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I feel that there must be some significance in designing the shut-off valve to have fuel pass thru it from the prefilter to the lift pump.If there was not they could have just designed a shutoff between the filter and ip which is also where temp is measured?
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The shut-off valve actually switches the direction of the suction side of the lift pump from the pre-filter to the inlet port of the IP thereby drawing fuel out of the IP.
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So assuming that the shut-off valve allows for two seperate fuel paths 1)from the prefilter thru the shut-off to the lift pump and 2)from the diesel filter thru the shutoff to the IP... for any fuel to cross contaminate it would mean that fuel that hasnt been filtered by the diesel fuel filter would be entering the IP which MB would not allow...
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Yes, the shut-off valve controls both fuel pathways at the same time (there is a ball valve on both ends of the solenoid). There is a third fuel pathway inside the shut-off value that is opened when the ignition is turned off that pushes the filtered fuel back towards the pre-filter. Its a neat design and I think the dual-nature of the solenoid is primarily for safety. If there were 2 separate solenoids the system would not be as single fault tolerant.
Sorry for the late reply...