Bad alternator?

[zhpboss]

Hey,
my 04 zhp was not driven for around a month (no keys) and when i finally got new keys it started right up. did it 2 more times that day and it worked fine. then when i was driving, if i went below 1500rpms the radio or my lights would shut of and the the dsc lights kept coming on and wouldnt shut off. so i drove it home parked it and took the battery to get charged at auto zone. they said it was good. so i put it back in and it worked. but after 2 days the car wont even start. im guessing its the alternator? and if it is can anyone tell me what the best replacement alternator is for my car?

[brettbimmer]

Sorry to hear of your zhp troubles. IMHO, car should still start and run (for a short time anyway) with a fully charged battery and bad alternator. I suspect it is a computer/key programming issue first, where the Immobilizer in the key/ignition system are not recognizing one another, and your car thinks it is being stolen (with the new key). Just my best educated guess, but we have some excellent computer wizards here, so hopefully one can help here!

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[brettbimmer]

Alternators can be bench tested if you really suspect it is the problem, but again, I still think it is a key recognition issue.

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[zhpboss]

i never even thought about the key being the problem. but would that explain the problem when i was driving with things turning off at low rpms?

[zj96sc]

What is your key off, key on, and running battery voltage?

[zhpboss]

key off was full charged battery. not sure exact voltage. but i can check tomorrow. then not sure about key on. and cant start it so cant get that.

[zj96sc]

If it won't even start with a fully charged battery, the problem is not the alternator.

Does it turn over?

[zhpboss]

i can hear the starter trying but it doesn't even get close to turning over. and the lights and everything flash

[zj96sc]

Interesting. I'd try a new battery, or have yours load tested somewhere else.

If the starter is engaging but doesn't have enough ass to turn the motor over, you've either got a bad battery, bad wiring, or a bad starter.

I'm not an expert on the theft protection circuit but I'm pretty sure if the starter is getting energized that is out of the equation. Anyone care to weigh in?

[zhpboss]

Well i think i figured out the problem. The alternator was only putting out 7.5 amps. it should be more around 14-15 right?

[zj96sc]

amps will be load dependent.

should be putting out 14.4 volts.

how did you get it running?

does jump starting it work?

[brettbimmer]

Interesting. I'd try a new battery, or have yours load tested somewhere else.

If the starter is engaging but doesn't have enough ass to turn the motor over, you've either got a bad battery, bad wiring, or a bad starter.

I'm not an expert on the theft protection circuit but I'm pretty sure if the starter is getting energized that is out of the equation. Anyone care to weigh in?

^This advice makes good sense to me if you are getting the starter to engage. I think that the anti-theft circuitry wouldn't allow it to get that far, though I am far from a BMW Master Tech.