1981LT1

iTrader: (1)

I just rebuilt my 700R4 and it won't engage into gear when i shift into OD. If I go from park to 3rd, it works. Every other gear works though and it shifts up through third without problem. The tv cable is adjusted fine so I am thinking it has to be a check ball or valve body problem.

Precision Trans

I assume you mean when you select D4(Drive 4) on the shifter, your car will not move. Your problem is the input sprag, You did replace the sprag element and de-glaze the races when you did your rebuild, didn't you?

Don

1981LT1

iTrader: (1)

The input sprag checked out ok and there was no heat checking on anything but the clutches and steels so I didn't replace it. I only rebuilt it because the guy my dad bought the car from said the tranny went out but when i pulled it apart, there was nothing visibly broken or damaged. So, i installed a new master rebuild kit and everything checked within tolerances and the "air pressure" check on the input drum worked like it should. Wouldn't the input sprag couse other things not to work?

Precision Trans

How did you check the sprag element? Physically remove it from its races, and eyeball the width of the flattened wear area on the "dogbones"?

If the sprag fails to hold, you will have no movement in the D4 position, but as soon as you move the shifter to the D3,D2, or Low position, the overrun clutch engages and bypasses the sprag enabling movement. This is what enables engine braking in these gear positions. If by chance you installed the element backwards, which is possible, you would also have no movement in the D4 position, D3, D2, and Low would work as above, but you would have a bind-up on the 3-4 shift if you manually shifted to D4 at the 3-4 shift point.

Sprags can be a source of grief in other ways, too. I have seen them hold properly when taking off from a stop, overrun properly when in 4th gear, but fail to hold on a 4-3 downshift, and then hold properly again when taking off from the next stop. Go figure.

So, the moral is: replace the sprag element and de-glaze the races during every rebuild. Cheaper than pulling the unit for a re-work. I realize you are trying to save money, but please remember: cheap is good, but good is never cheap.

If you have any other questions, please do not hesitate to ask.

Don

1981LT1

iTrader: (1)

Everything you said is what it's doing. I can shift manually or let it shift by itself through 3rd and when i shift up to 4th, it goes into nuetral. Does this sound like a bad part or an incorrectly installed part.

I did take the sprag apart and the rollers didn't have any flat spots and it only rotated in one direction.

I have sneaking suspicion there is a problem in the valve body.

Precision Trans

Am I correct in assuming that even if you were to take the vehicle up to about 60 or 70mph, that the trans still neutrals when you move the shifter from D3 to D4?

As for possible VB concerns, if you look at the hydraulic circuit diagrams, you will see that any leak in the fwd clutch circuit will also affect overrun clutch oil when in D3, D2, and Low. Curious also as to which model year 700-R4 you have. Is it an '87 up with the auxilliary valve body, or the early type?

Only a complete hard blockage of D4 oil, or a blockage of Fwd clutch oil between the auxiliary valve body and the Fwd clutch could be the only Vb problems.

Failure of the teflon sealing ring, on the input shaft, between the Fwd clutch circuit, and the ovverrun circuit would allow Fwd clutch oil to be dumped through the Ovverrun circuit back at the manuall valve.

However, since D4 oil also feeds the governor, any loss of D4 oil would affect governor pressure. When you have the shifter in D3, the leak is closed and the trans works normally. Moving the selector to D4, even at a speed high enough to allow a 3-4 shift would cause an immediate loss of D4 oil/Fwd clutch oil, but also a loss of oil pressure to the governor and the trans downshifts to 1st gear(not the same as Low) and just ovverruns.

So, how to check for this? Remove the pan. If yours is a unit with the auxillary VB, remove the Fwd Clutch feed tube. Using compressed air, about 40-50psi, and a rubber tipped air nozzle apply air to the Fwd Clutch feed hole on the pump( where the tube was). Do this with the shifter in D4 and in D3. If you hear a rush of air in D4 but not in D3, it's time to pull the unit, and make the necessary repairs to the Input hsg seals.

Let me know how you get along with this.

As always, should you or any of your I.M. Force be caught or killed........Oopps! Wrong job!! On second thought, maybe not., please do not hesitate to contact us.

