willys46

I recently rebuilt the Overdrive unit of my 85 with the Doug Nash 4 +3 with the Throttle Valve cable. 86 - 88's don't have the T. V. cable.
I the GM 1985 Corvette Shop Manual, and another set of instruction which look like a copy of a later GM shop manual.
One of the last steps in the rebuild process is checking the clutch pack clearance with air pressure.
The newer set of instructions says "On 1986-88 models, seat valve body check ball with suitable drift."
The 85 Corvette Shop Manual doesn't mention of seating the valve body check ball with a drift while checking the clutch pack clearance. If I seat the check ball with a drift I get 0.060" movement of the clutch pack. Right in the middle of the 0.050" - 0.070" spec. If I don't seat the check ball I get very little movement, about 0.010".
Which is the correct procedure for my 85? Should I be seating the check ball with a drift while checking
the clutch pack clearance? If not I have some other problem, maybe in the valve body as most of the air escapes
through the valve body check ball.
I assembled the Overdrive and installed it
The unit works fine in direct drive but slips continuously in overdrive. A slight touch of the gas pedal is
enough to make it slip. It stays in OD but the OD clutches are slipping. I rebuilt the overdrive to replace a noisy carrier bearing, it was working fine otherwise.
I disassembled and cleaned the whole thing including the valve body. I replaced the clutches and steels, the bearing and all O ring seals.
Any suggestions would be appreciated?

Thanks, Art

1985 Corvette

Why didn't the 86-88 4+3s have a throttle valve cable? I'm learning something new (and unexpected) about these transmissions everyday it seems.

willys46

The 86 - 88 Doug Nash 4+3 overdrive doesn't have a Throttle Valve cable. The newer ECM uses inputs from the Throttle Position Sensor to determine Wide Open Throttle. The site www.5speeds.com has quite a bit of info on these transmissions.

powerpigz-51

My 85 has a 88 OD, and it kicks down at full throttle with no TV cable.

rodj

Different years operate differently

http://www.5speeds.com/43table.htm

powerpigz-51

Mine does kick down at nearly full throttle, not I am curious to how this works, since my ECM is a 85, and my TV cable has been removed. I am assuming control of the OD comes strictly from the ECM. Everything does work as it should. Not sure why the cable was needed in the first place.

coupeguy2001

The TV cable is a manual kickdown. the later units are controlled electrically.
The earlier units use the cable as a failsafe way to ensure the unit isn't used with full power and destroy the clutches.

Here's one for ya:
I have an 85 overdrive on an 88 box,, and I took the TV cable off of it, sealed the hole, and used the 88 valve body.....it's in my '57 chevy pickup. I manually engage and disengage it....electrically.

I have a friend that worked for Doug Nash in the early 80's and he gives me hints on how to modify this thing whenever I'm up against a problem with it.
Although he never really put one together, he remembers some fixes that were done for some operational problems back in the day.
I asked him about the slipping problem, and he said he thought that the big servo bell seals were too hard to maintain pressure, and that the pressure was bleeding off.
Another thing he mentioned was that the cooler line restrictor might have been tampered with, and is bleeding off too much pressure.
Also, that the spring under the ball might be lazy.

willys46

Thanks for the suggestions.
I put all new 'o' rings in it when I rebuilt it including the big ones on the piston and the accumulator. I'll have to check the cooler valve but I'll need to drop the exhaust first to get at it.
I took the oil pan off last night and the fluid smelled burnt. I took the valve body off to see if I made some errors in re-assembling it. The little bit of the OD clutch plates that you can see through the bottom is burned a dark blue color and the clutches have almost no facing left on them.
The two manuals that I have also differ in the sequence of parts installation for the throttle valve parts. The 85 manual shows the sequence of piston, spring, valve, plug. The newer manual shows a
sequence of piston, valve, spring, plug. The locations of the valve and the spring are reversed. I re-assembled it with the sequence from the 85 manual where the piston acts to increase the spring pressure on the valve. I know, I should have paid more attention when I took it apart.
It seams to make more sense to have the piston act to move the valve
but that is not what the GM 85 shop manual shows. Does anybody know the correct sequence of assembly for the throttle valve in an 85 using the T.V. cable. I know I'm going to have to pull the tranny again to replace the OD clutches but I would like to know what went wrong first.

