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Mercedes Benz 300SD Repair Questions and Answers

Question:  1979 Mercedes 300SD Turbo Diesel mileage: 103,000.  I've never owned a diesel car before, so I'm not sure what this could be.  The other cold morning, I started the car, let it run for a minute then drove.  As soon as it shifted into 2nd gear, it started screaming - like a high-pitched horn sound coming through what seemed like the vents into the front of the car. When I slow down, the noise lowers in tone until I'm in first gear when it stops altogether.  I wasn't bold enough to try a higher speed.  I'm afraid to take it to a mechanic without having some idea first, because I don't know any around where I live. (I just bought the car a few months ago.)

Answer:   Without listening to the high-pitched noise, we can give you some suggestions of what to look for.  First, check the vacuum pump and also the vacuum hoses.  Next, check for loose or faulty fan belts. 

Question:  1979 Mercedes Benz 300SD mileage: 246,000. The connections on my glow plug relay seem to be fried.  Is there a way to over ride the relay allowing me to start the vehicle and then disconnect so as to not burn it out?  I just need it to work for a few more days so til I can get to the dealer to p/u a new one.

Answer:   Not a good idea to bypass the relay.  The glow plug should be replaced with the relay.

Question: 1983 Mercedes 300SD 5-cyl Diesel mileage: 155,000. Engine will not shut off when key is turned to off position. Also, braking very weak and right electric window will not go down. All happened the same day at the same time. Vacuum system? I tested the vacuum pressure with a gauge but what is a good reading? Also, checked vacuum lines for damage or leaking. Am I on the right track or is it something else?

Answer: Yes, you are on the right track. You probably have a vacuum leak somewhere. The vacuum pump should have approximately 20 to 25 inches of vacuum.

Question:  1981 Mercedes Benz 300SD mileage: 220,000. Do you know easy way to check for bad glow plug...possibly grounding and checking for a variance conductivity or resistance between the other glow plugs?

Answer:  Yes, you are right about the test procedure. Also perform the test with the engine cold.

Question:  1981 Mercedes Benz 300SD mileage: 220,000. Want to change transmission filter and pan gasket. Because of mileage and not sure when and if prior owner changed fluid, I am afraid to have fluid replaced. Present fluid clear and unburned. Should I chance or will someone just change filter and gasket. Second question, vacuum motor in trunk that runs door locks and gas cap cover, quits after about two weeks. If I pull out, disconnect, and then connect three-prong electrical connector, it works for another two weeks. However, I believe that if I disconnect the vacuum line to the motor, it works for unlimited period.

Answer:  If in doubt, change the transmission filter and gasket and at the same time replace the fluid in the pan. With the transmission pan down, inspect for metallic particles. A slight amount is normal. About your vacuum motor problem, a cracked and leaking vacuum hose probably causes this. Locating and repairing the hoses should fix the problem.

Question: 1982 Mercedes 300SD turbo 5 cyl diesel mileage: 92,000. Hi, I have just purchased this car and I had heard the A/C's on these cars are not the greatest! As a matter of fact, I do see an awful lot of these cars driving around with the windows open in the summer. I live in Florida and I can't imagine these other drivers 'just like fresh air'. I had the Freon 'topped off', it didn't require much (a mere 2+ ounces) and it does not seem to have helped. The best I can figure, from asking questions of local mechanics, is the evaporator is simply 'too small'. The A/C barely cools - Do I have to live with this or are there some suggestions you can make to improve the cooling capabilities? I'm sure you have been asked this before, but I did not see it in the archives.

Answer: The alternative to making the air-conditioning system "bigger" would be to better isolate the passenger compartment from the heat of the engine. Adding more isolation on the firewall and under the floor mats will help. If legal in your area, tinted windows will also help.

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