727 Goes into reverse but

[Dave6T4]

I takes about 3X the hydraulic pressure to operate reverse than the forward gears, so pump is working fine. What DVW said.

 

[BeepBeepRR]

If it has no forward gears there would be 2 prime suspects. #1 Rear clutch failure, piston seals or frictions worn. #2 Stripped front planetary splines where they ride on the output shaft. Both will result in reverse working with no forward gears. I've seen both of these failures. You won't find either of them with a pressure check. Both require disassembly of the transmission.
Doug

So I think I can check the pistons by dropping the pan and valve body correct?

 

[Rishi]

So I think I can check the pistons by dropping the pan and valve body correct?

You can check them, but if they work, you're still faced with dropping it to tear it down to figure out what's wrong. If they don't work, same-same.

 

[Geoff 2]

I would be looking at the rear clutch spring [ a diaphragm spring ] , which are known to break.

 

[Cliff Ramsdell]

Before you tear it apart start with some diagnostics. Get a pressure gauge and a factory service manual and do a pressure test as outlined in the manual before dropping the pan.

This could/will point you in a direction. If the car isn't moving and pressures are good drop the pan and inspect. If it backs up and not going forward the rear clutch pack is probably the issue but always diagnose first.

Cliff Ramsdell

 

[dvw]

Before you tear it apart start with some diagnostics. Get a pressure gauge and a factory service manual and do a pressure test as outlined in the manual before dropping the pan.

This could/will point you in a direction. If the car isn't moving and pressures are good drop the pan and inspect. If it backs up and not going forward the rear clutch pack is probably the issue but always diagnose first.

Cliff Ramsdell

View attachment 1773476

There is no real pressure check for the clutch pacs. Only line, front servo release, rear servo, and governor pressures. As you can see in the chart the rear clutch is the common component applied in all forward gears. Air checking the clutch pacs withh the valve body removed only lets you know if the piston(s) are moving. Worn frictions won't show up by using this test. When the diaphram spring breaks, it normally jams up the rear clutch. But it is possible that if the spring broke, the pac may have enough travel to unseat the piston in it's bore. The rear clutch drives the front planet annulus. That leaves the front planet carrier either being held or driven. If it slips on the output shaft (stripped splines) those two functions can't happen.
Doug

 

[Cliff Ramsdell]

Pressure test shows things like low pressure which can effect clutch pack engagement. Line pressure is 60 psi give or take but reverse apply is upwards to 300 and if you have low pressures reverse can still function but you will have issues with the forward gears.

I’m of the opinion that the rear clutch pack is tired, while I have replaced many burnt clutches over the years they out number the broken diaphragm springs, either way it’s coming out.

I base my diagnosis on my 40 years experience, 20 as a Chrysler dealer tech who did the transmission work at the stores I worked at. My statement was that I always pressure tested first before dropping the pan.

Cliff Ramsdell

 

[BeepBeepRR]

I told him to take his spare 727 and get it rebuilt and when its done I will swap out the one thats in it. The transmission that is in it now originally came out of the road runner. I knew it was working fine when I pulled it. But Mike wanted a new transmission so we replaced it. Anyhow 9 years later the RR trans is having issues. I guess a pretty good run. Anyhow the ball is in their court now and its up to them how they want to go.

Once I do end up pulling the old RR transmission out I will open it up and see what I see. I assume it won't be something they will be interested in keeping So I will just rebuild it for my 440 when I get around to building that, lol. I will keep this thread in my favorites so I can look back at all the great info when I do get into tearing into it. Thanks for all the replies and great info. This is why this forum is the best. Almost instant info for anyone needing to figure something out. Thanks again.

 

[R413]

Unless the valve body is very wrong then It can't be fixed without coming apart.

When it's taken apart the problem will be obvious.

 

[BeepBeepRR]

Yep was watching some videos on Youtube on how to tear one down and inspect. So I think I can handle that. I want to open it up and learn so its a good thing.