1996 E38 728I Alternator question

[GrandClam 1]

Hello there.

I recently had my old alternator die in my car and decided to order in a new one and replace it myself.

I put the vin number of my car into realoem.com found the alternator, grabbed the part number and ordered a new one from ebay as there were none in my country.

However something interesting happened when i started the replacement.

On realoem it states that my car takes the 120 amp alternator and that's the one I ordered in however when I took the old one out it was infact a 14v 140 amp alternator with a completely different part number.

i looked online and the general consensus seemed to be that if the alternator is made for the motor then it will fit (it does) and work anyway (it does).

i start the car up using a jump start (battery is completely discharged from sitting in my storage unit for 2 a bit months) and it starts up fine then cuts out.

There was very little fuel in the tank so i topped it up and now it will idle for as long as i want and also drives fine.

I also put the battery on a charger over night and got about 12-14 hours of trickle charge into it aswell

I checked the voltage of the battery while the alternator is running and its reading at over 14.4 volts which means the alternator is definitely feeding current to the battery

but now the car has issues starting. I let the car idle for 40 minutes and then drove about 15 minutes home and let it idle some more to let the alternator build some charge in the battery thinking this would be more than enough

the car splutters and turns over but has an issue starting fully. 

My question is this. does this seem to be a battery problem as it was completely discharged or is it an alternator problem?

I don't know why realoem would tell me to get a 120 amp alternator (i have linked the url for it below for reference) if that was wrong yet there was a 140 alternator in there already. its possible someone put a beefier alternator in my car at some other point as it is 25 years old at this point and i know that is something people do.

I will be putting the battery on charge again for a full day and see if that helps.

interested to hear your thoughts.

https://www.realoem.com/bmw/enUS/showparts?id=GE22-EUR-07-1996-E38-BMW-728i&diagId=12_2076

 

Edited October 18, 2021 by GrandClam

[jom 96]

Is it a calcium battery? If it is a standard charger won't touch it. If its fully discharged you'll need a calcium specific charger to kick it into life.

[GrandClam 1]

im not sure i will check that. I heard liquid sloshing around in it when i picked it up so figured it was a sulfuric acid and lead battery?

[jom 96]

The calcium battery is a lead/acid battery with calcium added to the plates. In my view it's not suitable for older cars that weren't designed for it, it needs a higher charging voltage and for the vehicle to be used regularly. Check the label, google the part number.

Edited October 19, 2021 by jom
speling

[KillerX 40]

have had E38 728s with both 120amp and 140amp alternators. I think it might have to do with how much "extra electronics" you have on board

[KillerX 40]

also E38 728 is very picky on the battery, had multiple electrical problems that where intermittent ,  got new Bosch Silver , most of the problems disappeared and never returned 

[GrandClam 1]

Right I see. Well I don't think this car has much extra in the way of electronics that I can tell. I havent had it for that long but nothing jumps out at me as a huge power draw past any normal car.

I will be checking the battery tomorrow so will have a look at the type then

 

[jon dee 500]

Car batteries do not take kindly to being totally discharged (like in the clock stops working). You really need to get the battery tested. If it is old and tired it may only be taking a surface charge. It will look like it is fully charged when to you test it with a multimeter, but it may only have enough stored charge to start the car once before going flat. A battery shop or garage should have a load tester that will check that for you in a couple of minutes.

Cheers...