[Steve25]

I have a 1974 C4 transmission that has a strange problem. All forward gears work just fine. But the reverse acts very strange. If I pull into the garage, turn off the engine, then place the shifter into park, the next time I reverse out of the garage, no matter how long of time it is, it backs out in reverse no problem.
However, if I pull into the garage and put the shifter in park, then shut off the engine, the next time I back out of the garage, no matter how long of time, reverse binds up and I have to give it full throttle to get it to move in reverse. It just started doing this one day.
It is binding up like it is in drive and reverse at the same time. I have pulled the transmission, completely disassembled it and can't find anything wrong. Re assembled it and same problem. Installed a different known to be good valve body, same problem.
I have rebuilt many C4s before and never had a problem like this before. Has anyone else had a similar problem or just me?

 

[galaxiex]

Parking pawl is sticking engaged.
Does it release with a “bang”?

 

[Steve25]

Parking pawl is sticking engaged.
Does it release with a “bang”?

No it is not the parking pawl. Its bound up like it is trying to go forward and backward at the same time. Just like a trans brake would act. But under full throttle will go backwards but don't like to. At other times, it goes into reverse no problem. It just started doing this one day. I don't know why.

Dissembled/reassembled the transmission, all new seals, checked shift linkages, replaced valve body, checked clutch clearances and band adjustments, gear train end play. All within spec. No difference.
All forward gears run just fine. I am thinking to get a whole different C4 transmission as a solution.
I have rebuilt several C4s and C6 transmissions in the past and never had a problem.

 

[PSIG]

Weird. I would check how things "set" when you do your problem procedure. Before starting (or conversely before placing in reverse) get behind the rear bumper and push hard forward. Test. Re-set and try again from the front pushing back.

Is your linkage set to the VB positions or the shifter positions (indexing)? If you double-shift (P-R-P-R), or other combo (P-D-R, etc), does it change operation?

It would seem a shame to ditch the entire trans not knowing what internal parts are part of the issue, or worse save something bad assuming it's not part of the problem… but you don't yet know for sure.

 

[galaxiex]

No it is not the parking pawl. Its bound up like it is trying to go forward and backward at the same time. Just like a trans brake would act. But under full throttle will go backwards but don't like to. At other times, it goes into reverse no problem. It just started doing this one day. I don't know why.

Dissembled/reassembled the transmission, all new seals, checked shift linkages, replaced valve body, checked clutch clearances and band adjustments, gear train end play. All within spec. No difference.
All forward gears run just fine. I am thinking to get a whole different C4 transmission as a solution.
I have rebuilt several C4s and C6 transmissions in the past and never had a problem.

If it's not the park pawl, then.....

In reverse 2 things are applied.
High/Reverse clutch and Rear Band.

That leaves the Forward Clutch and the Intermediate Band.
One (or both) of those 2 is/are getting applied during reverse somehow.

Cross leak somewhere that is causing the Int band, and/or the Fwd clutch to drag.

One possibility is somehow pressure is getting "trapped" in either Fwd Clutch or Int Band Servo when the engine is shut off.

Are you certain that all Check Valves in the stator are free and functioning properly?

Got a pressure gauge?
Put pressure gauge on the mainline pressure tap and see if any pressure remains when engine is shut off. (doubtful but worth a check)

 

[Steve25]

Weird. I would check how things "set" when you do your problem procedure. Before starting (or conversely before placing in reverse) get behind the rear bumper and push hard forward. Test. Re-set and try again from the front pushing back.

Is your linkage set to the VB positions or the shifter positions (indexing)? If you double-shift (P-R-P-R), or other combo (P-D-R, etc), does it change operation?

It would seem a shame to ditch the entire trans not knowing what internal parts are part of the issue, or worse save something bad assuming it's not part of the problem… but you don't yet know for sure.

I tried the pushing thing. Does not seem to make any difference. The shifter and VB are working. It is a Locar shifter direct connect and adjusted.
The part that I don't understand is why it only happens when coming to a stop, shifting into park or neutral, then turning off the engine. Does not matter if it is 15 seconds or sits a month.

If I come to a stop, turn off the engine, then put it in to park or neutral, the next time I start it goes right into reverse and works no problem. Does not matter if it is just 15 seconds or sits a month.
It is like something in the gear train is locking and unlocking. I put in a known good valve body, and it made no difference. I am starting to think it is something mechanical and not hydraulic.
As far as putting in a different transmission goes. I am just trying to fix the problem for good as I am reaching the end of my rope. Also, it does drive just fine as long as I remember to turn off the engine first before shifting to neutral or park. So, the car is still drivable.

 

[Steve25]

If it's not the park pawl, then.....

In reverse 2 things are applied.
High/Reverse clutch and Rear Band.

That leaves the Forward Clutch and the Intermediate Band.
One (or both) of those 2 is/are getting applied during reverse somehow.

Cross leak somewhere that is causing the Int band, and/or the Fwd clutch to drag.

One possibility is somehow pressure is getting "trapped" in either Fwd Clutch or Int Band Servo when the engine is shut off.

Are you certain that all Check Valves in the stator are free and functioning properly?

Got a pressure gauge?
Put pressure gauge on the mainline pressure tap and see if any pressure remains when engine is shut off. (doubtful but worth a check)

I agree, but the reverse problem happens no matter how long it sits. Main line pressures are normal and zero pressure with engine off. I did not check the check valves in the stator. I don't see how pressure in the intermediate band servo, or the forward clutch could be trapped for long periods of time.
I thought about cross leaks somewhere other than the valve body, which has been replaced.
Maybe a cross leak between the high reverse clutch and the forward clutch somehow. The shaft sealing rings are new PFTE and were not damaged. A cross leak at the pump gasket somehow, but I see no indication of this looking at the gasket and mating surfaces. May be a cross leak at the mating surfaces of the case to the valve body somehow. I never did hear of this before, but that don't mean it could not happen. The really odd part is that all the forward gears work ok. The transmission is going to be torn down for while so I can look into it further.

