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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53003
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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My 1985 300d won't shut off, When i put a hand vacuum pump

Customer Question

My 1985 300d won't shut off, When i put a hand vacuum pump on the shut-off valve and pump it shuts right off so I know that my shut off valve is good but I am not getting vacuum back from the ignition. What to do?
JA: Have you checked the fluids recently?
Customer: yes
JA: What is the model/year of your Mercedes?
Customer: 1985 123 300d turbo
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: yes. I checked vacuum in booster line measured 18 whatevers and I shut it off by attaching a hand vacuum pump to the shutoff valve where the grey vacuum line that comes through the firewall attached to the shut off valve. I know the problem is on the cab side of the firewall but I don't know how to test for it.
JA: Anything else you think the mechanic should know?
Customer: Yes, my door locks were working fine before I started to "fix" the shut off problem and now they are not and my cruise control hasn't worked for awhile so I know this is a vacuum issue
Submitted: 2 months ago.
Category: Mercedes
Expert:  Marty replied 2 months ago.

Hi, usually a large leak develops in the vacuum lock circuit and it depletes the vacuum to the shut off valve. Try locking the car and waiting a minute then see if the car shuts off. If it does you will need to locate and repair the leak in the door locks. You can also block off the yellow vacuum line for the door locks at the brake booster and see if the car shuts off. The brown lines are for the shut off. Sometimes the rubber vacuum hoses on the vacuum switch on the steering lock fall off and cause this problem as well.

Let me know if you have any questions.


Expert:  Marty replied 2 months ago.

Hi, I'm just following up with you on this question. Do you need any more help with this?