Page 77 of the Diesel B57 maual states it is a VTG/VNT turbo charger.
VTG = VNT, just two different terms for the same thing. VTG = Variable Turbo Geometry and VNT = Variable Nozzle Turbocharger
A VTG has moving shovels to restrict exhaust gas flow and speed it up this way. The moving shovels are mounted to a ring which is moved by the actuator. The shovels restricts or expands the pipe through which the exhaust gas is lead to the turbine wheel and it alters the angle how the gas hits the turbine wheel. Thereby the turbo can already compress air and generate pressure while the engine runs with low revs, as the speed of the exhaust gas is increased when the pipe is restricted. With increasing revs the shovels have to open up to balance the increased speed of the exhaust gas out.
It could be possible, that this is a cleaning mechanism, like you see it with the EGR. That is also moves when the engine is shut off, to keep it free of deposits (which works more or less bad in general). I personally never saw that, but that means nothing. But if it is a cleaning mechanism, all engines of that type, at least in Grenadiers, should do that, shouldn't they?
In general it is a good idea to check the actuator function from time to time. If the shovels get stuck while they are open, you just face a loss of power. If they get stuck while they are closed, you face a turbo damage due to overspeeding.
Usually overspeeding leads to an expansion of the compression wheel (the size of the aluminium wheel starts to increase due to its mass and the centrifugal force) because of too much revs. It starts scratch against the turbo housing and your cylinder will suck in small metall parts. Sometimes the compression wheel still look like it is new, the abrasion can be very clean and hard to see. Check the housing for scratches. The shape of the wheel is altered by that and it may start to make noises or to start whistling. Should one ever face that, check the back of the compression wheel. If it shows a lot of small dents, it has suffered from overspeeding.
You may want to check my (Google translated) article about turbos:
https://matsch--und--piste-de.trans..._sl=de&_x_tr_tl=en&_x_tr_hl=de&_x_tr_pto=wapp
AWo
