JJCIII

Does anyone know the diameter of the 4.0L alternator pulley from a 90 amp alternator? I have a high output alt but I still have the pulley from the 90 amp. I need to bump up the rpms a bit.

dave1123

I don't think changing the diameter of the pulley is going to change your output. Going smaller will increase the rpm for going slow, like in rock crawling, but will be too fast for highway use. There is a point when they can get fast enough to throw windings and burn up bearings. What amps are you pushing now?

JJCIII

It's going to get my voltage to stay at or near 13.8 at 800 rpm idle versus the NO CHARGE while im in stop and go traffic and never on the highway. I know all the pros and cons. I fix EVERTHING on my vehicle so I'm not worried about easily replaceable rotors or bearings. I asked for the diameter because the high amperage stator I have installed requires a higher rpm than the stock pulley can provide.

dave1123

Well, all I can tell you is my '00 WJ has a 2" pulley with the 90 amp alternator, but I've seen inboard boats with 1 1/2" pulleys on them because their engines generally run slower to eliminate prop cavitation. Unless you're talking about "go fast" boats.

When I say mine is 2", I mean effective diameter. It measures about 2 3/8" over the belt flange. It is also a serp belt.

JJCIII

Old pulley off, New pulley on. This has fixed my problem. I now don't have to put it in neutral at stops anymore now that I have the new pulley and reset the minimum throttle position. (Raised it to help with the voltage drop at stops).I don't really recommend this. However raising it slightly is just a bandaid and to much and you burn out your brakes at stops. Now at idle it sits between 13.8-14.0, no lower than 13.8 while at stops versus the no charge. And it sits at 14.2 while driving. Now to figure out if installing the new voltage regulator will affect the charging frequency.

dave1123

Okay, I told you I had a 90 amp alternator. I was wrong. It's a 136 amp.

From your picture, it's obvious yours was too big.

JJCIII

I know and that was from the original 90 amp. The new one is 1.7" or 44mm. I buddy of mine traded my original 90 amp for a rebuilt 136 amp. I sent that in to make it 200+. Maxed out at 217. And that came back with one bigger than the one I replaced. Lol had to keep it at 1500 just to charge it. It's nice to not have to hold the brake so hard and put it in neutral constantly. Plus it charges at least .2 volts higher than the minimal recommended charge rate for the optima which was also a problem.

dave1123

I just had a brain fart! Underneath the battery, there is a temperature sensor that tells the PCM to cut back the charge rate when the battery gets too hot. Your sensor may be faulty.