Charging issues with Toyota style Denso Alternator

billytuffnuts said
It's a 2 speed fan out of a early 90's Volvo 940

I looked into the pulley sizes & ratios and believe this is definitely the issue

Crank Pulley dia= 5.25"
Alt pulley dia= 2.63"
Ratio: 2:1

I looked again at the test report and they used a ratio of 3:1 (probably correct for a late 80's 4Runner). 80 amps @ 2500 RPMs (alt speed) / 3 (ratio) = 833 RPMs (Engine Speed).

With my actual pulley ratio of 2:1, at my idle speed I'm only pulling about 30 amps...

View attachment 1715545466

I'm not sure what the stock crank pulley OD is but bouchillon lists one with an OD of 7.375 and would get me to a ratio of 2.8. If my idle speed is around 900 RPM that should get back around 80 amps.

Your other alternative would be a smaller alternator pulley. Changing the crank pulley would change ALL of your pulley ratios, so that might mean all new pulleys. If everything else works fine changing the alternator pulley would be easier.

Are you running a v-belt or serpentine? March makes serpentine alternator pulleys down to 1.676, even with your current crank pulley that would be 3.13:1. V-belt down to 2.125, so that would be 2.5:1. Not all of them may fit that alternator but there are options.