It all started last week when one of my oil cooler lines started leaking on my 1982 300SD. I ordered the lines and waited until Saturday to do the repair. I finished the work and cleaned up the car and garage, but didn't go for a test drive - I just ran the car up to temperature to make sure there were no oils leaks.

We start to go out on Sunday morning and the car was shifting too firmly for my likes. I thought I might have swapped one of the vacuum lines which I had disconnected during the course of replacing the oil cooler lines. They were correct, but I wasn't getting the proper vacuum to the transmission modulator. I then took off the cover to the 3/2 valves on the valve cover and found the problem. One of the ports on the intermediate piece (Mercedes' term) had broken off. I had a spare port so I was able to circumvent this problem until I could get the proper parts. Modulation vacuum measured correctly. I still didn't test drive it.

Later in the day, I needed to run an errand. This time the car would barely make it up the hill and shifted late. I checked the transmission linkage to see that the Bowdin cable was operating correctly. It was. I then thought that the turbo boost to the injection pump wasn't working and quickly bypassed the switchover valve. That was it! The car ran great. I went in the house and was in the process of ordering the switchover valve when I wondered if the boost switch on the intake manifold (or its associated wiring) could be at fault. Sure enough the boost switch read 0 ohms when the engine wasn't running. I re-installed the switchover valve and disconnected the boost switch. The car continued to run great.

Moral of my story: always make a test run after you have made repairs!

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)