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2015 C300 4Matic, 1978 240D
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659 Posts
Discussion Starter · #1 ·
Well I've had my car almost 6 months so I guess I was due for a major failure. Yesterday I replaced my battery cables and when I restarted the car I noticed a continuous squealing sound from under the hood. I assumed it was probably just a belt squealing in the warmer weather.

Today on the way home from work I noticed my doors wouldn't work, then my power brakes wouldn't work, and I connected the dots for a vacuum pump failure.

The question is, what should I do now? I'm guessing it will be at least a week before I can get my in the shop and I have no idea how long it will take to get parts after that. First off, can anyone calm my nerves about whether this would cause engine damage if I keep driving it? It's a 1978 if that matters. Did I mention this is my only car and daily driver?

Has anyone else had the squealing? I'm curious if it's the whole $350 pump that needs to be replaced or if it's some simple fix.
 

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'79 240D, 83 300D
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1,046 Posts
I believe this type of pump can fail in a way that wrecks timing chain.

Driving without brake assist might not be a great idea either.

Did you test vacuum at pump, are you sure it is not a leak somewhere? I would get a vacuum gauge on it before I ordered any parts.

The squealing could be line leak or bad brake booster diaphragm also.
 

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1984 300D
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5,893 Posts
The squealing sound is not the description I have read happens when the Vacuum Pump Fails. If so it is a new one.

If the Vacuum Pump is torn up on side that faces the Engine when the Pump comse off you need to also check the in and out play of the Timer. If the Play is too much you in can ruin a New Vacuum Pump.

Changing the worn Bushing that causes too much end play will add to the Labor Cost and last I read the Bushing was $48 each.

But, as the other member indicated the squealing sound and loss of Vacuum could be in many places.

And, what year you Car is does matter because the newer models have a Piston Type vacuum Pump and the older ones have a Diaphragm type Vacuum Pump.

The Diaphragm one is cheaper to fix if only the Diaphragm is the problem.

Look at your Vacuum Pump. If you have a tube going from the Intake Manifold to the Vacuum Pump you have a Diaphragm Vacuum Pump.

Check under your Hood and look at the little 1 inch sections of Vacuum Hose that connect the Plastic Vacuum Tubes for Cracks or breaks. Don't put any weight on the main Vacuum line as the little plastic nipples are brittle. I broke one off during an Oil Filter change and did not know till My Engine would not shutoff.
 

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2015 C300 4Matic, 1978 240D
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659 Posts
Discussion Starter · #4 ·
I disconnected the big vacuum tube going to the brake booster and put my hand over it, and found no vacuum at all with the engine running. Could it still be a leak in a tube somewhere in that case?
 

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'79 240D, 83 300D
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1,046 Posts
You still could have a leak in that main tube on vacuum pump end, otherwise probably not.

I think pump outlet is too close to power steering pump belt for you to put your hand on that while running, so take tube off completely and inspect, blow in etc.
 

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1984 300D
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5,893 Posts
On the main Vacuum Tube I replaced the 2 large sections with Rubber Vacuum/Emissions Hose a I used longer Hose so I could route the Hose so it goes over the Fender Well and is less venerable to breaking the little plastic nipple/s.
It is also more out of the way.
 

· Registered
2015 C300 4Matic, 1978 240D
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659 Posts
Discussion Starter · #7 ·
It was definitely the vacuum pump. I had it fixed at a shop because I didn't want to take the radiator off myself. The brakes, locks and engine shutoff are all working perfectly now. The engine actually shuts off much faster, which makes me think the vacuum pump was probably already on its way out when I bought the car.

Now I start the countdown to the next repair :crying
 

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1984 300D
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5,893 Posts
On My 84 300D I have had the Vacuum Pump off for inspecton 2 time and I have never removedhe Radiator. TheRadiator Schroud yes but not the Radiator itself.

I can't recall if I removed the Fan or not.
 

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'85 Mercedes 300 D Turbo Diesel
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284 Posts
I have changed my vacuum twice, never removed anything but the power steering pump...Its funny but when the first one went out I bought a new one, and the new one did not last a year...On one of my Junkyard" excersions I saw a vacuum pump in the trunk of an old 300 sd, so I took it up to the counter to see how much it was. They charged me $20.00 so I took it home thinking I would someday rebuild it.
Anyway when my new one went out, I took it off and mailed it in for warranty replacement and put the junkyard one on without touching it... Well it is still working like a champ and I have a brand new one still in box waiting for it to die.
 
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