Do you know how to work on your truck?

wrong guy to ask about a JWAC bug motor, I’ll take that big cam no problem,

that hot garbage 15 can go in the scrap though

 

I'll take a 15 with a yellow motor all day long. But I understand with the Cummins you're gonna need a few more gears.

 

With only one out of every 25 built back in those days with Cat power finding 15 would be a life long quest lol.

 

A 15 with a Cummins works well with higher rear gears. I just cleaned up an ‘81 W900 for a friend that has a 3406A and 15. It was dealer ordered with a 12513 and 4.11’s. Original owner had the dealer change the trans to a 15 at purchase. Sometime over the years the rears were changed to 3.55’s. Geared too fast. The Cat struggles with it pulling welfare wagons.

 

I have no background in working on engines but when I became an OO I started watching a lot more YouTube videos and asking questions here. There are a lot of really helpful great people on this site. After about 5 years of running the same truck with an isx15 Ive learned a ton. Id say about 80% of the work I do the truck is related to the aftertreatment system. Have a spn 3936 I've been working on fixing right now actually.

But yeah, Ive learned that these things aren't magic and sometimes you just have to dive into a job and figure it out as you go.

 

you don’t really need more gears, but swapping in an RTlO will give you a lower low and a taller OD, I can’t recall what ratio low is on a RTO though, but they did have a shorter OD than the 15s of the time

it’s really only the last 2 I like being able to split any way

 

Yes I was being facetious, or maybe worse. I'm not an expert on gears, especially with transmissions. But I do know I had an 88 with a B model ATAAC cat motor that came out with a 15 speed and 3.70 gears. When the truck got close to 1,000,000 miles on it I switched the 15 to a 14 613 double over and 390s. That was the funnest truck I ever drove. It had a low medium and high and it sure would scoot.

 

I get that an A and a B are two completely different animals. A B with a 15 on tall rubber and 3:36’s is hard to beat. My brown truck has 3:55’s and a 15 it’s a freight train stock 4 an a quarter