Smaller alternator pulley

[77 K20]

I have a 63 amp alternator in my truck right now, and it is only a year old. Works great going down the road, but at idle- not so much.

Last night it was raining so I have the wipers on and I'm sitting at a stoplight with the headlights on. Everything is getting dim, volt gauge shows 11 volts or so. I power brake it a bit just to raise the RPM by 100-200, and the lights get bright again, and volt gauge shows 13-14 volts.

I've seen write ups on larger alternators, but all I need is more amperage at a low RPM (idle).

Putting a slightly smaller pulley on it would do this- make the alternator spin just a bit faster.

Has anyone done this and know of any part numbers? Am I overlooking anything here?

 

[350runner]

It's suggested as part of the bigger alternator upgrade as well

Sent from the dust in front of you!

 

[77 K20]

It's suggested as part of the bigger alternator upgrade as well

Sent from the dust in front of you!

Wouldn't I be fine with a 63A alternator though? I don't have a winch, no power windows, no off road lights, no power locks, no AC, no radio....

So far I've found some that have a .671 bore and I'm guessing this is the size of the hole in the middle of the pulley. Does anyone have any specs of a stock pulley?

One of the overdrive pulleys I found has an outside diameter of 2.342" and the other is 2.375"

 

[77 K20]

I *think* the stock pulley size is 2.6" with a bore of .67"


Found how to calculate the alternator RPM:
Alt RPM= (crank pulley dia X Engine RPM)/Alt pulley dia

My HT383 only idles at 650 RPM.

A stock alternator pulley then is turning 1,725 RPM at idle.
The smallest alternator pulley I found (2.342") would raise the alternator RPM to 1,915.

That should do it...

On the flip side IF I ever redline the engine I figured out the RPM of the alternator. I'll be ok. The 383 redlines at only 5,000 RPM. That puts the alternator at 14,730. (stock was 13,269)


$11.97@Summit Racing. Powermaster V-Belt Alternator Pulleys 111

http://www.summitracing.com/parts/pwm-111/overview/

 

[MadOgre]

I kept burning through those 65 amp alternators and when I upgraded to the 108 which is the same alternator just uses a 108 amp bridge, I have never changed it since. Its been on there 6 years now. Still works great.

I would think it would be more of a crusty wire syndrome though with it only taking a few hundred rpm to brighten the lights. Mine did that a little before I fixed the broken ground from the block to firewall.

 

[77 K20]

I haven't examined the alternator wiring yet, but I do know the truck is very well grounded. I've ran an extra 8 gauge wire from the alternator bracket directly to the frame. I've also run extra grounds to the bed and cab. Existing grounds have been removed and ground down to bare metal, then put back on. I've then put sealant over the top.

When the truck is in gear it idles at just a little over 500 RPM. I did find a performance chart showing different Delco 10SI alternators:

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If my truck is in gear at 500 RPM, the alternator is only turning 1,320 RPM. The chart shows it is only making maybe 15 amps? And a lot of crawling I do off road is in gear at idle.


I just ordered the pulley, so I guess I'll see what happens. I think it will work... and $13 is cheaper than a whole new alternator.

 

[77 K20]

I was looking at the CS 130 alternator, but they don't do well at low RPM either.

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[scottybaccus]

Read up on a self energizing jumper mod. You may just be losing the field at idle. Basically, I'm referring to making your alt a one-wire by jumping the field wire at the case with a diode. Google it.

 

[350runner]

Yes you would be just fine with your stock alternator. Most common reason for a failed alternator is too low of an output at idle. To answer your question yes you will see allot better charging at idle if you go with the pulley you mentioned.the self exciting mod is cool but you still won't start charging until the set point is reached as said in the graphs posted

Sent from the dust in front of you!

 

[89Suburban]

Great info here.

 

[J Knight]

I just had the same problem with my 78, the alt that was on it had a large pulley and wouldn't charge at idle. I just bought a stock replacement with a smaller pulley and works fine now. I didnt notice what the amp output was on either one though.

 

[MadOgre]

Just a note to all. The little bolt that bolts the bottom alternator bracket to the back of the alternator does make negative DC power. So if the bracket is not contacting well(bare metal to bare metal) Then the ground side of your circuit will have issues no matter what the rest of the vehicle wiring is like.

That bolt is the back side of the bridge and yes it does affect the ground side, although the battery will compensate a lot if the ground from battery to vehicle is good. There should be a good ground from alternator to battery for optimal performance. Normally this is achieved from the alternator bracket being grounded to the engine and alternator, thus either location works well for the battery grounding cable.

If though the alternator bracket is not grounding properly then you will have issues such as the voltage regulator not being able to adequately asses the voltage needs.

A simple fix is to run some 8 or 6 (minimum) gauge wire from that little bolt on the back of the alternator to where ever you have the battery ground cable and use the same connection.

 

[350runner]

Well said

Sent from the dust in front of you!