lil catching fox
Junior Member
- Joined
- Aug 21, 2025
- Posts
- 5
- Reaction score
- 0
- Location
- Kansas
- First Name
- Tanner
- Truck Year
- 1983
- Truck Model
- K10
- Engine Size
- 305
I have a 1983 K10, with a TH475 in it. It was a factory 700R4 truck, but the previous owner did a th400 conversion, with an output shaft that would fit a 700R4 transfer case. I ended up burning that transmission up towing a trailer, as it was on its last leg. I found a used th475 out of a late 80’s RV and swapped the output shaft from the old transmission into it and installed it in the truck, I didn’t touch anything else aside from new gaskets. Now that the transmission is buttoned up and installed in the truck, it’s doing the exact same thing the old th400 was doing. Shifts hard from park or neutral into reverse or drive also feels like it shifts hard from 2-3 possibly. I’ve checked the vacuum modulator and it hold a vacuum, I’ve unplugged and capped all vacuum lines other than the vacuum advance and the modulator lines. Even had the brake booster unplugged and the port capped. It still shifted hard into reverse and drive. I have check and made sure I have vacuum to the modulator, and I changed the vacuum source from the carburetor to the vacuum port on the intake to give it a consistent vacuum source. I installed a pressure gauge to the driver side test port on the transmission and these are the values I’ve gotten:
Th400 line pressure warm
Park idle: 70 psi goes up to 120-150 when revd up and come right back down. Does not move when throttle is slowly increased.
Reverse idle: 120 psi quick pressure drop to 30 psi then back up to 110psi to 130 psi fluctuates slowly
Neutral idle: 70 psi
Drive idle: steady 75psi after quick pressure drop to a 40 psi
2nd idle: 120-160 psi fluctuates fast
1st idle: 140-150 psi fluctuates slowly
From what I’ve seen online these values should be more at 100psi range at idle and go up to the 150 psi range when shifted to reverse and drive.
My question is are these values normal, if not is this a pressure regulator/pump issue or a valve body issue, or something else entirely? I haven’t dealt with transmissions very much, but it just seems odd that the last transmission did the exact same thing. Although I found out that the previous transmission wasn’t getting vacuum at idle only when the throttle was pressed.
Th400 line pressure warm
Park idle: 70 psi goes up to 120-150 when revd up and come right back down. Does not move when throttle is slowly increased.
Reverse idle: 120 psi quick pressure drop to 30 psi then back up to 110psi to 130 psi fluctuates slowly
Neutral idle: 70 psi
Drive idle: steady 75psi after quick pressure drop to a 40 psi
2nd idle: 120-160 psi fluctuates fast
1st idle: 140-150 psi fluctuates slowly
From what I’ve seen online these values should be more at 100psi range at idle and go up to the 150 psi range when shifted to reverse and drive.
My question is are these values normal, if not is this a pressure regulator/pump issue or a valve body issue, or something else entirely? I haven’t dealt with transmissions very much, but it just seems odd that the last transmission did the exact same thing. Although I found out that the previous transmission wasn’t getting vacuum at idle only when the throttle was pressed.
