Alternator Problems

[Mick]

Hey my alternator was a standard 80amp for my r33 gtst s2 and it just wasnt charging the battery properly so i brought a second hand 90amp one from a r33 gtst s1 and it fit in ok and had the same size pulley so all good there, but i noticed a problem with it, on a cold start it will sit on about 10.5v and stay there while idling until i give it a rev and then it starts to work and goes to 13.8v, now im getting this reading from the Apexi RSM which is connected to the ECU and it must be what the car is seeing cos even the battery light comes on when it is idling and sitting on 10.5v but goes away after i rev it, but the funny thing is that when i put a multimeter on the battery or the relay box near the alternator it shows me a different reading of upto 1v difference. So does anyone know why there is upto a 1v difference from the battety to the ecu and why does the alternator only work after it gets a rev, i read something about an exciter wire but im not sure if thats for this altetnator

[blind_elk]

I had this same problem many years ago, although I had to rev to around 3k before the alternator would start charging. IIRC, it was a blown diode in the alternator. If you can, you should return the alternator as faulty. I'm not sure if the fault is repairable.

As for the voltage difference, you lose voltage through the wiring. The further from the source, the bigger the voltage loss.

[Mick]

thankyou that sounds right about the diode, i had my original one apart a while ago to try fix and clean it and yeah i seen 4 big diodes in there so this one i got would have the diodes in it like my original one, im not sure if i should return it or open it up and put a new diode in cos im very good with electronics, would be an east fix for me to replace it and i would replace all 4 of them, i didnt make a note of the part number on the diodes when i had it apart but i assume they would have a part number on them and easily sourced, i also should try replace the 2 metal bushes i think they call them, the ones that are curved on the ends that are spring loaded and are located at the bottom of the rotor or stator or whatever the middle core is called again lol

[G37Sam]

I have the same issue, problem is I bought (what was advertised) a rebuilt alternator with completely new internals and had it shipped all the way here. Is the blown diode easy to repair? Or should I just live with this?

I live in Dubai so shipping the alternator back to the US and having it replaced is not feasible.

[Mick]

i can let you know how i opened my original one up when i seen the diodes that time but you will need a bit of electrical knowledge to be able to remove and solder in new ones, i think thats what i might end up doing with this one even tho i can return it cos they dont have another 90amp one there and if i repair this one i brought at least i know the diodes will be brand new, i think i took some photos when i opened it up if you want me to post them, it shows where the diodes are

[Mick]

oh and make sure your battery light comes on in the dash when its not working cos an autoelec told me that if that light is on it means the wiring isnt the problem

[jiffo]

I'd suggest it's a voltage reg problem rather than diodes, but I'd check brush length before digging too deep.

The brush lead limits extent of brush travel and once well worn, the brush won't make correct slip ring contact, it gets held back by the lead.

The fact it pumps out respectable voltage once you give it a rev. means it has the ability to generate OK, which it wouldn't do with inoperable diodes.

I've found with these old things, on engine start they don't charge, HICAS etc lights will flash as you move off.

Once you reach around 2000rpm the alternator will cut in and from then on she's perfect, charging at any rpm's.

Faulty regulator in those instances.

[89CAL]

I'd suggest it's a voltage reg problem rather than diodes, but I'd check brush length before digging too deep.

The brush lead limits extent of brush travel and once well worn, the brush won't make correct slip ring contact, it gets held back by the lead.

The fact it pumps out respectable voltage once you give it a rev. means it has the ability to generate OK, which it wouldn't do with inoperable diodes.

I've found with these old things, on engine start they don't charge, HICAS etc lights will flash as you move off.

Once you reach around 2000rpm the alternator will cut in and from then on she's perfect, charging at any rpm's.

Faulty regulator in those instances.

Agree with this. Pretty sound advice if you are willing to pull it apart to check those things

[Mick]

ok so does anyone know if it could be a diode or definatly not?

[89CAL]

We dont 'Know' what it will be

Youll either have to pull it apart and check or take it to someone

Or get a new one

[Dose Pipe Sutututu]

Then there's another issue of them producing next to no current/voltage as rpm increases too.

I've had two stock RB ones do just that, as soon as the RPM increased past 4500rpm voltage drops dramatically and once you back off the accelerator the voltage went back to normal.

Ended up just buying an aftermarket 85amp bosch unit from a VL Commodore with the bosch to Hitachi harness. Works great now, however because it has an external fan it's makes this stupid whine noise which I don't like lol