Is there a thermostat inside the transmission that will only allow it to go into overdrive when it has reached a certain temperature?
I have heard people say that it will only shift into overdrive after it has reached operating temperature.
After I adjusted my TPS, I took it for a test drive this morning (engine cold) and as soon as I hit 55 MPH, it shifted into overdrive. The engine had not reached operating temperature. Is this normal?
Thanks,
Wheelo

 

Yes. It shuts the overdrive option off when the sensor is below a designated set temperature.

 

The sensor measure tranny temp, not engine temp so it warms up very quickly.

 

This is wrong.

The sensor only kicks-out of OD if the tranny reaches 270*+. It does nothing to engage the OD at all. I know this because I have removed the stock sensor and have it zip tied out of the way, so it never gets warmer than the ambient air temp and the OD still works fine.

If it was used to engage the OD, that wouldn't work too well on a very cold morning when you first start the truck.

 

Mine warms up much slower than the engine temp.

 

It also will not let the transmission shift into O/D if the ambiant temperature is cold. The engine and transmission must warm up.

 

Ok then, at what temp does this happen? ("cold" is not a temp used by electronic sensors)

I mean, we all know what temp it kicks the OD off at, so if this is true there must be a temp published somewhere....right??

 

To call the item in question a sensor it giving it too much credit. It is simply a thermal switch that is normally closed at temps below 270*F, opens when the temps are above 270 or so. Thats it, very simple unit.

 

The computer controls the O/D. The computer depends on more than one sensor. It analyzes quite a few input signals and triggers the output devices.
Don't quote me but I'm gonna say that it uses engine temperature sensor when the ambiant air is cold and the transmission temperature sensor to prevent it from going into O/D when it reaches 270*. It also depends on vehicle load, speed to control when that shift valve allows O/D..

 

I thought the TPS controlled all OD functions except for the downshift out of OD if over 270*?

You didn't answer the original question though. To be honest, it sounds like you're just making this stuff up as you go. You have yet to give any exact numbers and/or any factual information.

I think it's pretty clear you're very confused as to how the tranny operates. The engine coolant sensor comment was the big giveaway there.

 

Here's what I know about as far as the PCM controls OD.

OD switch on dash sets a flag in the PCM wether or not to enage OD and turns the light on or off, it doesn't control it directly.
TPS, depending upon speed and throttle position it will either shift into or out of OD.
Speed sensor, see TPS above, I know for a fact that they won't shift into OD with a speed sensor working.
The thermal switch in the trans cooler line that opens at 270*F or so. This might not be connected into the PCM, but probably is. You newer guys have ones that actually can measure temperature.

I'm not completely sure, but I do remember reading a manual saying that it won't shift into OD until the coolant temp is 60*F or above, this doesn't take long to reach even on a cold day.

To answer wheelo90's orginal ?, yes its normal for them to shift into OD before it reaches operating temp.

 

My sensor is removed and it has never refused to shift into OD when ambient temps are under 60*. If I start it up and leave right away on a cold day the OD still works and kicks-in at around 35mph under light or no throttle. Coldest it's been since I got the truck and removed the sensor is probably 50*, so not really cold but well below the 60* mark. I did a lot of research here and at TDR on the effects of removing the stock sensor and all I could find was speculation on what it "might" do, and then several responses claiming they had removed it and can verify there is no lower temp limit for OD to engage.

Anyone have a manual they can check? If the upper sensor limit is published there then the lower limit would be too.

 

jrussell,
Could be that I'm thinking of the newer trucks. 94 and up, maybe thats where I read it. Wannadiesel has a FSM, I have orginal operators manuals I could check when I get home.

 

That would be cool if you or Wanna can check!

If I'm wrong I surely owe Rammer an apology. If it is supposed to work like that after all, mine sure doesn't.

 

From the FSM: