Hi Dave, (you da same guy from the DP forum?) welcome to My playground! it could be the flexible connectors that link the brown vacuum lines to the switch, or it could be the IP shutoff valve, or the vac valve in the ignition assembly itself. from your description, at a minimum you have an electrical problem in the ignition switch, as the spring that positions things after you start is in there. pullapart/pickandpull finds of any diesel after 82 will net you a good assembly, fairly simple to remove once you get access to the switch removing dash panels and such. if you pull an assembly from a vehicle newer than 89, you will have the ability to integrate the cable to lock the shifter with the key out, and require the brake to be pressed to move the shifter out of P... the key and tumbler can be ordered from any dealer matched to the vin of your vehicle.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!