The only thing I can't find is the pulley ratio for the 3.6.

 

Me neither. I was really hoping you had 🤞

 

Alternatively, I was hoping someone knows how to calculate the ratio from the pulley's outside diameter (approx 51.8 mm).
EDIT: Looks like it we also need to know the crank pulley diameter.

 

The pulley ratios are listed on the page with the blue and red graphs.

 

Those are for V8s (5.7L & 6.4L) in other RAM trucks. The PM's 3.6L V-6 likely has different alternator and crank pulleys, and therefore (maybe) a different pulley ratio.

 

I don't have calipers so I made some cad calipers
180 amp alt

pulley + belt
6.125
2.250

ratio of 1 : 2.7

 

I don't have calipers so I made some cad calipers
180 amp alt
pulley + belt
6.125
2.250
ratio of 1 : 2.7

Great! So if the alternator pulley itself is 2.04in (51.8mm) then the belt adds 0.21in. Subtracting that from your 6.125 makes the crank 5.915. Divide that by 2.04 = 1 : 2.9 ratio, which, as it turns out, is the same as the V8s after all. Apologies to @larry barello. I've amended the attach spec sheet.

 

@SteveSS
I'm confused, I couldn't measure the actual metal surface of the pulleys, so I measured the outside diameter of the belts, wouldn't the ratio of 1 : 2.7 be the same as the metal pulley surface?

 

@SteveSS
I'm confused, I couldn't measure the actual metal surface of the pulleys, so I measured the outside diameter of the belts, wouldn't the ratio of 1 : 2.7 be the same as the metal pulley surface?

It would if the pulleys themselves were the same diameter, but since they are different, belt thickness is a different proportion of each pulley+belt diameter. If that weren't true, the adjusted diameters would yield the same ratio as the unadjusted diameters (2.7). Someone please correct me if I'm wrong.

 

Here the alternator, are you referring to the angle of the belt as it meets and departs the pulley (red)? Not sure how to workd that
I realize a head on photo would be better.

 

Here the alternator, are you referring to the angle of the belt as it meets and departs the pulley (red)? Not sure how to word that

No, just the belt thickness relative to the diameters of the alternator and crank pulleys themselves. Since the pulley diameters are different, I think belt thickness has to be subtracted before calculating ratio. Our two ratio calculations, with and without belt thickness, seem to prove that. Do your diameter measurements include belt on both sides of each pulley? In other words, are your diameters equal to the metal pulley plus two belt thicknesses?

 

Great info, @SteveSS!

In your first chart, what is: "Minimum Output (Amps) @ specified test conditions"?

With the optional 220A alternator, the entry shows 150A. Does that mean at engine idle RPMs? I'm hoping the answer is "yes". If so, it could mean that if the PM's own draw for its various components including the headlights, radiator fan, etc., is as high as even 80A, that'd leave 150 - 80 = 70A for battery charging; specifically for my DC-DC charger. --which would be lots.

 

In your first chart, what is: "Minimum Output (Amps) @ specified test conditions"?

The test conditions are defined at the bottom of that table: hot engine, 2500 RPM, so not idle.

 

My question looking at the charts is that it appears that if the cooling fans (65A recently cited) kick on at idle (about 700-1000 rpm) the alternator isn't making any electricity.... so the fans are running off the battery alone...or...

Do I need to multiply the idle rpms by 2.7 to use the chart? (2.7 x 700 = 1890 rpms at the alternator) Which would indicate that the 180A alternator makes about 100A at idle...

 

Yes, the RPMs in those graphs/tables are alternator shaft RPM, which is why @phil and I are trying to nail down the pulley ratio. Right now, I'm leaning toward 2.9. I'm still scouring the internet hoping to find an official crank pulley diameter.

 

Because of the belt angles on the alternator my ratio is skewed, so 2.9 is good for me.

I know a guy on YouTube who probably has a crank pulley laying around, but I really don't want to ask him

 

Are you trying not to ask @Kip-on-truckin to measure the crank and alternator pulley's diameters so we can put this to rest?

 

The genie only gives you three wishes and I don't want to waste them

 

I just found another source for crank pulley diameter and updated the attached doc. I calculated the pulley ratio at 2.84. Practically speaking, that is no different from the 2.90 listed for V8s with the same alternators. I used an alternator pulley diameter of 51.8mm. 51.776mm yields a ratio of 2.9. Probably just rounding differences among the various data sources.

 

This was at a low SOC on my lithium bank at idle on a cool November morning in Big Bend NP.