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Rb25 Alternator Rating

BHDave

By BHDave,
in Car Audio & Electrical

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BHDave Grand Master

BHDave

Posted
Posted

When everything electrical is on in my car the alternator is incapable of supplying enough power at idle to keep the voltage above 12 and so my pfc has a cry and won't keep the idle anywhere close to stable.

So basically i'm looking to upgrade to a higher rated alternator.

2 questions: Whats the factory s1 rb25det alternator rated at in amps? And what can i upgrade to without the need to rewire half the car to suit the higher current?

Thanks,

Dave

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RANDY Veteran

RANDY

Posted
Posted

You might want to do some searches about this... but people have had similar issues.

Most of the time, it's due to poor earthing, and thick (8-guage?) earth and positive battery wires seem to solve most of those issues. I haven't yet read any post where someone has upgraded their alternator, but it IS possible that the voltage regulator is shot...

BHDave Grand Master

BHDave

Posted
  • Author
Posted (edited)

I haven't had earthing issues in the past.

I suspect the main culprit in my case is my adding of 2 14inch thermos which suck about 15amps each, combined with the 044 and walbro lift pump which are about 20amps combined, headlights, stereo amp, heater fan (which seems to make a fair difference to the voltage on the hand set).

I will check the earth straps though in the mean time.

Cheers

edit: forgot to mention i swapped alternators from my old rb20 one to an rb25 one with a smaller pulley and got a little improvement but not much really.

Edited by BHDave
RANDY Veteran

RANDY

Posted
Posted

Ah yeah, not much improvement because there was potentially another problem?

Getting an auto sparky to change the grounding wire over to a really thick one is probably cheaper and quicker to try first. It should really be done on ALL Skylines where amps are used... mine used to be crummy until I had the cabling on the battery changed, now everything is bright and dandy!

BHDave Grand Master

BHDave

Posted
  • Author
Posted

Yes, the problem being that i have alot of stuff sucking power which was why i asked about the alternator rating. I don't have a 33 so i'm only talking about 2 feet of wiring between the battery and the alternator and so expect the volts drop to be marginal with the size of the factory r32 cable.

The alternator was swapped to drive it a bit harder at idle to hopefully provide a bit more power without having to go an buy another alternator. Also the thought that the 25 alternator potentially had a higher rating than the 20 as it has a larger positive terminal which i assume was due to a heavier gauge wire being used from the factory due to the bettery being 3m away.

I added 2 extra earth straps as you suggested, one off the engine loom earth point direct to the negative terminal of the battery and one off the back of the head to the body in 12 gauge as i had some lying around. That is potentially 50 amps worth of better earthing. Marginal improvement generally so yes, it appears the earthing could have been better, but i still see battery voltage drop off by about 1.5V when the thermos kick in.

So back to the original questions.... I've been told i can upgrade by about 20 amps without changing the wiring but would like to confirm it, and i still need to know what the alternator rating is.

Turbo1958 Enthusiast

Turbo1958

Posted
Posted

[boy it sems to never suprise me with some of you would be machanics or auto elecs .. i have a r33 and when you start adding two decent amps the alternater in the skylines or vl cant handle it .. they are only a 45amp one .. i have one amp in my car which has a 150amp fuse and if i crank that up my amps drop under 10amps .. get a 70 to 120amp alternator ..

Nothing to do with bad earth or anything .. vl commodore alternaters ar the same as a skyline cost about $300 plus ..

RANDY Veteran

RANDY

Posted
Posted

Fair enough dude... in this case, it might not be the earthing.

Although my VK was quite happy to run 3 amps, 4 speakers, and a sub, with two 12" thermo fans on the stocker. The high-beams dimmed very slightly with a lot of bass, but other than that, no problems.

I actually suggested changing the original earth AND positive battery cables over, and cleaning the surface for a good contact. As for my R33... the power cabling is about 2 feet also. It goes directly into a fuse/relay box, and the earth straps to the body next to the battery. So even this upgrade for me made a huge difference... I figure that i'm drawing power from my battery, not my alternator... the alternator should charge the battery, and if it's really too weak and you run everything on full ball all the time, then eventually the battery will discharge completely as the alternator cannot keep up with it.

Then again, i've always had really good batteries in my cars. It'd be interesting to see how your new alternator goes! I wonder what will fit? Maybe a VQ item or something?

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