I recently purchased an all original 1985 with the 4+3 Manual. The overdrive switch is on the console and the overdrive activated lights up on the information panel, however, there is no drop in RPM's and still at 2800-2900 in 4th at 70mph. IS there some trick to engaging, or do I have an electrical issue in activating?

Thank you

 

hello and welcome!

how does it work for the lower gears/speeds - do you notice a drop when you are in 4th and transition to 50-55?

I am not sure that I have noticed the rpms at 70 - I am already in OD by then - there will be a "drop-out" if you are in od on the freeway and stomp on it - you will have a momentary pause then everything at once.

If you want - once it is warmed up (about 150 - if I remember correctly) you can try the OD in the other gears 2-4 should you want to see how it behaves then -

hope any of that helps

 

There can be a number of things wrong as to why it does not go into overdrive. The overdrive is controlled by the engine computer.
1. The overdrive is low on fluid
2. the wire on the overdrive fell off
3. the internal pressure switch is bad.
4. The internal filter is clogged.
5. The overdrive clutches are worn out.
6. Wrong type of fluid in the overdrive.
7. Wire to the solenoid in the overdrive is not connected
8. the internal solenoid is bad.
9. Bad overdrive relay
10. overdrive relay coil has a bad ground
11. the connector fell off the overdrive relay.
12. its really Just a 4 speed and nobody told you
😳
first thing to do would be to check fluid. It takes dexron lll automatic trans fluid. Synthetic is best.
these things are notorious leakers.
the 4 speed uses gear oil. Not auto trans fluid. There is a seal on each one to keep fluids separate.
second thing would be to clean the connector on the overdrive, and make sure it has a good connection to the plug that goes in the overdrive. Do not try to pry the plug out if it’s leaking.
it’s plastic and has 3 plastic fingers that break off when you pry on it. U have to take the pan off to get it out. There is an O-ring on it that shrinks over time.
i will sell you a connector for $18 if u break it😇.
since you are under there, make sure the connector is on the 1st/2nd gear shift arm switch on the side cover on the 4 speed. U can check the switch with an ohm meter. In first, the overdrive is locked out using that switch.
if you detect that the overdrive is functional in first gear, do not use overdrive in first. You can destroy the overdrive Because the clutches will slip and burn up.
Next is check for a ground and 12 volts at the relay, its over the battery, near brake booster.
while you are there, check the engine grounds.
they are on the left side of the engine on the bellhousing bolt.
let us know if you fixed anything.
i have a 4+3 in my 57 chevy truck.
manually selected, works great.

 

^^^This^^^^ is pretty much everything.
It’s a simple two speed auto like a Powerglide. When investigating why mine didn’t work I found a couple of things. Firstly my car is an 84 so some of the wiring may have changed (actually I know it did) over the years. Check the schematics in your FSM/Electrical trouble shooting guide, if the light comes on that means there is power to the OD unit and the ECM has commanded overdrive to be engaged. In my car a had this, I removed the pan and found the wire was off on the inside of the unit. I then put 12v across the solenoid but I didn’t click in. I removed the valve body and checked the solenoid on the bench it was open circuit. I noticed that the ball bearing that is part of the actuating valve had worked its way down the spring so would never work. I put it all back together and ordered the parts. They have arrived but I haven’t installed them yet. I don’t know if that will fix it but I found three reasons why it couldn’t work.

 

Take a look at the relay pigtail first.



Attachment 48337402

 

https://tech.corvettecentral.com/200...-transmission/