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Old 11-13-2007, 06:11 PM
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Location: Madison, Wis.
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How to: Fix shutoff problem with ignition vacuum switch

Probably old hat for most, but since I've seen this come up in a few threads I thought I'd post my experience solving a shut-off problem by replacing the vacuum switch at the ignition. (car is an '85 300TD Turbo, OM617 W123)

First, I determined that this was my problem from the following signs:

1. My car gradually took longer to shutoff, then wouldn't at all. The shut-off valve at the ignition pump (IP) had already recently been replaced.

2. There was black oil in the line (brown with a blue stripe) leading away from the IP shutoff valve. My shutoff valve had also been bad, and when these fail, the diaphram lets through oil, which clogs and ruins the vacuum switch at the ignition.

3. When I plugged in a meter at one of the lines teed off the brake booster line, it registered vacuum of 20-some inches when I'd plug the brown vacuum line that runs to the ignition. This meant there was a leak; the ignition switch wasn't holding a seal.

4. With my mityvac, I could shut off the car at the IP shutoff valve. The brown vacuum line going into the firewall and to the ignition (the engine compartment end connects to a four-way rubber piece) would not hold vacuum. Further evidence that stuff was wrong at the ignition end.

So, if all that sounds familiar, you'll likely be able to fix your car with a new ignition vacuum switch. I got mine from fastlane for just under $25. (http://catalog.worldpac.com/mercedes...Shut+Off+Valve)

The steps, in overview, are these:

1. Remove the panel under the steering wheel.
2. Disconnect a wire from its connection near the ignition for better access
3. Make note of which vacuum tube connects to which port in the ignition vacuum switch
3. Unscrew the two flat-head screws that hold on the valve.
4. Clean out the vacuum lines with a spray solvent like electronics cleaner.
5. Use fresh rubber tubing and connect the two vacuum lines to the new switch.
6. Put the screws back in and reattach the panel under the steering wheel.

Before you begin, You'll want to get a nice bright light that you can aim up underneath the steering wheel. I used a desk lamp with a flexible neck that sat on the center console and could be pointed where I needed it.

Secondly, you'll have to experiment with a confortable position to do a some fairly tricky finger work. Picture the Fossberry Flop position that a high jumper uses. I was kind of like that, with my back resting on the rocker jam, shoulders in the footwell, facing up toward the steering wheel. It was actually a comfortable working position. Lots better than bending over the engine compartment.

Now, the steps in more detail.

1. REMOVE PANEL UNDER STEERING WHEEL: There are two plastic screw-like connectors down low on the panel. One's near the hood release, the other is on the opposite side, near a low air vent. These are removed by turning 90 degrees with a phillips head and pulling out with fingers. You'll see how they work. Instead of being screws per se, the shaft is flat and smooth on two sides. Next, there are four little oval-shaped covers along the top of the panel below the steering wheel, light switch, etc. A tiny flathead screwdriver or a pocket knife can be used to easily pop these off. They cover normal metal phillips head screws. Once you get these off, pull the panel pretty much straight back. I started pulling the black plastic part near the vent on the right side. That offers the most resistance, and once you get it free, you're home free. It's not that tough once you do it. You don't have to muscle anything, but it takes a little deliberate firmness.

2. DISCONNECTING THE WIRE is optional. It's a white plastic box connection with two prongs inside. You'll see it. If you don't think it's in your way, no need to bother. It just pulls out, no tools needed, and pops back in.

3. See the TWO SCREWS holding on the switch? They are hard to get at. Diesel Wiki threw the term "near impossible" my way. This helps forge the patience and resolve you'll need. My advice: get comfortable and rest your forearms when they get tired of trying. On the lower screw, I used a flathead screwdriver bit meant for a drill. I put it into the screw's slot, then used a vicegrip to get a hold of it and turn it. Once I got it loose -- and my screws weren't on tight, plus were all oily -- they just spun off with fingers. The screw on the backside is really tucked away. I got at it with the screw bit in a socket. The bit and the socket made a tiny screwdriver and was able to get it free this way.

WARNING: once you get both screws loose, my little switch fell right out of the oily rubber tubes connecting it to the vacuum lines. So don't wait until you have it detached to figure out which line goes to which port.

(But if my warning comes too late, I'll offer you this: If your car has the same set-up as mine, which I believe is factory, the vacuum line with the blue stripe goes to the outside port, farther from the steering column. The plain brown line goes to the inner port, closer to the steering column.)

4. CLEAN OUT THE LINES. Disconnect them in the engine compartment, spray them out well. I used electronics cleaner because it seems effective and safe for other materials it's going to contact. Make sure they're clear. Test for obstructions with mityvac. Etc.

5. RECONNECTING THE LINES at the ignition is a little tricky. I used fresh rubber tubing which is always very tight for me. I connected the tubes onto the valve first. I couldn't do the vacuum line connections under the steering wheel with just my fingers, so I very carefully used a visegrips to hold the vacuum line as I slid the other end of the tubing over it. You don't want to crack the plastic lines, obviously. So be careful if you use my visegrip method.

6. Then put the SCREWS back on. For that top one, I attached it to my makeshift mini screwdriver with scotch tape. You got 'em off, you'll find a way to get 'em back on. If you detached the wire, remember to replace it to its connection.

Then you just put the interior panel back on, reconnect the vacuum tubing at the engine compartment and you're good to go!

I was so happy when this all worked, and I could go back to turning my car off with the key. Hope this helps!

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1995 E320 (2009 - current)
1985 300TD Turbo black (2007-current)
1983 300TD Turbo red (2008-2009)
1972 280SE (2004-2007)
1977 300CD (2003-2004)
1976 240D (1999-2003)

Last edited by ljsense; 11-13-2007 at 06:20 PM.
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Old 11-13-2007, 06:27 PM
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Join Date: Dec 2006
Posts: 111
this must be the problem with my car.thanks for posting this..........
by the way that link you post doesn't work.

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