Need confirmantion please > Alternator bad?
 

Afternoon all, I'm hoping to get a little confirmation on what I believe to be a bad alternator before proceeding with replacement.

Early last night I started experiencing the following symptoms:

1) Battery light comes on
2) Short time later mild but noticeable difference in shifting at low speeds (in sport mode) followed shortly after by...
3) "Trans fail safe" message > enter limp mode b blocks from home.

I hooked up a battery charger and started searching the forums. Later in the evening I decided to drive it to the nearby store, 2 blks to hwy then got on hwy for 1/2 mile to get speed/r's up a bit to see if that helped any possible charging issues. Left the X running while I ran in to grab a bag of ice 2 blks > 1/2 mile hwy stretch then as I exit hwy and slow to stop I get "Trans fail safe" error again, abs/battery/seat belt lights. In the final 2 blocks home the dash and headlights dimmed to the point they were effectively off.

To me that screams tired alternator throwing in the towel. I charge back up to 100%. Manage to drive 3 miles or so this morning before entering TFS limp mode as I arrive at in-laws house, charge to 100% drive home same distance TFS/limp mode as I pull up to my house. After charging back up to 100% voltage on unlocked OBC is 11.5/11.6 in key postion 1, 10.9 running.

Again everything screams alternator but I hope to hear from others based on their experience to see if I am overlooking anything before I throw money/time at a new alternator.

Thanks in advance for your help!

-Andy

If the alternator is charging you should see about 13.85 Volts. 10.9 Volts with the engine running indicates a bad alternator.
Can you test your battery after you charge it and before you reconnect everything and see if it reads over 12 Volts?
If not it is weak to failing also.

Also no more the 14.5 V, bad regulator will allow the battery to cook.

Thanks for the replies, always nice to have what you think confirmed by other with knowledge/experience!

As far as replacing the alternator it looks like I will loose a fair amount of coolant thus will need to add more. Probably a stupid question but do I need to do anything to purge air out of the cooling system?

Thanks again for all the help!

Just loosen the plastic screw on top by the plastic coolant tank with system cold, start motor. That allows air to escape from the system.

Sweet, you guys rock!

Painting the inside of the house last night until 5:00am, slept for 2 hours now it looks to be another late one. Cruel mistresses the house and cars are but impossible to say no to them.

If you're driving around with a dead battery, that is one way to kill an alternator. Alternators are not designed to charge up a dead battery. A dead battery places too much load on an alternator.

Def sounds like a bad alt. How many miles on your truck again?

There are rebuild kits available, as well as re-man alts which will likely be cheaper than buying new. IIRC, my alt was about $400 from BMW.

That's what was surprising to me is it was running fine showing no signs of a weak battery, no labored starts or anything and then all of a sudden...

Nonetheless always a good reminder : )

Good point. I change out batteries when they read around 55% (12.2/12.3V) after sitting overnight. I am still on the OE alternator after 10 yrs and 170k miles because I have not stressed it by having a bad battery.

Those who continue to drive a car getting jump starts are wreaking havoc on the charging system.