Umteenth vacuum question (1982 300 SD) - PeachParts Mercedes-Benz Forum

jtocwru · #1 05-28-2008, 11:49 PM

I'll preface by disclosing that neither the door locks nor the heating/cooling controls function at all on my 1982 300 SD. The transmission shifts extremely harshly on the 2-3 shift (and I believe the 2-1 shift, but only if I come to a stop slowly. The harsh downshift does not occur when I brake hard, interestingly.)

I should add that I believe the EGR system in this car was removed long ago. There are only two vacuum lines connected to the junction on top of the valve cover. I'm assuming that this junction is functioning simply as a loopback at this point, with a supply line coming in and a return line going out. Nevertheless, it looks like the throttle linkage opens and closes some valves that might restrict the flow through that junction. I have no idea.

I began by teeing in my Mityvac to the vacuum line that runs to the transmission. I teed in downstream of the green dashpot (and, obviously, upstream of the modulator on the transmission.) At idle, I am observing 15" Hg. I performed the driving test with the Mityvac resting on the windshield. As soon as I apply any throttle (literally, as soon as my foot taps the gas pedal) the vacuum reading drops to 0. If I'm coasting down a hill with my foot off the gas, the reading does come up to around 5", but it immediately drops to 0 as soon as I touch the gas pedal.

What does this mean, and what should I test next? If I'm able to get 15" out of the VCV, does that immediately rule out a leaky VCV?

Are the door locks on an 82 300 SD actually on a separate vacuum system? When I depress the driver's lock, I hear the pump turn on in the trunk for about 10 seconds. I also hear a gurgling sound emanating from the front passenger's door. That's all I know about the state of the lock system.

#2 05-29-2008, 12:05 AM

I don't know much about the SD vacuum system, but I do know that the door locks are a separate system powered by the vacuum pump.

Jeremy5848 · #3 05-29-2008, 12:07 AM

Pressing on the footfeed causes the VCV to reduce the vacuum to the transmission but it should drop gradually, not all at once as you describe. It sounds like your VCV is defective.

I believe (not having an S-class car myself) that the SD models have a separate vacuum pump for the door locks. There are many lines, junctions, and valves in the door lock system that can leak and cause the system to not work. It's a slow process to work your way through, starting with the driver's door, but that's what you have to do.

The climate control system is on the engine vacuum pump, I think. It may have its own leaks or simply not be getting vacuum due to disconnected lines. You can test it with your Mityvac.

Jeremy

Brian Carlton · #4 05-29-2008, 12:50 AM

Quote:

Originally Posted by jtocwru

I should add that I believe the EGR system in this car was removed long ago. There are only two vacuum lines connected to the junction on top of the valve cover. I'm assuming that this junction is functioning simply as a loopback at this point, with a supply line coming in and a return line going out. Nevertheless, it looks like the throttle linkage opens and closes some valves that might restrict the flow through that junction. I have no idea.

Remove all the vacuum lines to and from the black box on the valve cover. There is a vacuum supply line that originates at a T near the IP. There is also a vent line that also originates near another T near the IP. Both of these lines should be removed and the T's can be removed. Connect the two open lines that went to the T with a short piece of vacuum hose.

You've got a leak in the black box that shows up immediately when you open the rack.........even slightly. When you correct this leak, your shift quality will return to normal.

jtocwru · #5 05-31-2008, 01:22 PM

I removed the lines to the black box, and replaced the tees with a couple pieces of AutoZone vacuum hose, and VOILA! No more bone-jarring shifting.

Thanks a lot, Brian.

Now if I could only figure out why cabin air doesn't blow...

Brian Carlton · #6 06-01-2008, 09:39 PM

Quote:

Originally Posted by jtocwru

I removed the lines to the black box, and replaced the tees with a couple pieces of AutoZone vacuum hose, and VOILA! No more bone-jarring shifting.

Thanks a lot, Brian.

Now if I could only figure out why cabin air doesn't blow...

Good job.

The cabin air doesn't blow at all.........or it blows only out the defrost vents?

jtocwru · #7 06-01-2008, 10:03 PM

All I can say for sure is that there is definitely warm air emanating (definitely not "blowing") from the defrost vents when I have the temperature dial set to "Max." I assume that's simply convection.

No matter what button I depress on the heating/cooling controls, there is no air blowing out of the vents. With respect to the heating/cooling system, I don't understand exactly what is controlled by vacuum, and what is electrically operated. The (electrically operated, I assume) blower isn't controlled by vacuum, is it?

Brian Carlton · #8 06-01-2008, 10:05 PM

Quote:

Originally Posted by jtocwru

All I can say for sure is that there is definitely warm air emanating (definitely not "blowing") from the defrost vents when I have the temperature dial set to "Max." I assume that's simply convection.

No matter what button I depress on the heating/cooling controls, there is no air blowing out of the vents. With respect to the heating/cooling system, I don't understand exactly what is controlled by vacuum, and what is electrically operated. The (electrically operated, I assume) blower isn't controlled by vacuum, is it?

It sounds like the blower is non-functional. Now it's time to drop the passenger kick panel and connect 12V and ground directly to the blower via the plug. See if the blower works.