[atmazza]

Hello I recently bought a BMW x3 2004 3.0 75k miles, sometimes wont turn on so I have a jump starter that I used some times than I saw the negative cable of the batterie was little loose and I change it with a new battery to. I took to a mechanic and he told me the problem is not the battery or alternator.
He explained to me that the battery is ok with around 14v but the power that is arriving at the starter is only 8 volts so I loosing some power and that starter wont work. What Im doing know is plug in the jump starter in front of the car in the cables so that way the car turn on and runs smooth.
The thing is I have to take now to electrical mechanic to run another diagnostic and spend some money AGAIN.
Have someone ever had a problem like this? I check the fuses but didnt went deep on it like taking each one of them out.
I saw that have a space for maybe an extra battery under the hood, would you guys think will be a good idea have 2 batteries? That way I don't need to spend money with mechanics because probably they will say to re wire the car or something.
Do u guys have any idea what could it be?
Thanks everyone this forum help me a lot already with other problems.

 

[can0n4]

Sounds like battery. You said you replaced it, yet when it won't start you can jump start it? Doesn't add up.

 

[atmazza]

Yes I can jump start but if I plug the jump start cables in the trunk battery wont go but when I plug in the positive cable in the front on top of engine it works, there's low power in the front cables with a voltmeter I see 8 volts in the cables on top of the engine and in the back where the battery is located its saying 14volts. I have something draining the power.

 

[DangermouseX3]

It sounds like you have a high resistance connection somewhere, possibly caused by salty air, if the car lived near the water (you are in Miami).

As a first step I would disconnect the cable in the front and clean the connecting surface and the mounting location (or have your mechanic do it) - disconnect the battery in the rear first, to be safe.

 

[atmazza]

So as you guys know there is a place perfect to place a extra battery in the front, would you guys think is safe do that? I think that might fix the problem....

 

[can0n4]

That would be sketchy.

Look for a wiring diagram and I can help you figure out what might be causing the resistance, might be something you can fix yourself.

Does anyone else have a wiring diagram handy? Even if its for an E46?