How does the 6F50 "know" the line pressure?

[lolford]

I'm having some transmission issues and the line pressures are really erratic. I see in drive idling it's something like 5PSI and 15 when driving which are impossible IMO.

I'm wondering how does the PCM "know" what the pressure is as I don't see any sort of switch.

 

[bdcardinal]

Just skimming through theory and operation on the WSM. There is a Line Pressure Control solenoid.
Component Description

Sensors and Switches

The PCM controls the electronic functions of this transmission. The PCM receives input signals from engine and transmission sensors and uses these inputs to control line pressure, shift time, TCC and shift solenoids.

 

[lolford]

Just skimming through theory and operation on the WSM. There is a Line Pressure Control solenoid.
Component Description

Sensors and Switches

The PCM controls the electronic functions of this transmission. The PCM receives input signals from engine and transmission sensors and uses these inputs to control line pressure, shift time, TCC and shift solenoids.

Thanks! I've seen people get DTC's for bad line pressure but I don't understand why my PCM isn't throwing a code if it's saying they're a lot lower than the manual says they should be!

 

[Marcozi]

Your transmission likely has its own computer. Most OBD2 apps/tools don't read transmission computers.

 

[JavierH19]

6f50? No 6 speed automatic that I know is vacuum operated. Every single one will be computer controlled and will have a sensor somewhere. Might not be external but internal in the valve body.

 

[lolford]

6f50? No 6 speed automatic that I know is vacuum operated. Every single one will be computer controlled and will have a sensor somewhere. Might not be external but internal in the valve body.

I read the GM version of it has a solenoid that acts as a pressure transducer. I'm wondering if it thinks the pressure is low why isn't it throwing a code

 

[clinebarger]

Neither the 6F50 or the GM version "6T70" have a pressure transducer for line pressure, Must be measured with a mechanical gauge at the line pressure tap port.

Guessing you are seeing the Pressure Control Solenoid commanded pressure via scan data? All the solenoids are fed what we call Actuator Feed Limit pressure that's capped at @ 90-100psi, The solenoids then use this pressure to control their perspective valves.
In this case the PC Solenoid controls the Boost Valve-Pressure Regulator Valve.

Some examples......
PC Solenoid is commanded to output 30psi......This is @ 90-125psi of line pressure.
PC Solenoid is commanded to output 100psi.....This would be MAX regulated line pressure of 250-280psi.

You can't always trust scan data to be accurate, Using functional tests/bi-directional controls to command the PC Solenoid while watching the mechanical gauge is the only proper test!

Low line pressure will cause flared/lazy shifts & burnt frictions....Not just TCC apply/release concerns.

 

[carthusiast]

From what I’ve seen the 6F50 isn’t guessing line pressure, it has a pressure sensor that reads it directly. You can see commanded vs actual pressure with a decent scan tool, I noticed it when I was messing around with my youcanic and it helped make sense of some weird shift stuff

 

[lolford]

From what I’ve seen the 6F50 isn’t guessing line pressure, it has a pressure sensor that reads it directly. You can see commanded vs actual pressure with a decent scan tool, I noticed it when I was messing around with my youcanic and it helped make sense of some weird shift stuff

I see desired vs actual, but if my transmission is at 20 psi in drive I don't think it would work at all?

I had my TC replaced due to a "guess" by a transmissionmission shop so their next suggestion is the valvebody, since the engine will suddenly dip RPM's or even die when coming to a stop off the highway. He assumed it was the TCC stuck but now he thinks the valvebody is releasing pressure when it gets hot

 

[clinebarger]

From what I’ve seen the 6F50 isn’t guessing line pressure, it has a pressure sensor that reads it directly. You can see commanded vs actual pressure with a decent scan tool, I noticed it when I was messing around with my youcanic and it helped make sense of some weird shift stuff

It does not have a pressure sensor/transducer, Can't think of any GM or Ford transmission that does.