

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The alt goes to that post. So somewhere between that post and the battery you will likely find a corroded joint, maybe just loose. Clean the terms on the battery, speaking of that there have been a few posts on the + lead causing issues in the part where it explodes off. Inspect it closely then check the volts. Next move over to the box on the right side under the right trunk compartment door. clean and check there. Follow the - from the battery to where it grounds to the body and repeat.

That takes care of the trunk next is the engine compartment. First check the volts at the post choosing a ground on the engine next move the ground to the body and last move the ground to the - battery post, you wil need to extend that wire, jumper cables work well for that, just make sure they have good conduct. If there is any differences you will be looking for a ground strap issue. Report where the changes are and I can point you in the correct direction.

If there are no changes then you will have to follow the wire from the terminal back to the starter then back to the trunk, repeating the cleaning and checking.
 

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The alt goes to that post. So somewhere between that post and the battery you will likely find a corroded joint, maybe just loose. Clean the terms on the battery, speaking of that there have been a few posts on the + lead causing issues in the part where it explodes off. Inspect it closely then check the volts. Next move over to the box on the right side under the right trunk compartment door. clean and check there. Follow the - from the battery to where it grounds to the body and repeat.

That takes care of the trunk next is the engine compartment. First check the volts at the post choosing a ground on the engine next move the ground to the body and last move the ground to the - battery post, you wil need to extend that wire, jumper cables work well for that, just make sure they have good conduct. If there is any differences you will be looking for a ground strap issue. Report where the changes are and I can point you in the correct direction.

If there are no changes then you will have to follow the wire from the terminal back to the starter then back to the trunk, repeating the cleaning and checking.
Only 1/2 way through your suggestions. 12.8v at the battery. 12.8v at the box in the right of the trunk. All connections seem firm. Have not cleaned the battery terminals yet. 14.02 at the jump start post when grounding on a suspension bolt. This week I'll try to remove the engine ground (will have to lookup where it grounds) and use a jump start cable to ground to the battery and will report back. It's now at the point where the battery isn't charging enough on short around town trips so this is becoming urgent (to me)....


Edit: for archive purposes, grounding points in this thread: http://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/351945-where-grounds-located-m5.html
 

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Today's been a bad day for car starting for me, I needed a jump three times so far. I go to start it, the lights and gauges flicker and no starty-starty. A couple times I actually got lucky and it did start, but even then, I think it almost didn't. I juuuusssst barely got it turned over and it caught.

Got home and immediately tested the battery, 12.61v just sitting there. Likely I'll check it an an hour and see if it's dipped any. Is 12.6v good? OR is that battery change time? OR is my alternator being dumb and I need to change that instead?
 

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After the car sat for an hour, the voltage was down to 11.4v. New battery time. SO I went to AutoZone and bought the Duralast 95R-DLG. I set it to charge last night and will install it today. Yay for multimeters.
 
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