The hardest part of the job …

Several people were very complimentary about my achievements last weekend with the Mercedes 300D, but I have to be clear:  most of the achievement was Pierre’s.  I worked, but mostly I was there to learn while Pierre busted his knuckles doing the hard stuff, so I can’t take credit for most of it.

Today’s minor adventure in old car repair will demonstrate the true nature of my mechanical abilities.  As you may recall, we discovered a few minor needs toward the end of the weekend, for which we either lacked the proper tool or a Mercedes-only part.  I ordered a few things on Monday and they arrived today.  There were really only three tasks:

  1. replace a bad relay, one which controls the electric auxiliary engine fan.
  2. install a rebuilt kit in the mono valve. This is a fancy name for a simple valve that opens up to allow hot engine coolant to circulate in the heater core, thus providing heat to the cabin.  It has a rubber diaphragm that breaks eventually.
  3. replace one bad glow plug.  The glow plug warms the engine cylinders on a diesel, so that you can start it.

The relay was simple.  No tools involved.  You open a plastic cover, pull up the old relay, plug in the new one. Anyone who can change a light bulb can do this, so not surprisingly I managed to achieve it.

Then, buoyed by my success so far, I unbolted the mono valve and opened it up to reveal the internal plunger.  But I forgot that the engine was still warm from driving it 30 minutes earlier, so when I pulled the plunger out, coolant spewed all over.  Whoops! I quickly thrust the plunger back in.  Emma was standing by and got splattered, but fortunately it was not hot enough to burn.  That made me feel really stupid.

So I set that task aside and switched over to replacing the #3 glow plug.  I had a hell of a time getting to it.  You know how things that look simple often aren’t.  This happens to me a lot.  All you have to do here is unscrew an 8mm nut to remove the electrical connection, then unscrew the glow plug.  But I couldn’t do it.  The tools I had just wouldn’t fit in the space due to obstacles like the injector lines and injection fuel pump.

It was looking like I’d have to start removing injection lines, which would have brought the repair up to a new level of messiness and difficulty.  Instead, I finally managed to get the electrical wire and the glow plug by using a U-joint and a long extension on the ratchet wrench, and wrestling with it for a while.  It was frustrating because it seemed like it should be easy.  I dropped a nut three times trying to re-thread it, and once it fell into a spot beneath the injection pump where I thought I might have lost it.  Eventually the job got done, taking about three times longer than I had expected.

But in the process I made myself a new job.  I didn’t realize it, but I was leaning on the brake booster (vacuum) hose when I was fighting to get the glow plug electrical connection back on, and SNAP! a plastic vacuum fitting on the hose broke off.  This fitting goes to various transmission and engine accessories.  The brake booster is still getting vacuum, and I can plug the open fitting, but the transmission won’t shift right without vacuum, and the fitting can’t be glued back.  The hose was probably fairly old and brittle.

I could try to seal it up temporarily with some silicone tape, but why bother? The part has to be replaced anyway.  I sent the picture to Pierre and he confirmed that I need to buy an entirely new assembly, which includes the plastic fittings, vinyl hose, and metal ends.  The part comes only from Mercedes and it has to be ordered, so I’m lucky to wait only until Monday to get it.   Of course, installing it appears to be just a matter of two easily accessed nuts and two other vacuum hose connections.  I think I can do that without breaking anything else.

When the glow plug was done I went back to the mono valve.  Things were cooled down now, so it was fairly straightforward.  As expected the diaphragm was gone.  But unexpectedly, I found several 6-legged bug corpses inside the cylinder.  I’m not sure how they got in there, or why.  I cleaned them out and the rest was straightforward. Total elapsed time: about an hour.

So that’s the real glory of this type of project:  cleaning bug corpses, cursing at difficult nuts, and wearing Eau de Coolant.  With each step I feel like I’m learning, and simultaneously that I’m incredibly incompetent.  This kind of stuff isn’t easy for me, but in the end I do enjoy the sense of accomplishment and the gratification that comes from achieving something you’re not naturally good at.  So if you have any congratulations for me, let them be for having tried.  Turns out, that’s the hard part.


  1. says

    Rich, I’m the guy that managed to have a t-shirt I was wearing whipped off and over my head by the fan belt on a Volkswagen I was trying to tune, and I thought it had ripped my scalp off. So, for your efforts, I say, congratulations!

  2. says

    The Man in the Maze enters the Arena…

    Congratulations for having tried, because…

    “It is not the critic who counts: not the man who points out how the strong man stumbles or where the doer of deeds could have done better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly, who errs and comes up short again and again, because there is no effort without error or shortcoming, but who knows the great enthusiasms, the great devotions, who spends himself for a worthy cause; who, at the best, knows, in the end, the triumph of high achievement, and who, at the worst, if he fails, at least he fails while daring greatly, so that his place shall never be with those cold and timid souls who knew neither victory nor defeat.”

    Theodore Roosevelt
    “Citizenship in a Republic,”
    Speech at the Sorbonne, Paris, April 23, 1910