Bosch Alternator- change to a smaller pulley?

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Offline FickleLife

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Bosch Alternator- change to a smaller pulley?
« on: August 09, 2022, 10:03:05 pm »
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
« Last Edit: August 09, 2022, 11:02:56 pm by FickleLife »

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #1 on: August 10, 2022, 07:06:33 am »
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.
« Last Edit: August 10, 2022, 08:10:33 am by AussiePhil »
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Offline GEOFF289

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #2 on: August 10, 2022, 08:11:50 am »
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.


Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #3 on: August 10, 2022, 12:34:47 pm »
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.

Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #4 on: August 19, 2022, 06:12:45 am »
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.
« Last Edit: October 21, 2022, 07:40:12 am by FickleLife »

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #5 on: August 19, 2022, 06:30:07 am »
Hey Ficklelife,
Glad to hear!

 :fantastic:

Cheers Phil.
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But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline pmb0186

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #6 on: August 31, 2022, 09:37:47 pm »
Are you running the original voltage regulator?
Modern alternators have the reg built in so can be bypassed
Measure the voltage of the alternator rather than battery
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Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #7 on: September 02, 2022, 10:20:01 am »
Nope, not running the original voltage reg. Voltage at the alternator looks good

Offline pmb0186

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #8 on: September 02, 2022, 10:00:06 pm »
Do you have the third wire connected ?
That’s the excite wire without it the alternator will self excite but takes a stab of the accelerator
does anyone remember laughter

Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #9 on: October 21, 2022, 07:37:31 am »
Do you have the third wire connected ?
That’s the excite wire without it the alternator will self excite but takes a stab of the accelerator

I don’t, but after driving it for a while, even with the smaller pulley, at idle it isn’t charging the battery. Give it a tiny bit more rpm at idle and I get 14v, but I feel it’s idling too fast. The rpm at “normal” idle is not enough to get it to self excite.

I’ve been googling around and I’ll probably connect the 3rd wire to solve the low rpm charge issue. Since I’m a bit of an electrical noob on this topic, I’ll see if I can find the manual and where / how to hook up a 3rd wire but happy to take some advice :)
« Last Edit: October 21, 2022, 07:39:21 am by FickleLife »

Offline alan 70

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #10 on: October 21, 2022, 08:14:55 pm »
Hi Ficklelife, it sounds like you have a 1 wire alternator, they don’t start charging till higher revs 1000-2000 rpm on initial start up, once they have reached these revs they will keep on charging even when you go back to idle speed, I have a Bosch bxf 1237 and it works fine I only have 1 wire on it, I don’t have a alternator warning light on my 70.
Good way to check is get one of those 12 v voltage gauge that goes into you cigarette lighter, turn ign on check voltage this will be your battery voltage now start car and rev voltage should increase let it go back to idle and it should still be charging. Remember charging volage will always be higher than battery voltage maybe not a lot higher if battery is fully charged

Offline pmb0186

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #11 on: October 23, 2022, 10:15:23 pm »
Agree good plan
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Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #12 on: November 09, 2022, 07:29:01 am »
So I bought a cigarette light voltage meter and it’s not charging on idle. Give the Acellerator a tiny nudge and it hits high 13s so it’s just not spinning fast enough. At cruise speed it’s all good with 13.5+ going into the battery.

I did put l a smaller pulley on the alternator, a 68mm. I have a 140mm crank pulley, so 2.05:1 is just not enough at idle to get the charge going. Add to that a power hungry fan and at idle the battery is really under load.

I can’t find a pulley for the alternator that is small enough and even then when the fan is running and I get the revs high enough to charge the battery it still is around 12.x volts when I’d like it to be 13+ under fan load. I might go a new alternator such as  https://aeroflowperformance.com/af4273-1140-ford-black-140-amp-1-or-3-wire of which the manual states that it has a 44mm pulley for a 3.18:1 ratio, which means at 600 rpm idle it should be 1900 alternator rpm which is good for 42-60 amps according to https://imgur.com/a/R30c1K6

This is probably easier than changing out my crank pulley and all the belts and guarantees with a 140a alternator the power hungry fan has enough amps.

