Technical & General Discussion Area > Tech Torque Pre 1973
Bosch Alternator- change to a smaller pulley?
FickleLife:
I have a 66 with a Bosch alternator 70a BXF1238A. https://www.repco.com.au/en/parts-service/electrical-parts-vehicle-management/alternators/bosch-alternator-12v-70a-bxf1238a/p/A4230760
I swapped out the crankshaft pulley for a smaller one with more grooves to run a compressor for aircon and now the alternator doesn’t spin fast enough!
At idle the alternator isn’t charging the battery. Give it a slight touch on the throttle (maybe +100-200 rpm over idle) and we are at 13.5+ volts. This is especially problematic stationary at the lights as the electric fan kicks on and just flattens the battery over time as the higher revs when driving can’t make up for the deficit.
Does anyone have any experience with swapping out the alternator pulley? I’ve been googling around and this doesn’t seem to be a common thing to do. The one fitted is about 80mm in diameter. The one below is 68mm and should spin faster, and with 17mm shaft diameter should fit the alternator.
https://www.ebay.com.au/itm/Alternator-Pulley-Suit-A-Section-Belt-17mm-Shaft-Suits-Bosch-and-Delco-/264903472560
Any advice from the brains trust? Thank you
AussiePhil:
Hey Ficklelife,
Firstly I’m not a leco.
I fitted an overdrive supercharger engine pulley with smaller auxiliary drive.
This didn’t effect my alternator to my knowledge, but it is 90a and I have a big battery.
I did fit a smaller water pump pulley to compensate.
I was considering I may have to change to a 100 or 110 amp alternator, it all depends on cars amp draw?
I think maybe your 70a alternator could be bigger, is it older.
Changing the pulley to a smaller one is a cheap alternative to get it charging a bit better but in the long run I’d put a larger alternator on it.
Not sure how fast an alternator needs to spin to produce its voltage.
But if it’s a newish I’d fit the smaller pulley as you planned.
Cheers Phil.
Hey Ficklelife,
Do you know someone with a lathe that could machine the pulley down for you.
A machine shop would charge you more than what’s it’s worth.
Where are you located.
Cheers Phil.
GEOFF289:
Since I don't let not knowing what I'm talking about stop me from chiming in:
- I think some alternators need to reach a certain above idle rpm to actually begin charging. Even if this isn't the case, they'll be charging at a lower rate at idle than when driving and if you have a large load on the battery it may not keep up at idle
- Are you confident the battery itself is in good condition in terms of accepting and holding a charge? Supercheap will load test your battery for you and give you a printout of the result.
Your problem might be some combination of these issues.
FickleLife:
Thanks AussiePhil and Geoff, it’s a newer alternator as pictured in the repco link. Maybe 6-8 years old, so should have plenty of life in it. The higher amp alternators probably won’t work as, as Geoff says, rpm’s are too low for them to kick in.
I’m pretty confident the battery is good. I usually have it on charge/maintain when the car is not being used.
I think I’ll give the new pulley I linked a go and see what’s what. Thanks for your help.
FickleLife:
The pulley solved the problem [EDIT - no it didn’t, read on]. For anyone who might come across this post later, the pulley is OEX part no PLX001 and it’s available at repco, autobarn, etc. but the photo appears to be wrong on those sites, which is why I didn’t buy it from there - https://www.repco.com.au/en/parts-service/electrical-parts-vehicle-management/alternator-parts/alternator-pulley-a-section-id-17-x-od-68-x-w-22mm-plx001/p/A9526776
It came in black so the photo was also wrong on the eBay link I posted above.
Needed a few washers from the local bolt shop but went in and now it’s spinning fast enough at idle to charge the battery even when the elec fan is on.
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