Question: HAVE 1999 DIESEL NPR BOX TRUCK,NO PREVIOUS PROBLEM UNTIL LAST WEEK. TRUCK WAS IN SOME MUD ON ONE SIDE,TRUCK IN DRIVE MADE POP SOUND ,AT NEXT STOP TRUCK WOULD NOT PULL OFF IN DRIVE,YOU NOW HAVE TO PUT SHIFTER IN LOW RANGE TO GET IT GOING AND THEN SHIFT UP TO DRIVE ONCE YOU GET IT GOING,DRIVES FINE TO NEXT STOP , HAVE TO REPEAT SAME PROCESS. FLUID LEVEL OKAY. ANY HELP WOULD BE APPRECIATED
Answer: What I will tell you is an educated guess. It is imperative that you take the truck to the dealer or a qualified transmission replacement company for a diagnosis. Without a personal inspection and diagnosis, it’s all a bunch of meaningless words.
Dude, you broke the input sprag. That is the pop. Theoretically speaking, when you take off in drive (D) the forward clutches and input sprag are holding, thus producing a forward motion. When the input sprag breaks, it will not move forward in drive. When you put the selector in low (L) or first (1) the truck moves because the low/reverse band is applied at that point.
The idea behind applying the L/R band is for added holding power in situations like yours. Why you are not told this and no warning sticker is in your face to let you know about this feature is beyond me. This is not a unique design either. A few other transmission that don’t need to be mentioned right now have the same issues when forced to pull a too large load or used incorrectly, and they break the same parts.
NPR’s are really tough trucks, so either the tranny was ready to give out, or you were abusing it. In the future, put it in Low when you have a heavy load or are in a sticky spot when you take off.
As I said, this is pure speculation combined with 30 plus years as a transmission rebuilder and diagnostician. A diagnosis is the only way to prove it. Most shops don’t even charge the diagnosis if you get it fixed there.
My suggestion if you need to replace this normally very durable transmission is to start with a Call to GotTransmissions.com and discuss your needs and budget with a medium duty truck transmission specialist. We have a transmission waiting to ship out immediately, thus getting you back to work asap. Call now and let us council you so we make sure your best interests are covered.