2002 BMW M3: Time for a New Alternator?

Five years is a good life for a BMW battery if it still has the stock battery. It's cheaper than a new alternator, and Brent Romans can help you find it. :)

If it had a voltmeter instead of an idiot light, you would know instead of guessing.

makes one long for the G8 display! :) Just kidding.

Has Summit done your valve adjustment yet? were any valves out of spec?

I would load test the battery first; I've never had to replace an alternator on any of my eight BMWs. The closest I came was replacing the brushes on the Bosch alternator of my 1973 Bavaria 3.0. I'm not saying it can't happen, but it is extremely rare for a BMW alternator to go south before 120K. If the alternator is dead, you can go to BMA Auto Parts and pick up a Bosch rebuilt unit for $250+core. If the battery is kaput, go with an Interstate MTP-H7.

So we have 2 votes for battery. Anyone willing to put money on it?

I'm betting the alternator. The idiot light usually indicates charge voltage. In the stone ages, I seem to recall these idiot lights as being an indication of the voltage differential between the alternator output and the field coils. A battery bad enough to drag down the voltage that much wouldn't be able to run the starter and I don't recall Erin mentioning anything about a jump start.

Right, no jump starting required. Car starts normally.

If it's stock battery, I'm going to say battery is gone :)

If it's not stock battery, I say it's a possible bad ground or wiring problem.

I don't see it being alternator...it's not a known problem on these cars, and 5 years is a short life for an alternator.

Joe

When the battery in my E46 had a cell go bad, the car became hard to start, but this idiot light never came on.

"I've never had to replace an alternator on any of my eight BMW"

I've never had to replace the alternator on any car that I've ever owned, BMW or not.

Where I live, when a battery gets to be six years old, I just replace it. I don't need to be stranded at 30F below zero due to a dead battery.

Replace the battery. If the problem persists, have the alternator tested. If the alternator is bad, it was a good idea to replace the battery anyway.

"I've never had to replace the alternator on any car that I've ever owned, BMW or not."

I haven't had to either, with the exception of my 1984 T-Bird Turbo Coupe work beater.

I bought a 94 eagle talon 5-speed 1.8 liter 4-cyl for $500 off the side of the road.

I was on the way to sell it to a wholesaler, 5000 miles later, (for $800) and the alternator died.

Wholesaler agreed to take it for $300.

But hey, it got me around for 3 months and got 38mpg. :)

Joe

If the car starts fine, runs fine and does everything fine, but the light stays on then I vote bad connection somewhere, but the battery is still getting enough juice to stay charged...

Usually a bad battery will fail or make the car difficult to start whereas with a bad alternator the car will no longer run once the battery's juice is depleted. This is in normal simple and uncomplicated cars. Not sure if it works the same in a fancy shmancy BMW, but I am sure the basic need for power and delivery are the same.

"I'm betting the alternator. The idiot light usually indicates charge voltage. In the stone ages, I seem to recall these idiot lights as being an indication of the voltage differential between the alternator output and the field coils. A battery bad enough to drag down the voltage that much wouldn't be able to run the starter and I don't recall Erin mentioning anything about a jump start."

I agree - it's not the battery. Bad alternator or voltage regulator.

A car like this should have gauges though, not idiot lights only.

You know, my '02 Z06 has a full set of gauges including a voltmeter... (sorry, couldn't resist :p)

i believe you can check voltage through the OBD II port with the right scan tool...

I agree with milt721. You guys should own an OBD II scanner anyway given the amount of cars you test. A good one is about $300 and it comes in very handy in diagnosing problems.

Battery/bad connection or bust. I don't think the car would be driving normally at this point if it was the alternator failing :)

Joe

Definitely the alternator. This is a known issue on the E46 M3 and there's a sticky about it over on M3Forum.net. These alternators usually go between 30-50k miles, so consider yourself lucky that it has made it this far. Don't screw around with this one as all of the parasitic electrical demands can kill a battery right quick if the charging system isn't working properly (ask me how I know). Once the stereo starts acting up and the idiot lights start coming on one by one, it's time to find a safe spot on the side of the road.

I didn't know the alternator was a known issue on the e46 M3.

Random side note: I do know that Honda beefed up the alternator in the 06+ Civic SI to handle the engine spinning at 8000 rpms regularly. Since the M3 shares a similar redline, I wonder if this is a relevant design issue?

Joe

Ummm...just curious, but how much money have you sunk into this car since getting it?

"Ummm...just curious, but how much money have you sunk into this car since getting it?"

None of them care or keep track, since it is not their own money.

joefrompa,

"I don't think the car would be driving normally at this point if it was the alternator failing"

This is incorrect. Electricity is electricity. Car doesn't care where it's coming from, only that there's enough of it.

"i believe you can check voltage through the OBD II port with the right scan tool..."

Yeah, you can hook up a $500 OBDII scan tool to get voltage, but you could also use a $5 multimeter :-)

"Ummm...just curious, but how much money have you sunk into this car since getting it?"

None of them care or keep track, since it is not their own money."

We keep track of every penny spent on our LT cars (with the exception of fuel, we only keep track of volume) and post the full amount spent in the long-term wrap up. We also spend a lot of time on these cars debating how / where / what to repair.

-mike

When you're ready to sell this car, let me know.

7driver -

My quote"I don't think the car would be driving normally at this point if it was the alternator failing"

Your quote "This is incorrect. Electricity is electricity. Car doesn't care where it's coming from, only that there's enough of it."

When I've driven a car with a failing alternator (which was only once) it had trouble maintaining proper electrical delivery during normal operation. The result was dim lighting, occasional bursts of flashing lights on the dash (i.e. all the idiot lights would come on dimly and fade back out)....

Now, the battery could've been fried from what the alternator was doing....but I attributed it to the alternator itself (since the battery was at least jumpable).

The final failure was when the car stalled ON THE WAY to be wholesale'd, and would not stay started even when jumped out of the back of an old firebird with a ginormous battery and some serious jumper cables. Ah, fun times :)

Joe

I vote alternator or voltage regulator.