Please explain changing of gears on diesel automatic transmissions by vacuum control?

Question

As previously, vehicle would not run smooth when gear reduces to 3rd from 4th. There is a jerk and the engine sort of races. Before it was a smooth transition

Answer 1

there are actually 2 different types of transmission's for a diesel,one uses vacuum,,as with a modulator valve,,to help shift it,,what it does is use the manifold vacuum,to open a spring when you reach a certain ,,RPM in the car,and this allows the valve body to change positions,,which in turn changes a gear,,the other type is a shift solenoid,,it works similar but uses voltage,,it has a sensor that detect,s,, RPM,s and sends a signal to the solenoid to change it,s position,,this makes the transmission shift gears,,if it is jerking on you,,the filter in the transmission may be getting dirty,,and in need of change or the vacuum line is collapsing,,from age,,they will do this when the get old,,and real soft,,what happens is when they get a high amount of vacuum on them,,it sucks the line together,,not allowing it to pull the valve,,the way it wa intended to do,,all this stuff sound complicated,,but it really isn't,,i have owned and operated my own shop for 35 yrs,,and get a few of these to repair,,most of the time it is a simple thing check the line that goes to it,,and make sure it is good,,also look at the fluid,,if it has a burned smell to it,,change it and the filter,if you don't it will cause some damage,,i hope this help,s.

Answer 2

Vacuum is used to move the valves in the valve body of the automatic. You might need to adjust your linkage. Check for leaks from the vacuum modulator ( pull of vacuum line and see if fluid comes out. )

Answer 3

On a diesel transmission, there is a shift solenoid - if that goes bad, you will get the uneven shift as you describe.

Answer 4

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