urgent question RE: '79 300TD - PeachParts Mercedes-Benz Forum

nickydubs · #1 07-22-2004, 06:28 PM

hello all!

i'm new to the forum, but the answers i've read seem excellent. here's my situation:

i recently bought a '79 300TD wagon on EBAY. see here:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2483568331&rd=1&sspagename=STRK%3AMEWA%3AIT

it's in chicago and i'm in cleveland, so naturally i went there to get it. i picked it up no problem, it started no problem, and drove off the lot no problem. THEN, about 15 miles later, the car would not stop revving up. i've come to learn that this was because i had turned the unmarked knob on the dashboard all the way up, the one that apparently operates the throttle. then, after another 10 miles i lost all power in the brakes and was forced to do an emergency stop with E-brake and turning the car off.

here's the explanation i was given:

the knob was turned up, got stuck there, and while the car was unable to slow down it's RPM's i lost vacuum pressure and thus the brake booster.

here are my questions for you:

1. can i manually unstick this knob, or do i have to have the part? i can make it back home slowly, i.e. turn the knob down and replace it when i get home, if that's possible.

2. the car had problems with pick up, then it had problems with not accelerating, then it wouldn't stop accelerating in neutral or park, but would have no pick-up in drive. it would hold speed well, though. also, it had a slow response when i first drove it, before fiddling with the knob. why?

3. is that brakes explanation accurate, or is there a problem with the vacuum system?

4. what is the proper way to utilize this knob?

5. are there any other cronic mercedes problems i should be on the lookout for?

please keep in mind that i know what i'm doing around gasoline engines and cars, i'm just new to diesel. i'm very skilled mechanically. i've never owned a diesel before, and i didn't think i'd need to adjust that much.

also, the car is imaculate other than this problem. the suspension is gorgeous, it came from a rich suburb of chicago, and it was donated to the salvation army, which leads me to believe that someone's wealthy grandmother needed a new car and preferred the tax write off to the hassle of selling it for $1525.

the urgency comes with the fact that if i'm going ot go and pick it up again, i have to do it on monday, and i need to notify this guy of my decision by saturday.

please respond with any and all help, i'll really appreciate it.

nick

leathermang · #2 07-22-2004, 07:26 PM

I did not wait for all the photos to load.... I am on a slow dialup...
This is a Normally aspirated car ?
Amazingly clean......

You say " while" ... but are you asking if the rpms deal was the cause of the loss of vacuum ?

If you had brakes at first.... then the vacuum pump may have given out.....check for oil in the lines which go from it to the inside of the air cleaner from the bottom.... if your car is configured like that... check for oil in any of the vacuum lines... if yes, then the diaphram has ruptured in the vacuum pump....

dmorrison · #3 07-22-2004, 08:08 PM

That knob is an idle adjustment that repositions the "zero"set of the throttle linkage. Yes you can disconnect it and the car will run fine. Follow the black coaxial looking cable from the firewall to the throttle linkage just above the injection pump. Notice the steel line cable that extends outward from the cable end. It will attach to the throttle linkage in that linkage assembly mess that make up you throttle linkage. You will be able to see how the cable end is adjusted and disconnect it there. To operate it normally. Slightly depress the throttle. rotate the knob, counterclockwise to its stop. Start the car. the car will isle at a higher than normal RPM. I return it to the full clockwise position once the temp guage comes off the low limit.
What concerns me is the High RPM. The cable only increases the RPM a couple of hundred RPM. BUT if your throttle linkage is screwed up this could present a problem.
Check all the ball and socket joints. They should be be attached but "loose". They stay on by a spring C clip. Place a flat blade screwdrive under one and "pop" it off. This will give you a feel for it. They should all have rotation movement and a small amount of play while attached.
Do all of them look straight, attached, free of movement and are all there??

Vacuum, Yes vacuum is the most important thing in a Mercedes Deisel.
Vacuum runs the brakes as you discovered. It also runs EGR, door locks, AC controls, transmission shift, when and how hard/soft, fuel shutoff.

Your car is a non turbo. It has no power, It is slow compared to a gas car. So it may take some getting used to. I will include a picture of my 240D engine setup. Your will be somewhat simular. Except yours is a 5 cylinder. The throttle linkage is close to yours. So if yours is not identical donn't be allarmed. It should follow the basic setup.
The non turbo uses vacuum to determine shifting and at what point. It goes from the vacuum pump thru the large black line that goes to the booster. Its tapped off the at line and routs thru the injection pump vacuum control vavle on the rear top of the IP, thru the white air switch on the valve cover ( it is under the black plastic cover) and to the transmission modulator valve.

Your indication of loosing the brakes is not good. Either the booster failed or the vacuum pump failed, or a vacuum line came loose while driving. Owning a mityvac, vacuum guage is almost a necesity with the Mercedes. Whith this you can measure the output of the vacuum pump, the condition of the booster, the condition of the door locks, Ac elements, etc.

Look at the picture If included and see if all your throttle linkages are there. Are all the vacuum lines in good order and producing a tight seal. Older cars, the rubber 2, 3 and 4 way connectors cam allow vacuum to leak. You must ensure your vacuum system is in good shape prior to solving and other vacuum problems.

Heres a link to the vacuum diagrams I posted. find the one that is closest to your car.

Pictures of Vacuum diagrams for W123 and W126 diesels

Dave

engatwork · #4 07-22-2004, 08:48 PM

You should be fine with this car if you are mechanically inclined on the gassers. Just take your time and read/learn all about it here and other places. The first thing I would suggest doing would be to clean the engine compartment.