Transmission has no forward gears after rebuild

Kevin Culhane · October 15, 2019

Hello everyone, this is my first post here so please correct me if I have something wrong.

anyways I have a 1994 Chevy k1500 that I got with a blown trans. I have installed a rebuild kit with all the clutches and seals as well as replacing the internal trans harness and solenoids. Reverse works flawless but if I try to go into any of the forward gears [ D 321 ] it acts like a neutral. The rpms sputter and drop slightly when I shift to a forward gear but not nearly as much as I hear in reverse. If I unplug the transmission wiring harness I get it to work in reverse and third gear, I think these are the fully manually selected gears but I’m not positive on that. I know the Trans fluid pump is working because I have reverse and the fluid level on the dipstick goes down with the truck on vs off. Any ideas?

davester · October 15, 2019

read codes from the truck? you might be able to use "the paperclip trick" to read codes from the truck (you short two pins together on the odb connector and a light on the dash flashes to indicate the code) [i'm not sure if your truck supports that, or if it needs a proper code reader].

is the fluid level high enough (as it, it reachs the cold level with the engine running)?

past that, I don't know. when I rebuilt my 4l80e, it worked right afterwards

Kevin Culhane · October 15, 2019

6 hours ago, davester said:

read codes from the truck? you might be able to use "the paperclip trick" to read codes from the truck (you short two pins together on the odb connector and a light on the dash flashes to indicate the code) [i'm not sure if your truck supports that, or if it needs a proper code reader].

is the fluid level high enough (as it, it reachs the cold level with the engine running)?

past that, I don't know. when I rebuilt my 4l80e, it worked right afterwards

I don’t a obd 1 scanner but I’ll look into that paper c lip trick, I haven’t tried that yet.

carkhz316 · October 15, 2019

Does it move forward in 1,2 or 3/d range? If so, you have a problem with either the forward sprag or the low reverse roller clutch being broken or installed backwards. What was the initial reason for the transmission rebuild originally?

Edited October 15, 2019 by carkhz316

Kevin Culhane · October 16, 2019

12 hours ago, carkhz316 said:

Does it move forward in 1,2 or 3/d range? If so, you have a problem with either the forward sprag or the low reverse roller clutch being broken or installed backwards. What was the initial reason for the transmission rebuild originally?

It doesn’t move forward in any of the forward gears (drive, overdrive, 321) unless I unplug the harness from the outside of the trans case in which case I can barley move forward when I give it some gas ( I think thes is manual third ) I think the previous owner said that the truck was hard shifting and no first, but the flex plate was broken when I got the truck so I couldn’t confirm prior To tearing down the tranny. When’s I took it apart for the rebuild all the clutches were completely shot, fluid was black, and the planetary set towards the bottom of the case was damaged.

carkhz316 · October 16, 2019

So unplugging the main connector simulates a limp mode/ loss of power, which defaults the trans to third gear, which is what you have. Therefore, the 3-4 clutch pack is good, but since you have no forward gears in any other instance, I'm still concerned that there is a problem with the forward clutch or forward sprag, or poor line pressure from the PCS. Do you have a oil pressure gauge to check for line pressure on the trans?

Kevin Culhane · October 16, 2019

6 hours ago, carkhz316 said:

So unplugging the main connector simulates a limp mode/ loss of power, which defaults the trans to third gear, which is what you have. Therefore, the 3-4 clutch pack is good, but since you have no forward gears in any other instance, I'm still concerned that there is a problem with the forward clutch or forward sprag, or poor line pressure from the PCS. Do you have a oil pressure gauge to check for line pressure on the trans?

I don’t have a gauge to check for line pressure but I could probably get one, do you know how much pressure I should be looking for? I know the fluid pump is doing something because I have a noticeable difference in fluid level when the truck is idleing vs off but that’s not to say that I have enough line pressure to operate properly.