Repzard

I discovered a problem today
I drove into work and car was fine

But now when i left work
My 4+3 will not go into OD in any gear(2-4)
When i press the button on shifter OD light come on ok ,But will not go into Over drive.

Can someone tell me what may be causing this.Is the OD unit gone??
Maybe a bad selnoid??
FSM says there is a fuse but where??
Is there a Relay on the 4+3 anyone know of??

Please help


Thank you

racer85

The overdrive on our '85 happens to not work either, but its probably due to a bitten-through wire (we had mice living in the car).
I extrapolate from your avatar that youve got either an '84 or '85, so here goes: The OD relay is to the left of the brake booster, viewed from the front of the car. It takes maybe 10 seconds to remove the two nuts that hold it on. And, just in case you might want to know, the relay on a slightly higher level behind it the fuel pump relay. Good luck on solving the issue.

tarheelracer

Did you stall it out when leaving work? Mine won't engage after stalling, unless you let the car sit for a couple of minutes. Also, check the fluid level in the OD unit.

mikeb

There's also a cable that runs from the throttle bracket down to the OD unit. With OD light on, might be the relay or the cable.

scorp508

What was the coolant temperature saying on the dash? Yes, there is a relay for the overdrive mounted on the firwall kind of between the distributor and brake vacuum booster. The fuel pump relay and it are identical, so you got a 50/50 shot of pulling the right one.

Repzard

Thank very much guys and gals

My vette is a 1986 1/2

I did see 2 relays next to the distributor of the same part#.
Would one of those be the relay??

I drove the car on the highway home about est 75 miles in 4th gear and it tach around 2700 at 65mph and temp was great never once overheated or ran hot,Stayed around 168-172 degrees(i also have a chip in my ecm plus a 180 stat) so that pretty normal temps for my vette before this issue(just sucked up more gas)which is normal without OD on.

Does anyone think this problem is a BIG one? or something small?
I didnt smell any burning smells of tranny fuild nothing.
What i mean is should i pull the 4+3 is it gone???or something electrical??(like a relay )

Another thing im wondering is if it was a relay would the SES light come on and the OD light still work??

Thats what getting to me is the OD light still comes on when I press the button but doesnt go into OD.

Thank you all again for your replies it meens alot to me
I need a lift right now because im very depressed about my vette being almost the end of summer and not enjoying it as we speak

scorp508

Ok thanks, that helps a lot when diagnosing 4+3s.

Yep! Pull one, turn the key to 'on', but don't start the car. If you hear the fuel pump prime for 10 seconds then you pulled the right one.

Yeah, they run pretty good without O/D still. The reason I asked about temp is because the ECM has to see 122* (F) before O/D will be available. Since you saw the [D] light come on though, that means temp was ok for engagement.

More times than not it ends up being a small one.

My bet is electrical at this point.

No SES light for failed the 4+3 operation.

Then (correct me if I'm wrong guys), that pretty much means its the relay, pressure switch, solenoid, or something I am forgetting.

Siveck

I have a Vette book that says the most likely cause for a failed OD unit is a bad second gear switch. The second gear switch tells the unit that it can kick into overdrive. The book is called 101 Projects for Your Corvette by MotorBooks. It also says that you can bypass this switch and make the OD kick on only when you hit the button, which is also a cure for a bad second gear switch.

CentralCoaster

Gear switches, coolant temp, shift **** switch are all inputs to the overdrive relay.
The overdrive 12V signal and dash light are outputs from the relay... (ie, they won't come on unless all the inputs work, and the relay works.) If the dash indicator comes on, then the 12V signal is being sent to the overdrive unit.


Most likely, your connector came loose from the side of the transmission, or the fluid level is low inside the overdrive causing the internal pressure switch to open. Both are easy fixes. Is the back of the transmission covered in grime? Low fluid level typically starts causing the overdrive to cut out intermittently, and gets worse as the level gets lower until finally never coming on.

Other possible causes are mechanical failure in the overdrive, or a broken signal wire, or a bad internal solenoid.


While you're nosing around in there, check the wire harness to your overdrive relay and fuel pump relay. The heat tends to shrink the insulation way back and cause shorts.

Grenkepr

That is what I did. Fixed the same problem. Like this setup better anyway. Now I can shift when I want to.

scorp508

I can't remember right now, but does the dash light come on if the relay gets the 12V, or only when the relay activates? What if it is a stuck relay?

CentralCoaster

Comes on when the relay activates. I guess a stuck relay or a relay shorting as such would cause the light to stay on at all times. The pressure switch would be the only thing keeping it out of gear.

And if you put it in diagnostic mode, it ignores all the inputs and sends 12V but doesn't activate the solenoid if the pressure is too low. But it will activate the relay and light.

racer85

Quick Q: Did, before you did the second gear switch fix a'la 101PFYC, the Overdrive Indicator on the dash turn on when you turned on the OD switch? (I hope not, cause' that means ours will be an easy fix)

scorp508

A special 4+3 diagnostic mode or just the normal one while scanning etc.?

CentralCoaster

ALDL A&B