mitbpilot

1986 C4, Doug Nash 4+3.
Overdrive will not engage. New relay, new switch on shifter ****. The fluid and filter have been freshly changed. The OD light comes on at the correct time and the OD switch works like it should.

The overdrive is doing something... I'm just not sure what. When the light comes on the OD will not shift. Here's the strange part, when the OD light is on and I'm driving down hill the car seems to coast like it is in neutral. It will continue to coast until the car slows to the point where the engine RPM matches the wheel speed for the particular gear that I'm in, at which point it continues to drive normally with NO overdrive, regardless of whether the OD light is on or off.

Looking forward to hearing some ideas from the experts!
-Matt

jv9999

Inside the OD unit is a solenoid that engages it. If it were mine, I'd pull the pan and visually inspect, remove, and test it. Also check for lots of metal particles on the magnet. You might get lucky or it might be time for a rebuild.

They run hot have a short fluid life. If it wasn't changed for a long time or it ran low, it will fail prematurely. Most oil change places only checked the trans fill (if that).

This page from the SM might help: https://secure.flickr.com/photos/jvienneau/15374691087/

AJ123

When they are working they are a good transmission. Finally gave up and put in a TKO 600. Good luck.

SK has all the parts and great info call them

http://www.skspeed.com/manual-transmission-shop.aspx

mitbpilot

I pulled the pan and changed the fluid/filter myself. Everything looked good.
What is the best way to test the solenoid? It looked ok visually. The fluid looked good as well.

Does anyone know how much resistance the coil in the solonoid should have? Mine seems high.
Matt

Scott0757

Mine has the same issue as yours: slipping despite O/D being engaged. The transmission had been working fine, but has since stopped. I changed the fluid and filter approx 2K miles ago and have checked the fluid levels and all is fine. I know the solenoid is working as I can be cruising along in 4th about 3000 RPMs and turn on the OD. Then I can take my foot of the gas and the engine will drop RPMs back to where the OD would have been (about 1800 I Think) and the car will coast, but car will not drop in speed as it would if the normal drive was tied to the flywheel. There is no push from the engine until I reach the non-OD RPMs. Then when the engine RPMs are low say the normal OD speed, I turn off the OD and the RPMs jump back to where they would be without the overdrive. This tells me the solenoid is working, but maybe whatever is telling the ECU the engine is under load (TPS) is thinking that and leaving the OD overridden. Where is this OD relay located? Could this be the culprit?

mitbpilot

The OD relay is located near the wiper motor behind the fuel pump relay. However, if you get the OD light on your panel your relay is working. Im thinking our OD friction plates are worn out. I'm going to test by getting the wheels off the ground and seeing if the OD will engage without being under load.

jv9999

I'd say under 1000 ohms for sure, probably under 100. Put 12V on it and see if you hear it click.

Scott0757

I hope its not the pressure plates...i would be very depressed. Bought a nee relay today and am hoping it may do something. Mine was working fine and you knew when it switch in our out of OD as the was a thunk you could hear even with the clutch in. Then it started to take longer to kick in until one day it just stopped....it feels like it has the kcik down feature enabled...just put a new TPS sensor in as well.

mitbpilot

Anybody know of a good thread or video on rebuilding the 4+3 overdrive? All the links I've found on here are dead ends.

frank j. moran

This will get you started, my recommendation is a factory manual. The piece below is written by David Fulcher in Atlanta, he is an expert on the 4/3. Your biggest problem is going to be hard parts availability. I have done 2 of them, they are not difficult but you need to be aware of some key points, so following the book is a requirement.

http://www.corvetteactioncenter.com/...-unit-610.html

AJ123

Sk has all need parts instock

http://api.viglink.com/api/click?for...sion-shop.aspx

mitbpilot

For the people who have rebuilt the overdrive before...
Will I need any special tools for this project? I'm slowly mustering up the resolve to tackle this on my own.
-Matt

frank j. moran

You don't really need anything special. There are three 1/8 pipe plugs in the rear of the OD unit, have a good allen wrench socket to remove these. Some have been very tight and once they are buggered up they are a pain to remove. You will need three metric cap screws about an 1 1/2 long (I cant remember the exact size) these are used to hold the direct drive piston under slight compression. You can then remove every thing very easily. As far as all the info on bearing pre load and the measuring tools, my experience is the cases are so rough it's hard to get an accurate reading. Read what I sent you, if I can add any thing I will.

Hamblen

I Have to say that I have the exact some problem as Scott0757. I've changed the fluid and did not see any surprises in the waste. I know that it switches in/out of overdrive by the drop in rpm and the clunk when switching the OD button.

If I keep constant pressure on the gas on level highway while in OD, the car will slow down like its coasting. As soon as I apply any gas at all, it switches out of OD and starts to easily pickup speed.
Can this just be a TPS issue?

I'd like to believe a slipping clutch pack would cause the rpms to ramp up dramatically more than just the difference between normal and OD (like 2900 RPM at 65, VS 2000 RPM in OD) but it doesn't seem to shoot up above the normal RPM for being out of OD.

Thanks for the help!

Scott0757

I have not yet dug into mine and won't be as this car has just been sold as I shift focus to my 87 C4. My theory is related with the TPS cable linkage. The 4+3 has two methods of defeating the overdrive: The electric solenoid (switch and ECU controlled) and the Throttle position kcik down cable that pulls and arm that opens a hydraulic bypass circuit. I can hear the solenoid kick on as the RPMs do drop, so the electrical portion seems to be fine. The slippage might be due to that hydraulic bypass (sping loaded ball bearing in the lines) being hung up. As my Throttle kick down cable was broken and I could not find another, I capped off the hole on the tranny for it. This meant that the "hook" and small chain that would be attached between the cable end and the long kick down arm (you can see it when the pan and filter are off) may be hung up, keeping the bypass partially open. There is a possiblity that the ECU may also be wired to that hydraulic bypass as the 4+3 dropped the throttle cable in 85 and later years and went with full ECU control. So I would check to make sure that linkage/arm is not stuck holdng the bypass ball bearing in the open position.