No power brakes, engine won't stop with key - PeachParts Mercedes-Benz Forum

sarthur9 · #1 01-09-2007, 10:28 AM

Hello all,

I have a 84 300SD that I purchased in mid November with 74,400 miles, it looks all original, and now has 78,300 miles on it. Yesterday after driving about 35 miles it lost power brake assist and would not turn off with the key. Also, it has started to clunk when slowing down and going into first gear. I assumed a vacuum line had come off but could not find any disconnected.

Could it be the vacuum pump has gone bad? Any suggestions would be greatly appreciated. Thanks, Steve

TerryS · #2 01-09-2007, 10:49 AM

Whatever the cause, you have deffinately lost vacuum. The engine shutoff actuator vacuum comes off the power brake booster line, directly from the pump. Look for a large black line from pump to booster, with a tee in it. If nothing seems amiss, pull the small line off the Tee and check it with your vacuum guage there, or pull it directly off the pump and check the connection right at the pump for vacuum.

tangofox007 · #3 01-09-2007, 10:51 AM

Quote:

Originally Posted by sarthur9

Could it be the vacuum pump has gone bad? Any suggestions would be greatly appreciated. Thanks, Steve

Could be the pump. Could be the check valve where the hose attaches to the pump. The check valve is easy to remove with the pump still on the engine. If the check valve internal parts fall out in you hand, you have probably found the problem.

SD Blue · #4 01-09-2007, 11:14 AM

Only 78k miles! Wow!

If you rebuild the pump, ensure that you get the "later model" rebuild kit. There were some minor changes to the internal check valves. It is not really a bad job, just involves pulling a bunch of screws in order to get it off the engine.

sarthur9 · #5 01-13-2007, 02:04 PM

Thanks Terry, Tangofox007 and SD Blue for your replys.

I purchased a mitey-vac yesterday from Sears, cost a little over 60 bucks.

Anyway, I disconnected the large black line form the vacuum pump to the brake booster and checked the vacuum directly off the pump. It was 24" and came up as soon as the engine started, it held at 24" for several minutes until I disconnected the mitey-vac. I also hooked to the brown line going to the shutdown device and could shut the engine down with two pumps of the mitey-vac at 15" of vacuum.

Didn't know what else to do so I hooked everything back up. After that the brakes and engine shutdown worked properly for a few minutes then back to hard brakes and no shutdown. What happened?? Why did things work for a few minutes?? Is it the check valve??

kerry · #6 01-13-2007, 02:11 PM

I'd suspect your brake booster at this point. Remove the vacuum hose to the booster and use the mity vac to pull a vacuum on the booster. It will take A LOT of pumps. See if it holds a vacuum. If not, you've found your problem.
Brake boosters are expensive ($300+) so you want to confirm your diagnosis before replacing or find one at a junkyard. People say there is an o-ring inthe brake booster and that may also be a problem.
I'm having a similar problem. Not enough brake boost, shut down problems, and no door locks yet I seem to be getting vacuum from the pump. However, I have oil in the vacuum lines which leads me to think I have a tear in the diaphragm so I've ordered a vacuum pump rebuild kit and will try that.

sarthur9 · #7 01-13-2007, 03:03 PM

OK, I took the black line loose going directly into the brake booster and plugged in the mitey-vac, after a number of pumps I got it up to 10" of vacuum and it holds there with no drop at all.

I also disconnected the line where it meets the metal part nearer to the pump and cannot get it to hold vacuum from that point. What is the white thing near the brake booster with several vacuum lines coming off of it, could that thing be bad or maybe a leak on one of the lines coming off it?

I sure hope it's not an expensive part like a $300 booster.

Brian Carlton · #8 01-13-2007, 03:14 PM

Quote:

Originally Posted by sarthur9

Didn't know what else to do so I hooked everything back up. After that the brakes and engine shutdown worked properly for a few minutes then back to hard brakes and no shutdown. What happened?? Why did things work for a few minutes?? Is it the check valve??

Take the Mityvac and connect it up to one of the ports along the main vacuum line. Use a plastic T and a section of hose to make the connection.

