2002 4x4 2500 gmc

[MEM1971]

Hey guys I have an older model 4x4 gmc. One day the idler pulley or tensioner pulley came apart a block from my house so I was able to make it home and this was the night before I left for Toledo back in February. Didn't notice anything on way up there other than I hate the new clutch fan I just put on as it stays engaged more robbing hp and seems to make tranny shift a little different while engaged. I pulled a trailer back for my cousin he left at our camp and was the worst 1 1/2 hour drive I have had in that truck. Tranny didn't want to shift into overdrive and sometimes once it did didn't want to shift down and out of overdrive. Then I get back to town and the next day would take anywhere from 3 seconds to 30 seconds for it to go into reverse when cold, mostly. Did some research and didn't like was I was reading and then I rembered that belt coming off. It did get a little hot and I'm sure battery was too weak to crank back up. So I thought maybe since I have that bully dog tune on it the computer may not have corrected itself. I reprogrammed it back to the same performance tune and the tranny started shifting crisp and again and when I put it in reverse it immediately went into reverse. That is until this past Sunday. It went into reverse just fine when I left for work but then around 11 I went to the store and it took almost two minutes to go into reverse. I was about to give up but it finally went. Did not have a problem til the next morning and it's back to taking 3-30 seconds before it goes into reverse again. So I thought well it wasn't the computer after all but just in case I reprogrammed it again. Did not fix it. It's an old truck but it is like new inside and out with only 118,000 miles on it. My cousin bought it new then I got it in 2014. I want to drive it and put a few hundred miles on it to see if it goes away again but if not I guess I'll start with filter and tranny fluid. One thing the fluid has been about a quart high since I got the truck. Some say too low especially but too high could cause it. But why drive for 4 years and 30,000 miles with no issues. All fluids in truck are full synthetic. Cousin is picky and keeps his trucks clean like I do and is main reason I bought this truck. It shifts fine so far in all 4 gears goin down the road so I'm having hard time thinking tranny is goin out or needs rebuilding. If anyone knows or had this happen please post how u fixed it

[Broke Down]

Diesel or gas?
Was the trans fluid really dark and smell burnt?

[clay67]

I have a 2010 gmc 2500 diesel and my transmission control module went out causing it to get stuck in 3rd gear and shift hard from reverse to drive. I have friends that had different symptoms when theirs went out. Mine went out suddenly driving to work. Even if you have a gas truck you will have some sort of computer control for the transmission. Also check the altenator. High voltage can make electronics do strange things just like low voltage can. It's better to try and think of the cheaper and easier fixes first.

[MEM1971]

I agree that it's likely electrical cause it's almost all electrical computer controlled now and it shifts too good goin down the road. Was hoping it would go away like it did first time I even reprogrammed computer a couple times but no luck. Only has issue not engaging in reverse for 3 seconds to 2 minutes but only when cold or sitting for several hours. In between goes right into reverse gear. Fluid looks and smells fine. It is almost a quart too full but has been that way since I bought it in 2014. First I'm going to drain some fluid and add some trans conditioner or something like that. If that don't work I'll likely have the filter and fluid changed. I hate doing that cause I've had bad luck in past never working again after filter change but that was a Ford trans. Some say not to flush it so don't want to do that yet. If it was shuttering going into overdrive I may flush butthen again that's more of a Ford issue also. I'm worried it will lose reverse completely when I need to pick up boAt at launch. It is weird for sure!

[Broke Down]

Sounds like the check ball at the valve body has a groove in it. When you crank it hit the e brake and throw it in neutral for a minute or so then drip it in gear, I bet it goes right in. Sounds like it isn't holding the fluid pressure to engage when it sits. My old Dodge would do the same thing.

[MEM1971]

maybe idk. found a trick that has worked everytime so far. i drop it down to drive with foot on brake i give it some gas. not enough to roll tires or anything but enough to make it rev good then go to reverse and goes right into to gear. weird! worse time in peak fishing season so hope it makes it for awhile

[Broke Down]

That is usually the only time problems like that come up...lol

[MEM1971]

Update. After driving 7-10 days without pulling boat the truck went into reverse right away even in morning on cold starts. Started pulling boat again recently and it's back to taking 3-30 seconds before reverse gear engages. I don't let take that long and more cause I can drop into to drive with one foot on brake and with other give it gas and then reverse and goes right into gear. There is definitely some strange coincidence when pulling something and it acting up. Weirdest thing I've seen !

[MEM1971]

Transmission will do fine if I don't pull anything for a month. As soon as I do it starts again. It doesn't always do it but on most mornings I have to drop it into drive race it up just a little then go to reverse and goes right in. Now that I have boat back on boat lift and likely won't pull anything I'm curious to see how it does. When it first happened I was worried and it bothered me thinking transmission was surely fixing to go out. Used to it now. When it cools down in a couple months I'll change fluid and filter and see if it stops

[tboy]

Friendly advice here, but I WOULD NOT change the filter and fluid. Probably the only thing keeping it going right now is the dirt and gunk in the fluid. You change it, fresh fluid is high in detergent, it will clean all that nastiness out of transmission and drop it straight into the pan. I've seen several cars with the same scenario not pull themselves out of the shop with new fluid. Just my .02

[neverenuf]

Tranny is super sensitive to fluid level. Even one cup over or under can cause what you describe. Drive the truck until it's up to operating temp, park on level ground and check the fluid hot and engine running. Adjust as necessary. High fluid can kill your transmission quickly. Low fluid will cause shifting issues buy usually doesn't cause lasting ill-effects.