I finally got my car running today. Have some problems in the engine compartment (a different thread). I took it out for a test drive and my OD light comes on but the transmissin doesn't go into od. I don't know if this transmissin even works or not since it is not the one that came in the car, it is out of an '87 and my car is an '85. It doesn't have the TV cable on this transmission. I think the relay is good since I tried both of them on the fuel pressure plug and both gave me fuel pressure. How can I check to make sure everything is working correctly?

 

The 4+3's don't have the TV cable, only the automatics.

 

The '85's have a cable that goes from the throttle butterflies down to the overdrive unit of the transmission which is an automatic.

 

84 and 85 cars had the TV cable on the 4+3 OD.86-88 cars did not. You should learn how to put your car in diagnostic mode. If you do, in that mode the OD relay is powered up which will light up the OD light and send power to the OD unit. With a volt meter, check for voltage at the connection to the OD. ( the plug next to the fill plug ). If yes, then open up the pan and check for loose wires, bad pressure switch, bad solinoid. You can jump the pressure switch to check the solinoid. If you have no power, usually the connectors at the relay are crudded up.

Good Luck,

Paul

 

Thanks. Now I have an idea what to do. The wires on the OD relay were missing some insulation so I cut them and put heat shrink tubing on and spliced them back together. I did them one at a time so I know I didn't cross any wires. I had the pan off to change the fluid. I will check for voltage there before I go into the pan again.

 

Yes, check the connection at the transmission. You may have to cheat the relay to test it because you won't get a signal without the car going down the road.

I have troubleshot this problem before by connecting a wire to the transmission lead and then running the wire into the cockpit, connected a DVM, then drove the car.

If you have voltage at the transmission connector, use an ohmmeter to test the coil in the transmission. If the coil is open you will have to pull the pan and replace the coil. If you do, don't lose the check ball that goes in above the coil.

Good luck,

 

Again ... putting the car in diagnostic mode will energize the relay. You don't have to go down the road.

 

Thanks again. Will try that this weekend after I get the half shafts out for new u-joints.

 

Do you mean shorting A and B or grounding one of the other pins?

Have a great day,

 

I built a ALDL cable to hook my laptop up to it. I have various resisters installed on a rotary switch so I can go from direct short, 10k ohm, 3.9k ohm to no short. I think when I hit the 3.9k ohm spot it is in Diag mode and I see the OD light come on on the dashboard.

 

Just jumper pins A&B with a paper clip.

OOOOBA
OOOOOO

If you get 12V at the side of the overdrive, then you're possibilities are a bad pressure switch, bad solenoid, pump sucking air due to dislodged filter or torn grommet... or bad overdrive unit.

I tested my pressure switch continuity by hooking it up to compressed air. It has some hysterisis, meaning it doesn't open and close at the same pressure, but that's fine.


Don't use ecklers grommets, they're cheap rubber and get soft and tear from the atf.

 

Ok, I put my car in diag mode today and tested my power. I have 12 volts going down to the transmission. I checked continuity on the connector on the side of the tranny to ground and I have absolutely no connection. Am I supposed to have some sort of connection? Would this be a sign of a bad relay? I have another tranny that the OD worked ocasionally in. Can I take the relay out of that one and put into this tranny even though they are different years (85 and 87)?

 

If you have 12V at the overdrive unit, then the relay is fine, and the circuit it 99% most likely ok. For future reference, the fuel pump relay next to it is the same part number and the plugs can be swapped to test either relay.

That external spade terminal passes the 12V signal into the overdrive case to a normally open pressure switch, then to the overdrive solenoid. (Normally open means the pressure switch only closes and sends the 12V thru to the solenoid if there's enough fluid pressure.)

The trans case is the ground side of the solenoid.

When the solenoid gets 12V across it, it forces a piston out to change the hydraulic path and activate the overdrive. Note, the solenoid will NOT do anything in diagnostic mode, because the fluid pressure switch isn't closing the circuit path. If you hear a clicking, it's the overdrive relay on the firewall, not the overdrive solenoid.