Don

1981LT1

iTrader: (1)

the trans is an 86 unit and doesn't have the auxilary valve body. I don't have a problem pulling the trans but I want to make sure I trace the problem correctly so I don't have to.

Is there a possibility the valves are place wrong in the valve body itself or a backwards gasket, or would that cause other problems as well.

And no, no matter how fast I go, the trans won't go into OD.

Is there any way to pressure check if there isn't a auxilary valve body.

Precision Trans

Does your trans have 3 pressure test ports on the right hand side? One immediately behind the servo just above the pan, one a bit further up, and one just above pan level to the rear of the first 2? If yes, then proceed as follows.

Install a pressure gauge ( and hose) into the upper port immediately behind the servo. This is 3rd oil. Route the gauge and hose inside the car so you can read it easily. Test drive. When the trans shifts into 3rd, you should see line pressure on the gauge. Move the shifter to the D4 position. If pressure goes to zero, the trans has downshifted to 1st. That should confirm my earlier diagnoses of a crossleak between the Fwd Clutch circuit, and the Ovverrun Clutch circuit causing a loss of D4 oil which will cause no pressure to the Fwd Clutch circuit, and the governor.

Are you using scarf cut teflon rings on the input shaft or solid teflon rings? The solid rings are the updated ones, but require special tools to install correctly. Please let me know.

Don.

1981LT1

iTrader: (1)

it has the split rings.

I will try the pressure gauge this weekend and let you know.

1981LT1

iTrader: (1)

I did notice something else too.,

While i was driving it the other day, i let it get into third and accelerated to get the passing gear engaged and when it downshifted, it started to shudder pretty bad. Could this be bad tv adjustment or another reason to tear back into the trans?

Precision Trans

Shuddering is never good! Sounds like you have insufficient pressure to a clutch or band servo, or one again, the old bugaboo, skipping sprag. Lack of pressure could be caused by improper TV adjustment, worn boost valve in the pump, eyc. It will allow clutch slippage, and eventually shuddering.

Let me know how your tests go this weekend, and get as many of the problems as possible worked out before deciding to tear back into the trans. Remember, you can't road test it on the bench.

Don.

PS. Never use the scarf cut rings on the input shaft, always the solid ones.

1981LT1

iTrader: (1)

Well, I had to order a test gauge so i won't be able to do the road test until next week sometime. On the other hand, let me give the exact rundown of the trans.

Master overhaul kit with red alto clutches (with new lip seals etc)
upgraded pump to 10 vane unit
new (stock spec) torque converter
new front pump bushing
kevlar band
all hard parts inspected and cleaned
no cracking on sun shell
planetary gears in good condition
roller clutches had no flat spots
corvette servo (pending installation)
B&M shift kit (pending installation)

Also, when i rebuilt the trans, the video i was using recommended that instead of using the 7 ckeck *****, I leave out 2 of them for a firmer feel.

This probably doesn't help in any way but I figured you should know exactly what I am working with.

1981LT1

iTrader: (1)

Well, I finally got a chance to drive the car again. Here are the results from the pressure tests:

Park = 55psi
Reverse = 120 psi
D4 = 55psi
D3 = 55psi
D2 = 55psi
D1 = 55psi

From D1 to D2 the the trans shifts at about 150 psi
From D2 to D3 it shifts at about the same.
From D3 to D4 the pressure drops to 55psi and basically goes into neutral.

I am still getting some shuddering when i accelerate hard and it downshifts, but when i accelerate gradually, it does fine.

These pressures seem a little low according to my shop manual. This doesn't make sense because the pump has been upgraded along with the rebuild.

Precision Trans

I still think your problem is a crossleak from the fwd clutch circuit into the overrun clutch circuit.

Your pressures are on the low side because your pressure regulator spring is probably a bit tired. This spring is what establishes base line pressure. Line pressure is boosted when TV oil is fed to the pressure boost valve.

I am curious as to how you got any pressure reading at all in 1st and 2nd gears if you were reading the pressure at the 3rd oil port as I suggested. Please clarify how you proceeded with your pressure check

Thank you
Don

1981LT1

iTrader: (1)

i used the gauge at the port above the shifter lever