Alternator light still on after new alternator

[Brockallen91@ya]

Battery light has been flashing for a year but didn’t seem to cause any problems but now if the car sits for a couple days it’s not completely dead but needs a jumper so I replaced the alternator with a new bosh and now the light is on all the time and really bright and don’t charge at all .any ideas before I spend a bunch of money,I’m a mechanic but not familiar with bmws

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I’ve charged the battery and get 13 volts but then when I start it it drops to 6 and wants to die,I’m lost it was better before I put the alternator on it

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I’ve been reading a lot my next move is to check the cable under the car to see if oil has loosened it after that should I get a battery or is there something else I can test I have a volt meter ,what else can go wrong?

[acspecialists]

Double check the connection to the alternator and the fuse.

Charge the battery, disconnect the charger and check the voltage. Going from 13 to 6 that quickly has me wondering about the battery. You can also unlock the gauge panel to read the voltage while the engine is running. If it was fine before and there is a problem now double check your work.

[CPHES]

Maybe a bad replacement - lights on usually means bad voltage regulator or diodes

[Timm]

Dead battery AND a dead alternator - unless the B+ connection at the alternator is not tightened down correctly...

[Brockallen91@ya]

Maybe a bad replacement - lights on usually means bad voltage regulator or diodes

I had the battery tested,it was bad I bought a new one and did the diagnostic on the dashboard and it putting out 11.6 .when hammer it it goes almost to twelve so I’m thinking maybe I did get a bad alternator but it was a new Bosch,I have a voltage regulator I’m going to put in my old alternator and see what happens,I’ve never heard of so many people getting bad new alternators,I guess this is common with the water cooled ones? I did get it on eBay for 176 $ new from the part store they are 375 should I get the more expensive one,is there a difference,I would just like to hear from experience I know about cheap parts but it said brand new Bosch,or is the belt tensioner really that big of deal,the belt seems plenty tight enough and it’s not running hot so I really don’t think that’s a problem but any information would be highly appreciated thank ya

[Timm]

A new (exchange) Bosch water-cooled alternator is available from BMW at over $1000 - all the others are 'refurbished' - and that can mean anything from a Chinese one to a second-hand one from a scrapper that's been wiped with an oily-rag. And yes, they are an unknown quantity and half the 'battery' problems we have seen recently are due to dodgy replacement alternators...

[acspecialists]

Since you mentioned the belt tensioner. You have to preload the tensioner and then tighten it. On domestic cars you load it, put the belt on and then let go and it's tight. On and E38 you put the belt on load the tensioner about 1/2 of the full travel and the tighten the bolt to lock it in place. The reason I says this is because you are getting some voltage, the belt may be slipping?

[Brockallen91@ya]

Thanks a lot,I had no idea they were that expensive and I’m sure that’s the problem,I have a new voltage regulator,do you think if I put the new regulator in the original alternator that would fix it? But the regulator was also an eBay item but it is new,is it worth a try in your opinion ?

[Timm]

Never tried replacing the processor/regulator while the alternator is in place - but worth a go - apart from the diodes the rest of the alternator only fails on bearings...

[Brockallen91@ya]

Has anyone tried wiring a voltage regulator off like a harleyor any external regulator to a watercooled 740
alternator ? As long as it puts out no more than 14.4 volts is there any reason that would not work,I’ve done it many times on American cars but a bmw is a different animal,has any one done this successfully ? It would be so easy and you would not even have to remove the alternator,I’m going to try it on my old one and bench test it,but just curious if anyone else has trued it,a genuine bmw regulator is over 400 bucks and I’ve got a new 40 dollar one but it’s going in the garbage,I don’t understand how they sell parts that just don’t work,I’m just thinking out of the box.im sure someone out there has tried it,they even have adjustable ones now,just a thought someone please give me some advice before I do something stupid

[CPHES]

That sounds sketchy. The electrical systems in these cars are finicky, and the water cooled aspect adds to the complexity. FCPEuro gives a good warranty, and they have a complete unit for $396 right now. you should at least test the replacement unit to see if it is the regulator. Since you seem to be undercharging, I don't think a piggyback will help.

[acspecialists]

I VR is a VR, excitation to control voltage output. It is a 12VDC system the same as any other. If you are sure of the connections and voltage control give it a shot. The water cooling is just a little BMW overkill.