300D runs 3 to 5 seconds before stopping. - PeachParts Mercedes-Benz Forum

nomorehurry · #1 04-25-2008, 02:27 PM

I have read information on this subject before, but I want a second opinion. I have a 1982 300D turbo, automatic transmission and 183,000 miles. Owned for about 4 months and generally runs OK for its age. I know (think?) the B-something valve on the driver side of the transmission leaks, but that will be another question.

Last night I drove home through a heavy rain and the eingine didn't shut off immediately when I removed the key. Thinking back, it has taken about 1/2 to 1 second before comming to full stop. After last night it is taking 3 to 5 seconds after removing the key from the ignition.

My investigative work with a vacuum guage and vacuum pump and other implements of destruction showed me the vacum to the ignition (brown) was about 7 inches Hg, from switch to shut off valve (brown w/purple stripe) was 7 inches when running, and fell to zero when the key was turned off. The vacuum would then climb slowely to 7 - 8 inches and the engine would stop. Vacuum applied directly to IP shut off would cause engine stop immediately at about 8 inches Hg.

The black hose connectors to the vacuum switch on ignition "look" fine, and when I plug one end (Brown w/purple stripe) and apply vacuum to the other (brown) it holds. Except when the switch is turned. I think that means I have to replace the vacuum portion of the ignition switch.

(It sure sounds like I know what I am talking about; sure wish I did!)

My question is could this have been caused by driving home in a heavy rain storm? It dosn't seem likely, but my knowledge of MB Diesel is just below the beginner level. Next is what is the procedure and is there a shop forum guide on how to do it? What parts will I need, etc. etc.

(The last back yard mechanics I did was on a 72 VW Bus that died in the garage.)

Assistance would be appreciated (especially from a knowledgable source - as most if not all of you are).

Serious comment - this forum is fantastic and has helped me a lot. Thanks!!

rrgrassi · #2 04-25-2008, 02:33 PM

Sounds like the ignition switch is fine. You need to get a mity vac and check all of the vacuum system. Faulty HVAC or lock systems can and will affect the engine shut off. It appears you have a vacuum leak somewhere,

The mitiy vac can also tell you about the health of your vaccum system and brake booster.

nomorehurry · #3 04-25-2008, 06:11 PM

I know it is a vacuum problem, but my problem is where. (I'm not trying to be a smart-A, even though I'm told I am very good at it.)
I have a vacuum pump, have worked through everything that is connected, (the door locks are disconnected and plugged), and it is pretty tight. The HVAC sysem seems tight, but I have not checked all the valves individually. I can pull 15+ in Hg when the motor is off and transmission line disconnected and plugged, and the vacuum will hold steady. With the transmission connected, I'm don't get a total seal, but the only time the guage drops to zero (like a rock) is when I turn the ignition switch on or off. So I know I have a small leak in the line to the transmission valve - at the valve itself. When connected, it takes a longer time for the vacuum to go to zero inches, and the rest of the system does hold.)
This problem started suddenly after coming home in the rain. I don't see how the rain could cause this problem - but stranger things have happened. (Maybe it washed out the wrong piece of dirt!) I have ~20 in Hg at the brake booster and ~10 in Hg of the tee to the rest of the system. The vacuum in the ignition switch will hold when the engine is running, OR when it has stopped. But vacuum will suddenly go to zero inches Hg when the key is turned. I tried hooking the vac-pump to the inlet side of the ignition switch and starting the engine, then turn the keyoff and removing the key. The engine continued to run till I reached about 8 in Hg vacuum which stopped the engine. That's why I think it is the switch and the act of turning the key causes an opening to the outside air (if it was electrical I would call it a direct short to ground). BTW, I think the electric part of the switch is fine.

nomorehurry · #4 04-25-2008, 06:13 PM

I forgot to mention that my wife wanted the computer and then I fell asleep. That's why I wasn't at the computer when you answered.

zeke · #5 04-25-2008, 07:55 PM

Others have described this better than I can and with photos, (do a search) but if you kill the ignition switch and suck on a tube hooked up to your shutoff valve (brown tube near the oil filter housing connected to a metal disc shape) and can't kill the engine - it's the valve. Likely you will kill it that way, so start moving up the line. Check the vacuum on the brown line, then plug all the other connections on the T, then move upstream towards the vacuum pump.

Good luck -

toomany MBZ · #6 04-26-2008, 01:09 AM

Check for oil in the brown vac line coming from the shut off valve, if you have some, new valve time.

running-snail · #7 04-26-2008, 08:35 AM

Quote:

Originally Posted by nomorehurry

Vacuum applied directly to IP shut off would cause engine stop immediately at about 8 inches Hg.

The black hose connectors to the vacuum switch on ignition "look" fine, and when I plug one end (Brown w/purple stripe) and apply vacuum to the other (brown) it holds. Except when the switch is turned. I think that means I have to replace the vacuum portion of the ignition switch.

(It sure sounds like I know what I am talking about; sure wish I did!)

I agree, sounds like your vacuum switch is bad. I don't recall what color my hard lines are going to and from the ignition switch; as long as you're certain you're on the opposite ends of these lines, vacuum should hold regardless of switch position if one end of the line is plugged.

nomorehurry · #8 04-26-2008, 10:23 AM

That's what I was thinking. The brown line and the brown w/stripe are the two vacuum lines comming from the back part of the ignition switch. When I disconnect both in the engine compartment, plug one and pull vacuum on the other, the vacuum holds if the engine is on, it holds if the engine is off, but falls to zero when the ignition key is turned.

Do you know what the vacuum part of the ignition switch is called? Or can it be repaired (by a klutz) or does it have to be replaced? From what I have read, access is through the instrument cluster.

Mustang_man298 · #9 04-26-2008, 10:33 AM

On the vac line to the trans having a slow leak, I believe that is supposed to be that way, since it was designed as a metered leak through an orifice and variable valve (the white one on top of the IP).

Looking at his results on the ignition switch, it sounds like it is possible that it is leaking as the switch changes positions, but holds at the ends. Seems like the vac in the system would absorb that very quickly though. The other thing he might be seeing, is the absorption of the ambient air in the shutoff side of the switch, since that goes to atmosphere when in the run position.

nomorehurry · #10 04-26-2008, 11:02 AM

On the vac to transmission leak - the leak is between the green metered valve in the engine comparment and the transmission. I replaced the small connector tube on the B-1 (or B-2?) and the volume of leak(?) - the speed at which the vacuum needle fell to zero on the guage - decreased.

I will probably have it replaced by a mechanic since it is twenty five years old and everthing I read says they don't last that long and I am afraid of entering an automatic transmission with two left feet.

I am assuming that is what is causing a flare between 2-3 and 3-4 shift points. (It works well when manually shifted.)

I found the part number for the vacuum valve part of the ignition switch on another page - MB #0004600284, cost between $25 and $30. Difficulty of repair is questionable, they say it is iasy once you get to the valve.

Time I went and took another look at taking the instrument cluster out and what all I gotta fix there.

Thanks many time to all.