Reverse, no forward

[DancingAvocado]

72 C20

Trying to get this truck moving. Sat for 12ish years when I got it. Didn't shift into any gear once I got the motor going. Trans fluid was low, put in about 4qts to get it right. Now it shifts into reverse very tightly, drops RPMs and moves backwards. When I shift past reverse, the shifter glides right through N, D, L2, and L1 with no resistance and back to normal idle RPMs. No forward movement.

Is there a chance this is just linkage inside the trans? Or the forward clutch pack burnt up?

[Mnguy]

What transmission are we talking about? It would be unusual to have reverse and not get at least gear engagement in drive, especially if you don't feel the detent positions. Look at the shift linkage at the transmission while you have someone shift through the gears.

[DancingAvocado]

Th350. I had someone shifting while I looked and from column all the way down to trans looked fine. I guess I have to disconnect linkage at the trans and try to slide lever by hand.

[SkidmoreGarage]

Long shot, but when my trans fluid was very low, I had this happen to me. Any chance you didn't add enough, or that it leaked out already?

[DancingAvocado]

Originally Posted by SkidmoreGarage

Long shot, but when my trans fluid was very low, I had this happen to me. Any chance you didn't add enough, or that it leaked out already?

I had been filling it and shifting gears to get it this far. Was stable at "Full."

[Accelo]

"shifter glides right through N, D, L2,"

Interesting as typically you can feel the rooster comb detents.
Time to pull the pan and inspect the linkage.
debris in the pan will give you a clue on the foreword clutch condition.

[Bigdav160]

Likely forward clutch seals are hard and toasty

[HotWheelsFan]

Forward clutch assembly should engage when gear selector is in low, second, and drive. If the seals have hardened the fluid will bypass the seal preventing the clutch pack to engage. Vehicle will not move forward without this clutch pack engaging.

Problem may also be low fluid level, low internal fluid pressure, broken part, or other...

If you feel the shift detents while moving the gear selector from park to low, in each gear and recheck the fluid, you could try add a transmission conditioner that reconditions seals. The seals that apply the clutch packs work similar to the seals in brake wheel cylinders and master cylinder. When pressure is applied, the seal will expand outward to seal against the outer bore that the seal rides in and thus builds pressure in the system. If the seal becomes less pliable, it will not expand and instead of the system building pressure where needed it allows the fluid to leak between the seal and outer bore that the seal sits in.

A trans fluid conditioner can soften seals so they can expand when needed. How stiff seals can get when not being used for extended periods of time is like comparing rubber in cold weather and how stiff it gets to how rubber feels when warm. Just like a garden hose may get real stiff on cold winter mornings compared to using the same hose during the summer.

If you follow the directions of a bottle or two of conditioner, run the fluid through the trans, warm up the vehicle and trans, may take a few days for the conditioner to sit in the trans... This may help, and if it does work, regular use of the transmission for a few days could get the seals back in shape.

You don't want to run the trans if you hear noise from it, you don't want to force the trans in any way such as trying to get it to roll forward if the gears are slipping. If the trans does not work and needs to be disassembled for repair, forcing things will only cause more damage and add to the cost of an overhaul by damaging parts that do not come in an overhaul kit.

Bob

[DancingAvocado]

Originally Posted by HotWheelsFan

Forward clutch assembly should engage when gear selector is in low, second, and drive. If the seals have hardened the fluid will bypass the seal preventing the clutch pack to engage. Vehicle will not move forward without this clutch pack engaging.
...

Bob

Thanks for this. I took the pan off and looked around. Everything looked normal. Had my wife roll through the gears and the roller spring moved into the rooster comb like it should. Didn't see much gunk in bottom of pan. I will put it back together and try some conditioner. New filter and gasket while I'm down there.

[Steeveedee]

Pop the line off your vacuum modulator and make sure no fluid runs out.

[HotWheelsFan]

Looks good in there so far, I think you may have narrowed down the failure area a bit. Not much more you can do other than the conditioner.

I don't have my TH 350 book close by but there may be another test you can do with an oil pressure gauge. The side of the transmission will have 1/4" pipe fitting plugs for pressure tests. These ports lead to certain fluid circuits that have pressure under certain conditions. Just as the fluid can bypass a seal, if faulty, it will show low pressure for that circuit. When the seals hold, you will see high pressure in that circuit. Here's where you need the specs. and info. for the test ports...

The testing information will show what the pressure should be under certain conditions. I'm not sure, since my book is not handy, but you may have a port that you can connect an oil pressure gauge to that may be connected to the forward clutch pack. If so, you should have pressure at the test port when the gear selector is in D, 2, or low.

If you do test the pressure circuits, get the specs for maximum pressure readings you may get and make sure the pressure gauge and line to the gauge are rated for the same pressure. Pressures found on test ports vary from under 150 PSI, up to 300 PSI depending on vehicle, transmission, or test port. Testing chart will show normal pressure and maximum pressures for each port. SAFETY ALWAYS if you do test pressures... You don't want to get run over by your own vehicle if under the truck while in drive performing a pressure test.


Bob

[Accelo]

Having been through this many times I am going to recommend you save your new oil and pull it out and have it rebuilt. It hardly matters what the oil pressure is if it doesn't move. You have a soft to no reverse and no drive. It could be the pump is now slipping on the drive notches? As the forward clutch and the reverse clutches are different it points to a pressure issue. If the oil level was correct it needs work. Looks to be original as the filter is OEM style and the converter isn't painted. Your choice but I'd bet $20, worth of oil, it has to come out.
Cheers.

[Bigdav160]

Reverse requires higher pressure than Drive. Since the vehicle backs up it's not a pressure issue.