Start the engine and observe the Mityvac reading. It should be 24" as you originally noted. Then, watch the gauge after a few minutes and observe what it does and when it does it. You should see the gauge start to fall in vacuum as the problem develops. The question is where the vacuum leak is coming from..........or whether the vacuum pump is truly defective and only provides vacuum for a few minutes on startup and then loses capability.

You've got a more difficult problem than the usual............

tjohn · #9 01-13-2007, 04:00 PM

I admit this one is tricky. Normally on the 126 the doors are a separate issue because they have an independant source of vacuum. If you have an federal version 84, try this vacuum diagram and see if its of assistance. California is different, and there are some differences in model year to model year. If yours is different, there are some other vacuum diagrams at the same site.

Good luck

http://www.peterschmid.com/vacuum/1977_1985/617_95/1981_1984_small.jpg

sarthur9 · #10 01-14-2007, 02:41 PM

Hey Brian, Took me a while to find a tee and get the set up to connect the miteyvac into the booster line.

After I got things setup so I could drive the car with the miteyvac in line I found that it would idle with the gauge bouncing between 15" and 16" and would rise to a fairly steady 19 1\2" at 50/55 mph. I drove about four miles starting and stopping with good brakes until the gauge dropped quickly to zero while running 50 mph and stayed there.

Twice during that trial run it would drop to 5" when I gave it the petal to start off from a dead stop then go back to 16" or above until it reached 19/20" as speeds increased. The other times I started from a dead stop it never dropped below 15". Also, it did not make the clunking noise when the transmission went into first gear while slowing to a stop. If the car is pulling when it goes from 2nd to 1st it does not make the clunking sound.

After that one time I could not get it to repeat. However, each time that I checked it at the pump it would have 24" of vacuum I rechecked all the lines and could not find any disconnected or loose.

Does any of this make sense? What's next?

Brian Carlton · #11 01-14-2007, 03:06 PM

Run it at idle for about 5 minutes.

Note the vacuum reading.

Then, step on the brake pedal and hold the pedal for one minute.

Note the vacuum reading.

Release the pedal.

Note the vacuum reading.

Wait one minute.

Note the vacuum reading.

Report back with four vacuum readings.

Something is leaking vacuum but it's not doing it consistently. We'll see where the above tests take us.

kerry · #12 01-14-2007, 03:07 PM

Something odd about losing vacuum when you step on the accelerator. Could it be a bad check valve in the vacuum pump? I'd be leaning toward a vacuum pump rebuild.

sarthur9 · #13 01-14-2007, 03:48 PM

OK, may have discovered something. At first startup with miteyvac inline, zero reading then would go up to 2", then drop back to zero.

After several times stopping and starting engine I got the same results.

Then I took a wrench and tapped on the check valve at the top of vacuum pump with the engine running (not a real good idea, too close to the PS belt) but the vacuum came up to 15 1/2" to 17 1/2" bouncing.

I let it idle for about 5 minutes, got 16/17" bouncing.

Brake held down 1 minute got 10/12" bouncing.

Released pedel got 13/15" bouncing.

After 1 minute 16/17" bouncing.

Maybe I should call Phil tomorrow and get a check valve and try it.

I don't want to start throwing parts at it, especially when I do not know which parts to throw, but this seems logical or does it? Thanks, Steve

Brian Carlton · #14 01-14-2007, 04:07 PM

Quote:

Originally Posted by sarthur9

Maybe I should call Phil tomorrow and get a check valve and try it.

I don't want to start throwing parts at it, especially when I do not know which parts to throw, but this seems logical or does it? Thanks, Steve

Yep, good job on troubleshooting.

It very much sounds like you've found an intermittent condition with the check valve.

I would definitely change it and report back with results. Nothing else makes sense at this point.

sarthur9 · #15 01-22-2007, 09:08 PM

I received the vacuum pump check valve form Phil today. I installed it and still have not corrected the problem.

When I removed the old check valve I only found the metal part that connects to the side of the piece at three places. I did not see the plastic part that is on the other side. Could it be down in the vacuum pump causing the pump not to produce enough vacuum?

I disconnected the booster line where it meets the metal line coming off the vacuum pump and hooked up the mityvac, it pulled 23" of vacuum but it took about 15 or 20 seconds to get there. Does that seem slow? I could not get any vacuum at the first line coming off the booster line just before the white plastic piece with several lines coming off it.

Any suggestions???