I have read many posts on the importance of monitoring vacuum in older Mercedes. I have even read posts about people plumbing vacuum lines into the cabin to either connect an "economy" gauge or a MityVac.

I was unable to find any pictures of how to get vacuum from the engine to the cabin. To that end, I am attaching my efforts this evening.

I decided to connect off the transmission line of the blue saucer as I have been hunting down a transmission shifting problem. It seemed as this was an "end-of-the-line" vacuum connection (terminating at the transmission), that it would be a good spot to splice.

The first picture is before the splice. The second picture is after the splice. Picture 3 is my spool waiting to be fed through the firewall.

I would have pushed to get it through the firewall except that when I buried my head under the steering column, I found a cover. I did not feel like wrestling with it tonight so it will wait for the weekend.

I will post more info as I complete the project. Of all the mods of which I have read, this seems to be one of the simpler ones... Any thoughts or advise are always welcome.

Attached Thumbnails

     

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1987 300 TD - Kraut
1987 300 D - Hans

Normally you don't want "end of the line" vacuum, you want to measure as close to the vacuum pump as feasible. To that end, tee into one of the lines coming off the main line between the VP and the brake booster.

There are lots of different vacuum gauges you can get. Vacuum is very important in your MB as it runs the brakes, climate, shutoff, doors and transmission. Once you've got your gauge set up, it's pretty easy to plumb the source temporarily to various components for troubleshooting. Once set up, cap all other users temporarily to get a baseline vacuum number. Then plug in and you'll get an indication whether you've got a leak somewhere. Keep in mind your transmission is always bleeding vacuum.

I monitor vacuum as part of my bluetooth engine monitoring system and it's very helpful to have.

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I added the economy gauge from an 190E to my 190D. I plumbed it to the vacuum input to the BUFO, not the modulator output, but it's the same idea. I pulled the line to through the firewall via one of the existing grommets. It's probably a bit different on each model, but on a 190D, it required pulling the dash pod. You can read about it here:

An Economy Gauge for my 190D - Mercedes-Benz Forum

Assuming your car is like mine, there will be a multi-purpose grommet on the top left side of the firewall, behind the electrical center. There are multiple holes in the grommet, the ones that aren't in use are sealed and will need to be pierced to run your line. Much easier to reach from behind the dash pod than from below.

To mach4's point, I have no idea why you would want to measure vacuum close to the pump...it's not modulated, and won't change unless the pump falls apart or you develop a big leak. Either way, you'll know without a gauge. A perfect example of TMI. Reading the vacuum supply doesn't tell you what sort of proportioned signal is being given to the modulator, so it would do you no good. The transmission modulator doesn't "bleed vacuum", but the VCV does.

If you're looking to test your transmission modulator, you're tapping into a good spot. In my case, I wanted a proportioned vacuum signal to drive the economy gauge, which is why I'm picking up the VCV signal underneath the BUFO.

If you want a general sense of supply vacuum, you can tap into the line that feeds ACC (red?). This is easier in a sedan because the line splits in the driver footwell to the rear headrest release. There's also the vacuum supply to the key switch (brown?) which sources from the big brake booster line, not the auxiliary port at the pump. It depends what you want to know.

Sixto
MB-less