What Causes Different Trans Line Pressure

[ExplorerDMB]

This is more of a hypothetical question - just to get your theory on things.

If your not familiar, a transmission flush machine hooks up the the trans lines and then exchanges fluid (old to new). Anyhow, when the machine is hooked up it reads the line pressure. Now, before and today the line pressure was 10 psi. Why? What causes line pressure of only 10 psi.

When I've done flushes on other vehicles I've seen it as high as 40 psi. Just trying to get more indepth with it.

-Drew

 

[IZwack]

I dont quite understand the question 100% - are you asking why perhaps two different vehicles would have two different line pressure? or the same vehicle having two different line pressure on two different days?

 

[unclemeat]

Pressure is directly related to velocity. The higher the pressure, the lower the velocity(Bernolli's equation). Velocity is related to the flow rate and the area of flow (continunity equation).

With that being said, I am not a mecahic, but an engineer. I am only giving you some basic fluid mechanics properties.

It would seem to me that if the the flow rate and pressure delivered by the flushing device (pump) is constant then the variation would be in the fluid passage size and roughness. Smaller passage (flow area) and greater roughness would create a condition of higher pressure loss.

I do not know how your flushing machine operates, by reading higher or lower pressure. I would asume that the gage is reading pressure loss, so the greater the pressure loss the more likely the transmission is in bad condition. I would check the manual, because the pressure reading is likely a simply was to assess the condiion of the transmission.

 

[BrooklynBay]

There are a few different factors involved, such as the transmission type, fluid viscosity, filter, cooler's condition, and if your valve body has the thermostatic valve which cuts off flow to the cooler when it is cold. Was this machine used with the engine off, or the engine idling?

 

[410Fortune]

RPM's? Just a thought....

 

[ExplorerDMB]

i'm talking about two different vehicles. Say my explorer is one and a jeep cherokee is another. I know RPM will raise pressure but I'm just wondering why the Explorer will idle at around 10 psi trans pressure when the jeep says idles at 25/30 psi

-Drew

 

[BrooklynBay]

Check this pressure when it is cold, then hot. It should be higher when it is hot because the thermostatic valve in the valve body should open then. I don't know if Jeeps use this same set up like in the Explorer.