 

[galaxiex]

I'd suspect the PTFE rings for the cross leak.

Try some metal rings like the factory originally used.

I know they switched to PTFE rings at some point, but then switched back to metal.
Must have been a reason for that.

 

[Steve25]

I'd suspect the PTFE rings for the cross leak.

Try some metal rings like the factory originally used.

I know they switched to PTFE rings at some point, but then switched back to metal.
Must have been a reason for that.

After going over every part all this afternoon, I found something, I don't know if it is causing the original problem or not, but it is a problem for sure. Seems the input shaft is grinding into the output shaft. Closer inspection and comparing with another input shaft show that the input shaft has longer splines on the forward clutch end than it should.
This causes the input shaft to stick out past the forward clutch drum and grind into the output shaft. I have never seen anything like this before.
Apparently, there are two different types of input shaft spline lengths out there. Whether fixing this solves the original reverse problem I don't know but I am going to give it a try. I have a like new C5 input shaft I am going to try.

 

[galaxiex]

C4 26 spline input shafts have a long spline end and a short spline end.
Short spline end goes into the forward drum.
Long spline end to torque converter.
Be sure the splines in the forward drum are not “hammered” causing the input shaft to sit too deep into the drum.

 

[Steve25]

C4 26 spline input shafts have a long spline end and a short spline end.
Short spline end goes into the forward drum.
Long spline end to torque converter.
Be sure the splines in the forward drum are not “hammered” causing the input shaft to sit too deep into the drum.

Right, but this input shaft has clearly longer splined on the forward clutch end causing the end to stick out of the forward clutch. It was machined this way. Why I don't know. Maybe someone here knows.
The C5 input shaft I have fits perfect and is flush with the surface of the forward clutch.

🙂 oops, I almost said front clutch. It is a slip from my old Ford -O- Matic days.

 

[galaxiex]

Right, but this input shaft has clearly longer splined on the forward clutch end causing the end to stick out of the forward clutch. It was machined this way. Why I don't know. Maybe someone here knows.
The C5 input shaft I have fits perfect and is flush with the surface of the forward clutch.

🙂 oops, I almost said front clutch. It is a slip from my old Ford -O- Matic days.

Do both shafts have the same overall length?

“Front clutch” is also a Chrysler term.

 

[Steve25]

Do both shafts have the same overall length?

“Front clutch” is also a Chrysler term.

Yes they are both the same length. It is clearly made this way that the forward clutch end has longer splines. May be for some special application, but I don't know what that would be. Otherwise, both shafts look identical. This transmission was rebuilt before I bought it, so the shaft could have come from anywhere. Also, I install the input shaft last, so if it looks ok, I just install it and don't think anything about it. Not anymore.

The C5 shaft looks just like the good C4 shaft except for the dip in the middle of the shaft. I understand this was done for better oil flow in the C5. But the C5 shaft will work in a C4. I think the C5 shaft is a real hardened shaft not just case hardened. I would have to check on this.

 

[galaxiex]

Yes they are both the same length. It is clearly made this way that the forward clutch end has longer splines. May be for some special application, but I don't know what that would be. Otherwise, both shafts look identical. This transmission was rebuilt before I bought it, so the shaft could have come from anywhere. Also, I install the input shaft last, so if it looks ok, I just install it and don't think anything about it. Not anymore.

The C5 shaft looks just like the good C4 shaft except for the dip in the middle of the shaft. I understand this was done for better oil flow in the C5. But the C5 shaft will work in a C4. I think the C5 shaft is a real hardened shaft not just case hardened. I would have to check on this.

Interesting....

Any chance you could post a pic of that shaft?

 

[Steve25]

Interesting....

Any chance you could post a pic of that shaft?

I am not sure how to do this.

 

[Steve25]

Interesting....

Any chance you could post a pic of that shaft?

Here are some measurements.

Performance Automatic hardened C4 shaft spline length forward clutch end .690"
Stock C5 spline length forward clutch end .726"
C4 problem shaft spline length forward clutch end .795"

Distance of shaft past the face of forward clutch drum.
Performance Automatic hardened shaft. Flush with face of clutch drum.
Stock C5 shaft .010"
C4 problem shaft .083"

 

[Steve25]

 

[Steve25]

 

[Steve25]

I am not sure how to do this.

Here is the pictures of the input shaft and forward clutch drum. After looking at these pictures and comparing input shafts, I tend to agree with you that this shaft has had a rough life and could very well have hammered out splines causing the shaft to stick out of the drum.
You can see that the input shaft was grinding into the output shaft. The splines measuring longer may be because they have been hammered out to become longer. Or a manufacturing defect.
Never seen one like this before so I don't know. The two other input shafts I have don't have this problem. In any case it is going into the scrap bin.

 

[galaxiex]

Thanks for the pics.

That shaft spline looks mostly "normal" for the short spline end.

Look carefully at the splines in the forward drum.
Look from the front to rear.
I bet the splines in there are "shorter" than they should be,
allowing the input shaft to move toward the rear and then contacting the output shaft.
I've seen that before on units that got driven hard.

That forward drum hub where the washer sits looks kinda chewed up.
I'd seriously consider replacing that drum.