Anyhow, if you have 12V between the spade terminal and the case, the next step is to open up the overdrive unit and jumper the pressure switch to see if the solenoid plunger pops out in diagnostic mode. Its easy to observe if beforehand, you push the solenoid shut (by hand). DON'T use the solenoid any longer than necessary to test it with the pressure switch jumpered as it will overheat and damage the solenoid.

If the solenoid doesn't move with 12V across it, then it needs replacing.

If the solenoid plunger pops out, then either your pressure switch is bad or the overdrive pump isn't putting out enough pressure. Common causes for low pump pressure in order are:

low fluid level
cut or dislodged filter grommet
dislodged pickup tube or oring
mechanical failure in overdrive unit

Obviously test the pressure switch before you assume it's mechanical failure, I hooked mine up to a compressed air source and used a continuity meter. I don't remember what pressures it opened/closed at, but it's not like you can just blow into it.



And I have absolutely no idea where to find a replacement pressure switch or solenoid besides from another 4+3 transmission

 

That helps tremendously. I Now have a better understanding of what to look for now.

 

Also keep in mind that the ECM will not allow overdrive operation if it sees coolant temp below 140 degrees. The ecm gets it temperature reading from the front of the intake base. (the sensor on the left that doesn't have a fuel injector type plug)

This dash gets its coolant reading from a different sensor.

But, I think the ECM will throw a code if that sensors unhooked and ignore it.

If you try to turn on the overdrive below 140*, the dash indicator light won't come on. Dash light only comes on when the overdrive relay is activated by the computer and sending 12V to the trans.

 

It is refreshing to see that I am not the only one who understands the 4+3 overdrive. This transmission has gotten a bad reputation due to technician ignorance in MANY cases.

I do, however, suspect that there must be a difference in some 4+3 units. Centralcoaster is talking about a solenoid in which you can see a plunger. My 88 has no such plunger. With the pan removed there was the pressure switch and a hockey puck shaped coil. It had no visible plunger. When the coil is removed there is a check ball above it that is actuated by the coil. On models with the coil like mine you just need to pull the pan and use an ohmmeter on the coil itself. If it is open it will need replacing. If it does indeed have continuity it is in all likelihood okay. At that point you would need to test the internal wiring and the pressure switch.

Thanks for the great explanation CentralCoaster. It sounds like insane is moving along well in the troubleshooting process.

Have a great day,

 

For what its worth.... 1987 is 122*, not 140*.

84, 176*
85, 140*
86, 122*
87, 122*
88, 122*

 

His car is an 85, his trans is an 87. Sucks to have an 84 though! I couldnt stand waiting till 140 to use overdrive on the freeway. Sucks cruising along at 2600rpm waiting till it'll let you shift... especially with no mufflers.

 

Put the half shafts back in today after getting them back from having new u-joints put into them. Car shifts into overdrive no problem. While I was checking for voltage the other day I cut off the original connector and put a regular spade connector on the wire and plugged it back in. Seems that maybe it wasn't making connection? While I was working under the hood I repaired the relay connecting wires. The insulation was shrinking away so I put heat shrink tubing on them and then encased them in electrical tape. On my other tranny the OD light wouldn't always light when I put it into overdrive. So far it has been working every time. I actually drove her to work today for the first time in about 6 months. Feels good to be back in her. Can't wait until I get the new intake on it and then get Alvin to finish my chip tune.

The u-joints went bad from someone replacing them and not putting in the correct u-joints. Parts, labor and tax cost $261.06 to have all four u-joints replaced.

Scorp508 - Thanks. I was wondering why my car went into overdrive when it was 142*

 

Great work insane1!

One of the most important traits of a good troubleshooter is his grabbing the problem by the throat and not giving up until it is fixed. Perserverence is the key!

Enjoy,