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The TH350 is about as simple as a GM automatic gets, and that works in your favor when reverse quits. Unlike the Turbo 400, the TH350 does not use a band in reverse. Reverse is two clutch packs: the direct clutch (the front drum right behind the pump) and the low/reverse clutch (the rear pack that splines into the case). The forward clutch is released. If either pack does not apply, the car will not back up.
Each of those packs also works in a forward range, and that is the shortcut. The direct clutch is applied in 3rd gear. The low/reverse clutch is applied in manual Low (selector in 1), where it gives you engine braking. So before you touch a wrench, drive it and answer two questions: does it shift into 3rd and hold it under throttle, and does it engine brake when you coast down in manual 1? Those two answers usually point straight at the bad pack.
Step 1: Fluid, Linkage, and the Obvious Stuff
A lot of TH350s come across the bench with "no reverse" that turns out to be something outside the case. Rule these out first.
- Fluid level, hot and running. Reverse needs the most apply pressure of any range, so a unit that is a quart or two low often loses reverse first while forward ranges still feel okay. Burnt smell and dark fluid with fiber in it means a pack cooked.
- Shift linkage and detent. Put the lever in R and go look at the outer manual shaft lever on the side of the case. It should sit firmly in the reverse detent. Worn column linkage, a floor shifter cable stretched out of adjustment, or a loose clamp can leave the manual valve short of reverse.
- Manual valve link inside. The manual valve is driven by a link from the detent lever. If it pops off, range selection goes erratic or dead. It is a pan-off check.
- Valve body pattern. Race cars and hot rods often run a full manual valve body. If the previous owner installed a reverse-pattern valve body and kept a forward-pattern shifter, the detents no longer match the gate. Confirm what is in there before you buy hard parts.
Step 2: Line Pressure in Drive and Reverse
Plug a gauge into the line pressure tap on the side of the case. Read it in Drive and Reverse at idle, then at a slightly raised idle. The TH350 boosts line pressure in reverse, so reverse should read clearly higher than Drive. Specs vary with the modulator and model code; our line pressure test guide walks through hookup and what the readings mean.
- Low in every range: this is not a reverse problem. Look at the pump, pressure regulator, a plugged filter, or a leaking vacuum modulator.
- Drive fine, reverse does not rise: suspect the manual valve, the reverse boost circuit, or a spacer plate or gasket problem in the valve body.
- Reverse pressure good, still no movement: the oil is getting made but lost between the valve body and the clutch. Go to the air check.
Step 3: Air Check the Two Packs
Drain it, drop the pan, pull the filter and the valve body, and keep track of the check balls. With regulated shop air and a rubber tip, feed each case passage one at a time. The full passage map is in our air check and cross leak guide.
- Direct clutch passage: you want a firm thump as the piston strokes, then near silence. A loud steady hiss means the oil is escaping past piston seals, a leaking drum check ball, or worn sealing rings and bushings at the pump stator support.
- Low/reverse clutch passage: same deal. A good pack clunks and holds air. Air rushing out of the rear of the case points at the low/reverse piston seals.
Direct Clutch Failure: No Reverse and No 3rd
If the car also will not take 3rd, or it flares and falls back to 2nd, stop looking at the rear of the case. Both symptoms share the direct clutch. What we find, roughly in order:
- Hard or cut piston seals. The direct piston runs an inner and outer lip seal plus a center seal on the drum hub. Heat turns them to plastic and they stop sealing.
- Pump stator support rings and drum bushing. Direct clutch oil feeds from the pump stator support into the drum. Worn sealing rings or a sloppy drum bushing bleed that oil off before it ever reaches the piston. Check ring side clearance in the grooves and the bushing fit on the support.
- Drum check ball. The ball in the drum exhausts oil on release. If it is stuck open or the seat is damaged, the pack cannot build pressure.
- Cracked or worn drum. Look hard at the lugs and the hub. A cracked TH350 clutch drum gets replaced, not welded.
- Burnt pack or a snap ring out of its groove. Measure pack clearance on reassembly; our clutch clearance guide covers the method.
