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I had my alternator replaced with a rebuilt one. Everything was fine for a week. Then after a week when I put my AC on and I am at low speeds, like going to a drive thru, etc. The car's power starts going low. Its been doing this ever since, this before mentioned happens. Also this happens when I turn my AC on for some reason.

My mechanic said to get another alternator. Any tips?

Thank you.
 

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You can use the hidden OBC menu to view the alternator output voltage.
I don’t like reman alternators, I’ve had 2 bad ones in a row before. I strictly use NEW alternators now for that reason.
It does sound like you need a new one tho


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E30M3 Race F10 535 R1150Rt M Coupe
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You say low power. Can you be more specific. With the above what voltage when it's low?

Also how old is your battery? Correct size?

All the positive connections; 4 of them: alternator, starter, jump post and battery are clean and tight?

Battery negative cable is tight at both ends and the engine ground strap the same?
 

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Where did you buy the alternator? Nowadays it’s a good idea to buy only an alternator that was rebuilt by the manufacturer.
 

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I had my alternator replaced with a rebuilt one. Everything was fine for a week. Then after a week when I put my AC on and I am at low speeds, like going to a drive thru, etc. The car's power starts going low. Its been doing this ever since, this before mentioned happens. Also this happens when I turn my AC on for some reason.

My mechanic said to get another alternator. Any tips?

Thank you.
Measure the voltage across the battery when you are just idling without any aux on. Should be 14.3 VDC - 14.5 VDC. If it isn't take it back.
 

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Measure the voltage across the battery when you are just idling without any aux on. Should be 14.3 VDC - 14.5 VDC. If it isn't take it back.
If that checks out ok, see what it does with a load on it - all the lights, brake lights, blower motor on high. The voltage will drop but should come back up with in a few seconds

If your battery is more than a couple years old, or has went dead more than twice - replace the battery.


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2016 340i xD 6-spd
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When you say 'lose power,' I believe you're talking about engine power and not electric power...like it sounds like it's stalling out or something?

If so, I'd consider checking all the pulleys, but also, in case your mechanic is an idiot like me...look at the top bolt of alt. It's possible to mount alt with only one bolt holding. I did somehow. That might lead to less 'push' on alt which would lead to lower elect. power which then might affect the fuel pump.

Alt is an easy-ish diy, fyi. If you have a garage, I'd suggest $100 in tools and some time/aggravation reading/watching diys/vids. Many mechs are unfortunately under the gun to make money so will do a lot of work as quickly as they can.

It's not necessarily coincidental that you had this work done and you're now 'stalling,' but it might be. Not sure where you're at with other basic maintenance, but it is all necessary--plugs, air/fuel filters, belts/pulleys, etc.
 

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Parts store alternator testers are junk.

What you need is a multimeter. It will tell you if you are getting ~14V to the battery for charging. If not it'll enable you to track down the problem better than some idiot box.

It is possible that the voltage regulator in the ECU is bad... but I would look at the Optima with more suspicion. Their quality control has absolutely nosedived.
 

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If that checks out ok, see what it does with a load on it - all the lights, brake lights, blower motor on high. The voltage will drop but should come back up with in a few seconds

If your battery is more than a couple years old, or has went dead more than twice - replace the battery.


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Should drop about 1.2 VDC leaving about 13.2 VDC to keep your battery charged. To keep your battery from having to supplement demand and eventuall draining.
 
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