Thanks for your help,
Art

jhammons01

Damn Art, you are deep into it. Good for you....albeit you are having troubles....at least you are doing it yourself. That part is really encouraging.

willys46

I'm still trying to figure out what went wrong. I have the Overdrive out and disassembled. I found a couple of tablespoons of pink foam in the back of the overdrive case near the air vent. Is that normal or a sign that the trans fluid is contaminated with coolant? I flushed the cooler and lines to make sure there were no particles of the old bearing in them. Maybe I opened a crack in the 25 year old cooler in the radiator. I didn't notice any coolant in the trans fluid when I drained it. Would even a small amount of coolant cause the overdrive clutches to burn ? The direct drive clutches are OK.

Thanks for any thoughts or ideas.

Art

willys46

I have been meaning to post about what I eventually found to be wrong with my OD unit. I had switched
two of the springs in the valve body when I assembled it. Two of the springs are the same length and diameter. However one of them is of a heavier gauge wire and applies more pressure. There are two springs on the relief valve. An inner and an outer spring. The heavier gauge spring is the outer spring on the relief valve.
The lighter spring of the same length and diameter goes on the shift valve. Without enough pressure on the relief valve the unit won't develop enough pressure to firmly engage the OD clutch pack.

When I first assembled mine incorrectly it seamed to work but then it began to slip in OD and it burned the OD clutches. I pulled the trans and disassembled the OD unit again. I couldn't find anything else wrong so lastly I disassembled the valve body again and realized it was possible to switch those two springs.
The manual doesn't mention the different gauges of the spring wire so I contacted Brian at S&K and he set me straight on the correct positions of the two springs.
Mine is working fine now with the springs in the correct position.
Some of the manuals also have the order of the T.V. Valve and it's spring switched. The correct order going from the end with the pins towards the solenoid is pin, end plug, T.V. Valve, spring, piston, bushing solenoid.

Let me know if I'm not clear.
good luck,

Art

Art

Benny42

Art, are you a transmission mechanic or just a very brave
soul? I follow these threads as my OD unit is out. I
appreciate the update. benny

willys46

I'm not a transmission mechanic just a old hot rodder that has been doing most of my own work for years. I made a living in the computer industry.
I do have a heated workshop and pretty good set of tools.
With a factory service manual and the right tools or a substitute I'll tackle all most anything. If you decide to tackle yours yourself definitely get a copy of the factory manual. I was able to buy some of the special tools from another forum member. I made my own spring compressor from a couple of pulleys and a length of 1/2' threaded rod I purchased at Tractor Supply company. I used a piece of 2' X 2' angle iron drilled to fit the bolt holes that attach the C-beam to the OD unit and my bench vice as a holding fixture. I already had seal drivers and I used my digital caliper instead of the depth micrometer. You could use a hard wood dowel with a slot cut in it for the valve spring compressor.
The air line adapter is the same as the fitting used to pressurize
a home water system.
The only things I can't think of a substitute for would be the seal protector and throttle setting gauges that are needed for 84 and 85 cars with the throttle valve cable. If you need the gauges I think I have an extra set of them. I had to buy a long torq head socket to reach the oil pump bolts.
Two of the pipe plugs that plug the holes where you insert the pressure plate bolts were seized and I had to drill through them and use an "easy out" to get them out. S & K Speed on Long Island is an excellent source of parts. Brian there was very helpful.

Art

RPOL71

HI Art, I replaced the gear and cluthches on my 85 od unit. I cannot seem to get the planetaty gears to align with the sun gear. Not to mention the brain damage the clutches are causing trying to get it all to line up and fit. Is there a trick?

frank j. moran

There are very faint marks on each planetary gear, the marks have to be positioned a certain way. Two face one another and one points up and one down. The manual has a picture that will clear this up. Even with that you will still have to move each gear a bit to allow the sun gear to drop in, you should not force it.

willys46

It has been over 5 years and I'm a bit rusty on this subject now but I've attached an article from another site that I found helpful back when I rebuilt mine. I also have some other information I could scan in. Let me know how you are coming along.

Art