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #13 on: November 09, 2022, 07:46:18 am »
Hey Ficklelife,
If you want to send me your old alternator pulley I’ll machine it down to your required measurements at no charge!
Let me know if that suits.
Bathurst, New South Wales.
Cheers Phil.
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But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline FickleLife

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #14 on: November 09, 2022, 09:30:13 am »
Wow that would be amazing Phil! There may be a problem with the old pulley - it has this recess which would have to be removed but then it would make it too narrow. See https://imgur.com/a/f6YlgKF

This pulley is 82mm in diameter. Reckon it can be done down to ideally 45mm?

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #15 on: November 09, 2022, 03:33:36 pm »
Wow that would be amazing Phil! There may be a problem with the old pulley - it has this recess which would have to be removed but then it would make it too narrow. See https://imgur.com/a/f6YlgKF

This pulley is 82mm in diameter. Reckon it can be done down to ideally 45mm?
Hey Ficklelife,
I see what you mean!
I could machine you an new aluminium one but whether the V belt would be harmed with that much bend radius.
I’ll look into it, is it the usual A section V belt.
You may have to look at a larger crank pulley, I’ll need to reread the post about the size of your pulley.
I’ll keep you posted.
Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

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Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #16 on: November 09, 2022, 05:11:59 pm »
Hey Ficklelife,
Minimum bend radius V-Belt   A Sec.   13 mm   57.2mm I’d imagine that’s the Outside Diameter.
I see that you’ve added a three groove crank pulley with smaller auxiliary pulley diameter.
Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

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Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #17 on: November 28, 2022, 05:34:08 am »
Hey Ficklelife,
How did you end up with your alternator.
I’ve just fitted Flex-a-lite dual thermo shroud fans that draw 19.5 amps.
With a Bosch 044 fuel pump and surge tank fuel transfer pump and a brake vacuum pump, ECU and etc. the added amp draw pushed my 3G alternator over the top, in time if this happens an overdrawn alternator the engine will just turn off.

I have a massive supercharger pulley with smaller auxiliary pulley, at the time it ran fine until I fitted the Flex-a-lite thermos, so over the weekend I machined my serpentine belt alternator pulley and manufactured a new power steering pulley to get back to original pulley ratios.
It worked a treat, the alternator runs a beautiful 14 volt and the power steering before was a bit lazy at low revs, now it’s back to normal.

Hope this helps.
Cheers Phil.

« Last Edit: November 28, 2022, 12:56:39 pm by AussiePhil »
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

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Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #18 on: November 28, 2022, 05:57:02 am »
Hey Ficklelife,
This information is off PowerMaster web site.

How to Determine Ratio and Rotor Speed

The alternator rotor RPM is not necessarily the same as engine RPM. To calculate the actual alternator RPM, determine the ratio between the two pulley diameters.

Ratio = Crankshaft Pulley Diameter/ Alternator Pulley Diameter

Now that we know the ratio, we can now determine the rotor speed:

Divide engine pulley diameter with alternator pulley diameter to get a ratio.
Mine is Crank auxillary pulley 116.84mm.
New machined Alternator pulley 58.6mm.
Equals 1.99:1 ratio.

Alternator rotor rpm = Pulley Ratio x Engine Speed
(example; 2.1 x 870 = 1827 Rotor RPM)

Tech Tip
Generally, For street use, we recommend 3:1.

Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #19 on: November 28, 2022, 11:54:52 pm »
Hey All,
Took my car for a bit of a drive today, the flex-a-lite fans really work well but effects my idle at lights.
Sounds even more lumpy but can’t be good for the battery.
And guess what, hade to fill with fuel and hit every red light.
I machined my alternator pulley to 58mm giving me about 2:1 ratio, seems like it needs to be machined even further to about 48mm.
I’ve seen some 48mm PowerMaster pulleys advertised on eBay so it must be ok for the bend radius of the belt.
Easy job to do, that will give me 2.4:1 ratio.
That should help.
If not I’ll disconnect one of the fans until I get a bigger alternator.
The biggest killer of batteries is the cycle draw from an insufficient alternator.
Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #20 on: November 29, 2022, 05:08:27 pm »
Hi all,
Machined pulley this morning to 51mm o/d. That gives me 2.3:1 ratio.
But when the second fan kicks in the engine stalls.
I disconnected one fan and engine runs fine.
I’ll machine it to 48mm as planned. That gives me 2.4:1 ratio.
If that doesn’t work I’ll have to buy a bigger alternator.
It currently runs a 95A Ford 3G alternator. 3G are mainly US stocked.
I’m off to Jaycar to buy a current clamp multimeter to measure the fans, they are supposed to be 19A total.
A big increase from the old Spal fans.
Any suggestions from brains trust.
Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #21 on: November 30, 2022, 07:44:47 pm »
Hey all,
Funny story, working under the car today going to remove the alternator pulley (yes it’s underneath) to machine the pulley down further I noticed the cable from the alternator was very small (8mm2). :lmao:
That was half my trouble! :pepper:
When I fitted a CRV starter motor I deleted the starter solenoid and ran the battery cable direct to the starter motor as per the CRV instructions? and ran this small cable back to the alternator. :cry:
I stuck my head over the fence to see my neighbour who’s a leco and said this would support much amperage would it! :agree:
So I connected the battery cable direct to the alternator and then ran a 35mm2 cable to the starter motor, in doing so removed about 500mm of excess cable.
After completing that job I tested it again and with two fans running the engine ran fine!
But when I turned the headlights on the engine stopped.
So, tomorrow’s job is to machine the alternator pulley down to 48mm and check the thermo fans amp draw with the cable clamp multimeter I bought from Jaycar.
Just checking on eBay the fans draws between 17.5-29 amps, I was working on 19 amps, but worth it, they blow like hurricanes.
Cheers Phil.
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

Two for one is good but four for two is better!

Offline pmb0186

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #22 on: November 30, 2022, 09:46:02 pm »
Since you bought your multimeter check the voltage across the battery
Simple over 12.6v charging good
Under 12.6v not charging bad
If the engine dies and you have electric ignition the voltage must be dropped under 10v.
Battery voltage is key and will tell you more than current
does anyone remember laughter

Offline AussiePhil

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #23 on: December 01, 2022, 06:26:27 am »
Hey Pete,
Thanks for responding.
I’ll check out as say.
Yes I have a Haltech ECU.
As it turns out I did a very average job deleting the starter solenoid, The CVR starter motor has an internal solenoid.
I didn’t get very long life out of a big Delcor battery, this may have contributed to this?
It’s also a long run to the battery in the boot, with a little work I could reduce that running the cable into the back of the engine bay to the starter motor and then onto the alternator, I’m using 35mm2 cable. The negative cable goes to an isolation switch and then is connected to the chassis.
Also turns out it’s not a Ford 3G alternator, it’s a Jaylec 65-0007, it has the same type of internal regulator as Ford 3G.
It has a different clock face that tipped me off.
Cheers Phil.

« Last Edit: December 01, 2022, 06:28:12 am by AussiePhil »
If you've never buggered anything you've never done anything!
But if you've buggered a lot maybe you're not very good at it!

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Offline pmb0186

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Re: Bosch Alternator- change to a smaller pulley?
« Reply #24 on: December 01, 2022, 06:03:42 pm »
I’m not a big fan of batteries in the boot due to the length of the run in relation to staring.
It all about volt drop, so a good little test is to measure the volt drop from the alternator to the battery when running. Volt drop from the battery to the fans
Volt drop from the starter to the battery when ranking
Now how to undertake get your meter on volts and measure + to + ideally you will measure 0v
You can do the same with ground
Check good ground connection between chassis, engine battery. You can use the meter to test volt drop while cranking 0v ideal
Where are you located ?
does anyone remember laughter