What I've Been Up To At Work

Your mention of the 220S reminds me of an incident a long time ago. At the time, I had a lot of contact with a Motor Vehicle Dismantler. I use that term out of respect for him. One day, he called me and said he had a car for me. A customer had sold him a 1965 220SE that would not start and run. I was reluctant to buy it, but did so as soon as he suggested I pulled out a spark plug and smelled it. As you will know, the SE's of the time had mechanical fuel injection. Something back then which was rare...the man that sold the car had known this and thought it was a diesel....so tried to run it on diesel fuel!

 

He wasn't the first person to make that mistake and probably not the last!

 

Continuing the work on the gearbox of the 250S the next job to tackled was the leaking modulator valve. This is operated by engine vacuum and it controls the quality of the gearchange. On this car it had a serious oil leak. So it was removed and taken apart to reveal an o'ring that had been poorly fitted...............
Modulator valve oil leak by longk48, on Flickr

Modulator valve oil leak by longk48, on Flickr

Typically it is not a standard sized o'ring but a local bearing supplier was able to track one down and it arrived this morning. Here's the valve reassembled and refitted to the gearbox...........
1966 Mercedes 250S gearbox modulator valve by longk48, on Flickr

The next job on the list to address the gearbox issues is actually part of one of the carburetors. The gearbox solenoid is actuated by two switches - one under the throttle pedal for the kickdown and the other one on the carburetor which operates the solenoid when the engine is at idle. Whilst the solenoid worked perfectly when tested on the bench once fitted to the car the idle function did not work. As the carburetors had been overhauled by someone fairly recently I hoped that the throttle switch had been incorrectly adjusted (the adjustment should be checked every time the idle screw is touched) but it turned out that the contacts in the switch were filthy. If it were a modern car this would mean a new switch but on these old cars it is just a matter of four screws, a quick clean and then refitting and adjusting.............
1966 Mercedes 250S gearbox throttle switch by longk48, on Flickr

1966 Mercedes 250S gearbox throttle switch by longk48, on Flickr

And after....................
1966 Mercedes 250S gearbox throttle switch by longk48, on Flickr

1966 Mercedes 250S gearbox throttle switch by longk48, on Flickr

We've been blanketed in thick fog today so the road test will wait until that lifts. But the gearbox engages smoothly on idle now so hopefully all that is needed now is to adjust the modulating pressure and adjust the solenoid rod to optimize the shift points.
Of course, having done all this it is still possible that wear on the gearbox internals could be causing the poor gearshift quality but at least we know that the basics are correct now.
One other job that will need addressing is slowing down the fast idle from cold. It is idling far too fast and some gasket sets have been ordered.

 

So the gearchange on the 250s is much improved in terms of shift quality, timing and kickdown operation. The change from three to four is still delayed though. One of the checks that you make before adjusting the modulating pressure is the vacuum that the engine is producing and on this car it is lower than it should be. Usually this would be a little concerning as it is an indication of engine wear but I was not panicked by this because as I said in an earlier post the carburetors look to have been recently overhauled and I had already decided that they need to be redone. A precautionary compression and cylinder leakage test told me that all was well with the engine condition so it was out with the small propane torch. What I did with this (please don't try this at home) was to open the gas valve on the gun but not light it and then you direct the gas where you think that there may be an air leak. I was pretty sure that I knew where it was sucking air in and sure enough I was right first time - the carburetor bases are distorted and not sealing correctly. This has been caused by the hotspot flaps in the exhaust manifolds being stuck open due to seizing.
So the parts are on order and this job goes on the backburner for a few days. Here is a photo of the car which I took whilst on roadtest the other day...............
Mercedes 250S by longk48, on Flickr