Okay, I'll try to be as straight fwd as possible without insulting anyone because God knows I dont know everything... If you took it to a shop and they checked your " transmission Gears" and then said they are just fine without having to remove the tranny and disassemble 3/4 of it to get to the rear sprag unit and rear clutch basket then you need to take it to another shop. To the best of my knowledge as of today there are no ways to check the gears without disassembling it and besides the gears have nothing to do the the transmission slipping, if thats what it is doing. The only thing the gears do in the transmission is to give you your gear reduction manually if down shifted in 1st or automatically if left in drive. Just about all gear drives in rear applications are just a planetary gear and a sun gear. Depending on what transmission it is this is done by clutchs or a band. High pressure fluid is forced through the middle of the main shaft through your valve body and then out a small hole in the rear clutch pack, the pressure of this fluid is what compresses all the clutches together and now your planetaries will turn on the inside of your ring gear creating a gear reduction, some units will use a band around the clutch basket and when pressure is applied to the outside of the pin it compresses the band around the clutch basket holding it in place while the sun gear or plantaries turn... either way, if the gears dont engage then its either 1 low fluid pressure getting to the band or clutches to engage the unit, or your clutches are burned out or the band. If you took it to the tranny shop and they said everything checks out ok but your converter, obviously the diesel doesn't have one and they were exempt from emissions then they dont have a clue. Simply put the gearsdont have anything to do with your tranny slipping if they told you this I would let them even sit in my truck. Ammco is usually a reputable shop, and everyone I have heard of gives you a warranty on rebuilds as long as you take it back to them, I believe they may be a little high. If you dont go with them, find a reputable shop that has a ASE transmission tech. If they dont I would let them touch it, transmission have alot of little parts inside but any monkey can put all the parts back in and bolt it together. Another could sign to look for is any shop that that is a member of the ATSG group. I belonged to them for yrs and just about any transmission rebuilder tell you its a must to join if your in that business just like joining here if you have a MV vehicle. A good shop will plave your vehicle on the lift remove the vacume module install a adjustable vacuum pump with gauge, remove both trasmission lines and install 2 T fittings with gauges to check the pressure going to the transmission and then back out of it. That will pretty much let them know if its a external problem or internal. The flat rate book pays 8 hrs to remove your TH400 tear it down , clean everything and rebuild it, and reinstall. I dont know anyone that will remove it start to tear it down and only replace a few parts that are messed up and then put it back together. I would pretty much say Ammco is a rept. shop, they are everywhere, I believe all there Techs. are ASE certified and I know they belong to ATSG, I have had customers in the past I had to redo thier transmissions and they were previously rebuilt from Ammco.. All outlasted thier warranty and the only causes I found they needed to be rebuilt was from the owner modifying there truck and not hooking stuff back up, people forging through water and thinking thier tranny was 100% water proof, ect. all some type of driver error. I will tell you if you go with them and they give you thier warranty, just make sure how many months and mileage, its in writing, signed and keep track of the records yourself, keep the documentation, also some places require you to take your tansmission back to them for service, if you do it yourself its void, just check all that stuff out. I believe every center will check your transmission out for you, they wont charge you the rebuild price for just having it checked.... and last but not least, They hate this when you ask for it but they can't refuse it. If it needs to be rebuilt, after you have agreed on a price tell them any part that is replaced you want the old part saved and given to you !! Nothing like someone charging you 1000.00 worth of parts and they only give you 500.00 worth back, it almost forces any place to be honest with what they remove and replace, if not then they are going to have to spend alot of extra time damaging parts that were good to begine with. Keep me posted with what happens !
Your post just took up 3 pages. I think I put enough up where I can rest