Low/Reverse Clutch Failure: No Reverse and No Engine Braking in 1
If 3rd gear is solid but reverse is gone and manual Low freewheels on a coast, you are almost certainly on the low/reverse side. The low roller clutch still carries 1st gear in Drive, which is why the car pulls away normally.
- Low/reverse piston seals. This piston sits in the back of the case and uses multiple lip seals. They harden, roll on installation, or get nicked on the case bore. A single rolled seal is enough to lose reverse.
- The case cup plug. The low/reverse feed passage in the case uses a cup plug. On units that have been apart before, check that it is present, fully seated, and not leaking. A missing or loose plug dumps reverse oil.
- Burnt clutch pack. Frequent on tow rigs and cars that got rocked out of mud or snow in R. The frictions glaze or strip, and the steels blue.
- Return spring retainer and snap ring. A distorted spring seat or a snap ring that walked out lets the piston cock in the bore.
Fixing either pack means the transmission comes out. The low/reverse piston is at the rear of the case and the direct drum is at the front, so there is no in-car shortcut.
Symptom to Cause Table
| Symptom | Most Likely Cause | Next Check |
|---|---|---|
| No reverse, no 3rd, 1st and 2nd fine | Direct clutch: seals, stator support rings, drum bushing, check ball, cracked drum | Air check direct passage |
| No reverse, 3rd fine, no engine braking in manual 1 | Low/reverse clutch seals, burnt pack, case cup plug | Air check low/reverse passage |
| No reverse, 3rd fine, manual 1 holds | Reverse oil circuit: manual valve, spacer plate, gasket | Reverse line pressure, valve body inspection |
| Reverse engages late or slips when hot | Worn seals or pack starting to go | Pressure test hot; see delayed reverse guide |
| Selector in R but acts like Neutral | Linkage out of adjustment, manual valve link off | Check detent at the case lever |
| Low pressure in all ranges | Pump, regulator, filter, modulator | Full line pressure test |
If reverse does engage but takes a few seconds to bite, work through our delayed reverse engagement guide first.
Parts and Cost
Parts-only ballpark numbers. Pulling and reinstalling a TH350 in a typical car or truck is most of a day, with teardown and assembly on top.
| Repair | Parts Needed | Parts Cost Range |
|---|---|---|
| Linkage adjust or bushing | Shift rod bushing, clips | Under $20 |
| Low/reverse clutch reseal | Piston seals, frictions, overhaul kit | Roughly $80 to $180 |
| Direct clutch rebuild | Master rebuild kit with frictions and steels | Roughly $120 to $300 |
| Cracked direct drum | Drum plus master kit | Add roughly $70 to $200 for the drum |
| Worn pump and stator support | TH350 pump | Roughly $100 to $250 |
Our take: once a Turbo 350 is on the bench for either reverse pack, do the whole unit. It costs very little more in parts than resealing one clutch, and a 40-year-old transmission has plenty of other seals waiting to fail. A Banner rebuild kit handles a stock rebuild. If you want firmer, quicker shifts on a street car, drop a TH350 shift kit into the valve body while it is apart.
Shop the Part
Direct drums, low/reverse clutch parts, pumps, seal kits and complete rebuild kits for the Turbo 350 at our sister store, CoreTransmissionParts.com.
Shop TH350 Parts →FAQ
Why does my TH350 drive forward but not reverse?
Reverse in a TH350 uses only the direct clutch and the low/reverse clutch. First and second gear in Drive use neither, so either pack can fail and the car still pulls away normally. Check whether it shifts into 3rd and whether it engine brakes in manual Low to find out which pack is bad.
Does a TH350 have a band in reverse?
No. The TH350 intermediate overrun band is only applied in manual 2nd for engine braking. Reverse is two clutch packs, which is different from the Turbo 400 that uses the rear band in reverse.
Can I fix TH350 no reverse without removing the transmission?
Only if the cause is linkage, low fluid, or a valve body problem. The direct clutch is at the front of the case and the low/reverse clutch is at the rear, so any clutch or seal failure means pulling and tearing down the unit.
Will a shift kit or additive bring reverse back?
No. A shift kit changes shift feel and an additive might swell a tired seal for a short time, but neither repairs a burnt clutch pack, a cut seal, or a cracked drum. Fix the failure, then add a shift kit during the rebuild if you want firmer shifts.