help please. 4+3 problem.I can turn on od and od engaged light comes on ,but it does not physically engage. it shows its on ,but its not.
changed the fluid, it was low. but still no actual od.
been driving as a 4 speed for a couple of weeks now.
any help is appreciated. 85 Z51

 

As you have found out the light only indicates the computer is "trying " to turn the O/d on ;
is not a indication it is engaged as there is no feedback.

Wiring problem somewhere
Check O/d relay on firewall first , relay connector also goes bad on older cars
Check wiring from relay to O/d connector through 1st gear switch .Switch goes open in 1st gear , preventing operation; possible it is locked in open position or has bad contacts

Putting 12V on terminal F of the ALDL connector with car not in 1st gear should make the solenoid in the O/d unit click

 

Have you checked the power connection at the OD to see if it's getting power when switched on? (put the car up on jacks, connect a test light to the OD connection, run car and switch OD on.)

 

What you can do, jack up the car,
Put a wire in the little plug that is on the rear part of the trans on the left side, and clip the red lead of your volt meter to it, and the black to a ground somewhere.
Get in the car with this hooked up, get the car up to speed, switch the overdrive on, and check for 12 volts.
If you don't get 12 volts, it is probably the wire or relay or fuse.
If you get 12 volts, it is the trans.
If it is the trans, it may be one of several things.
A. the pressure switch inside is bad, and is not forwarding the 12 volts to the engage solenoid.
B. engage solenoid is bad.
C. the filter is plugged up and cannot get enough fluid to engage.
D. The seals are hard, and the unit is bypassing fluid pressure internally.
E. The trans has worn clutches, and won't hold the weight of the car in forward propulsion.
F. The internal valve body has a broken link rod or stuck piston.
G. a fluid galley plug has worked loose, and is just dumping fluid internally.
H. The cable from the gas pedal is broken at the trans.
All of these things can be fixed, but it will cost you in time and inconvenience. and about $1400 for a rebuilt overdrive.
You can change the pressure switch for about $30,
You can change the solenoid for about $45-$65.
Forget those costs if you can't do it yourself.
Only an honest trans guy can fix those items for $150 including labor.

To check for old, hardened seals, jack up the car. run through first and second gear.
make the speed 40 MPH. note your speed VS RPM.
put clutch in, flip OD on, let the clutch out, get the speed up to 40, and note the tach VS RPM.
If the overdrive works, it is the seals or clutches. They won't hold enough pressure to push the car.
Basically, there is a pump inside that pumps fluid to a common area. There is a drum with clutches in it. The fluid is just recirculating until the solenoid opens, and ports fluid to the piston that squeezes the clutches against a bunch of springs to take up the shock, and then the gearset rotates with the drum. When that gearset rotates, the trans output shaft begins to rotate faster than the input shaft, and you have overdrive.

 

thanks guys this is all great advice.I'll check it out.
thanks for the help.