Has anyone ever heard of a bad / defective stop light switch causing my OD to kick off and on by itself ? My brake lights seem to operate normally but a trans shop told me that a stop light switch could cause this.

 

Is it overdrive kicking in/out, or torque converter clutch? How much of an RPM drop are you seeing?

 

It is definitely kicking in and out of overdrive. I'd have to check the RPM drop. I'm retired now but was a mechanic for over 45 years and did auto and manual trans repairs but mostly on Toyota and Mitsubishi. When it drops out the OD light on the dash warning light for "OD OFF" COMES on and sometimes I can hit the switch and turn the OD back on and sometimes not. Sometimes it just will go out of OD for awhile and then sometimes it will go back into OD by itself. By reading other notes, I replaced the TPS just to see and I checked the OD switch on the column with an ohmmeter and it seemed to not make good contact all the time so I replaced it as well.

 

Ok, that changes things dramatically.

If you have a scanner that can read live data, monitor trans temp when the problem presents itself. My guess is, the trans is getting hot, and the first thing that happens is, the PCM disables O/D. If it gets warm enough, it will turn on the trans temp light in the dash. It's possible that the check valve in the trans line is clogging up, so, the trans isn't being cooled sufficiently. I would be tempted to just pull that line, drill out the check valve, clean the line out good, reinstall it, and see if the problem goes away.

 

The only problem is that I have no trans temp light in my dash. I have a scanner at home that reads DTC codes and there are none found. I"ll have to see if I can find a scanner that will read live data. I'm not sure where this check valve is but the truck does not use the trans cooler in the radiator. It has a bigger cooler added in front of the radiator. The trans was slipping and I did overhaul it about 90k ago and has been operating normally after that. This problem just started recently. I agree with the overheating but it still seems very random and intermittent when it goes out of overdrive and then back in. By the way, I checked the RPM drop when it kicks out of overdrive and it seems to be about 7-800 RPM at about 65-70 MPH.

 

Yep, that really is O/D kicking out.

Check valve is in the trans cooler line, just in front of the engine on the passenger side, right where the metal line transitions to rubber. In stock form, that hose went to the lower fitting on the radiator. They are notorious for clogging up, and causing the symptoms you are seeing.

 

I think I see where it is. Is it best to clean it out or just replace the line? Is there some trick to fixing it?

 

This won't damage the pressure, will it? Why do they even put that in there? I'm going to check the line this week.

 

Nope. Is there to stop the fluid from draining back into the tranny. Although thing that will change is you can't go into D right away. Have to give it 2 or 3 seconds

 

You can deal with it however you care to. Drill it out, replace the line, doesn't matter. I took off the line with the valve in it, and just replaced it with some rubber hose rated for trans fluid.

Valve is to prevent fluid drain-back when it isn't running. The only thing you *may* notice, is a bit of a delay going in to gear if the truck has been sitting for a while. (several hours.)