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Old 12-04-2000, 07:43 PM
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Join Date: Oct 2000
Location: Jacksonvill, FL, USA
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This is for a 1981 240D. I believe that I have traced the vacuum leak to a green/yellow tube that connects just above the yellow check valve that goes into the passenger compartment. I took off the bottom panel to the dash on the driver side and saw that it went up, so I took the insterment cluster out and saw that it continued up into the insulation at the top of the dash. My guess is that it controls a ventelation valve, probably the defrost valve.

Does this sound right? And assuming it does, how do I get to valve to replace it.

Is any additional information required?

The rest of the system seemed to check out.
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Old 12-05-2000, 12:10 AM
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Location: Venice, FL - "sharktooth capital of the world"
Posts: 712
Sounds like it may be going to an actuator for one of the vents. If it is, it can be a bear cuz you will need to take out the dash. I have a '85 300D and the dash is a royal pain in the rear to get out. There is one actuator that controls the center console vents and the actuator sits smack dab in the middle behind the center console.

Good luck,
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Old 12-05-2000, 02:42 AM
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Join Date: Apr 2000
Posts: 166
Looked at the vacuum section of the MB manual I have for my 1979 240D. There has been some indication that things don't change too much between model years.

It shows a green-yellow line delivering vacuum into the ventilation system all right, but the actuators seem to be located primarily on the right side of the car. Remove the panel over the passenger's legs and, I think, you will see several vacuum actuators quite clearly. The manual specifically recommends carefully checking the hoses and connections as well as the actuators. Hopefully, this will help get you started in your checking. If you don't find the problem and want copies of the manual pages (which go into considerable detail), let me know and I will try to send them to you.

Good luck. Hope this proves helpful.
1979 240D
165,000 miles
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Old 12-05-2000, 10:20 AM
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Location: Jacksonvill, FL, USA
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Is there anyway to test the brake servo by itself?
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Old 12-06-2000, 10:06 PM
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Join Date: Apr 2000
Posts: 166
Sorry it has taken me so long to respond. What with my wife's insistence on doing the Christmas shopping and a broken water line, there just hasn't been a lot of time for MercedesShopping.

The manual's description of testing the brake vacuum cylinder is complicated. It entails use of a vacuum tester, a pressure tester (which is inserted between a brake line and a caliper), a pedal pressure tester and a self-made tool to be inserted into the vacuum line leading to the vacuum unit allowing for connection of the vacuum tester. Overall, I'd say the MB manual procedure is impractical for the DIY.

Perhaps someone in the forum can recommend a simpler, more practical procedure for a DIY to accomplish this job.

Regret being the bearer of bad tidings. Hope someone else can help.

1979 240D
165,000 miles
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Old 12-06-2000, 11:18 PM
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Join Date: May 1999
Location: Charlotte, NC, USA
Posts: 688
For about $30 you can purchase a hand held vacuum pump with a guage to test most lines. It comes with different fittings for most size hoses and goes to about 25" of vac.
which is more than enough. You can watch the guage for how quick the bleed off is and since the motor isn't running you can hear it bleed...if it does. Most NAPA, Autozones and PepBoys carry this.

Bad news again, if you need to get to the valve/potentiometer assembly at the top of the dash. If the car is old/high mileage think about just cutting a flap over the center area to work in that section. When finished just put a Coverlay dash top in place. If you need to gain access again, it's real easy to remove cover and replace.
This way you don't have to pay/do remove the whole dash and possibly screw something else up doing it!

Good luck...
Tobias MB
4 MBs
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Old 12-07-2000, 11:50 AM
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I had to trace a broken door lock vacuum line in my '79 300SD. I found the break by removing both speakers in the top of the dash, and using a mirror.

Can the brake booster in a diesel Mercedes be tested in the old fashioned way? It will give a false reading if your vacuum pump is leaking or you have a leak elsewhere. If you can isolate the other leak by capping off that section, you should be able to test the booster as follows:

Booster test on other vehicles:

Test for a system vacuum leak as described below:
1. Operate the engine at idle without touching the brake pedal for at least one minute.
2. Turn off the engine, and wait one minute.
3. Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. Light application will produce less and less pedal travel, if vacuum was present. If there is no vacuum, air is leaking into the system somewhere. (most likely a faulty inline check valve, or leaky booster.)

Test for system operation as follows:
1. Pump the brake pedal(with engine off) until the supply vacuum is entirely gone.
2.Put a light, steady pressure on the pedal.
3. Start the engine, and operate it at idle. If the system is operating, the brake pedal should fall to the floor if constant pressure is maintained on the pedal.

I don't know why this test would not also apply to the Mercedes power booster.

I would give it a shot.

The booster receives it's vacuum directly from the vacuum pump. It can certainly rob a lot of vacuum with a leak. I would make sure you have good vacuum from the grey line that feeds all of the other components first.

Your year and model may be different but basically the same.

[Edited by patsy on 12-07-2000 at 12:12 PM]
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