I recently purchased a 1984 300 Diesel Turbo and the cruise control, although functional, seems not to be able to hold the car at a steady speed. The car speeds up and slows down trying its best, I assume, to lock in the correct speed.

Has anyone had a similar problem? What should I look at repairing?

I have read here, that (believe it or not) incorrect wattage lightbulbs can cause erratic behaviour in cruise controls. Check your manual to see what belongs in there, and then check the bulbs.
Good Luck!
-Larry

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

JoeMB, check out this site it might be what you are looking for. http://www.fix.net/~gdl/testproc.html Good luck.

Does anyone have an idea where the "BRAIN" of the cruise control is? I was told it was under the "KICK PANEL" but I am not sure where that is either. I was told at a parts place that I should check this out first since my cruise doesn't engage.

-SB

Silverbullet, I take it that your 81' 240D is a W123 body style. The are three major componets which make up the brainy part of the system. 1. transducer located either on the speedo cable behind the instrument panel or it has one screw holding it on the back of the instrument panel just above the speedo cable attach point, mine is black plastic with a two wire electrical connector. 2. The amplifier is located on the interior firewall just forward of the foot brake. It is a aluimium box 1.25"X4"X6" at the bottom of the box is a 14 pin electrical connector. You will need a 10mm socket and a 1.5" extension on a 1/4 drive rachet to remove the one bolt. Its not hard to do, as long as your not 6'1" and weight 260 lbs (don't ask how I know this). Oh yes you will need to remove the lower dash cover to access this area. Take you time! 3. The actuator is located at the front of the engine on the drivers side. It has a electrical wire that goes to the dirvers side fender well, just forward of the tire hump. The actuator has a lever which goes aft to the throttle linkage hookup on top of the value cover. Please check out this site for testing information. It will save you alot of grief.
http://www.fix.net/~gdl/testproc.html

cruise control....any comment on my 87 300D? The cruise sometimes will engage but then disengages. At other times it will not work at all.

I've heard that about the lights a number of different places. Check the MB dealer and I think you will find almost all of the bulbs of different from each other. They have silver colored bases as compared to brass colored aftermarket. What this means I have no ideal. While your at that, clean the fuse contacts and change all the fuses.
I think the kick plate you're looking for is the panel above the driver's feet, but I've always thought the kick plate was the panel to the left of the driver's feet.
I think a lot of fixes on the cruise control can be accomplished by resoldering the amplifier. The best site on that is http://www.mindspring.com/~houlihan/merc/cruise.htm
I wish I took care of my records the way this person does.
Good Luck:

__________________
Dan Stadt
'85 300D turbo
'79 300SD
'73 220D

Just an update on my own troubles with the cruise:

1. I tested the actuator by hooking up the electrical connection to the battery (better if car is not on). It moved good and smooth.

2. I then started the car and hooked up a vacuum gauge I picked up from Kragen. It reads only in "in Hg" or "cm Hg" while the specs of the Service Manual are in units of "mbar". Well 1 mbar = .75006 mm Hg I found out later on the internet. The service manual says "at approx. 300mbar the vacuum control linkage should be at the full throttle position".

Well when my car was at idle, the vacuum to the actuator was closer to 50 cm Hg = 500 mm Hg or 666.6 mbar!!! I could understand the system not getting enough vacuum, but too much?? Perhaps I performed the test wrong... I will try it again.

3. At this point I was happy the actuator was working and decided to by the amp because the female end of the connection all showed voltage when moving lever on the steering column.

Unfortunately they did not have the exact same amp as my original... although it says it is made for my car. So since it is 10 pin and everything and the parts guy said he was confident it was for me... I bought it.

4. When I got the car on the road with the new amp... it accelerated... setted... and appeared to maintain its speed... all with the cruise control. But after a while it seemed like it was searching for the correct speed. Or as the troubleshooting pamphlet says "Vehicle surges or slows down and speeds up." They say this is usually do to a vacuum leak being applied to the actuator. I will try to check this out soon.



Moral of the story is that it is usually the amp that goes out. If it looks like the cruise is on its way out... it could be that the amp is on its way out... or maybe the vacuum problem I have.

QUESTION. Is it normal to see a little bit of oil in the vacuum hoses?

-SB

Here is some more info. I have.

I called a local MB dealership to ask about the oil in the vacuum lines... he said this is probably due to a bad shutoff valve that is located on the back of the injection pump (one vacuum line goes straight to it).

I also asked about the surge and he told me to check the actuator cable. You should provide 2 - 3 mm of play when in the idle position. I am not sure if I provided this... so I am going to loosen mine a bit and see what happens. I am also going to change the vacuum hoses and add the clear vaccum filters to track oil flow.

I'll let you guys know what happens...
Thanks for reading.

-SB

It is my understanding that oil in the lines is not normal and can come from either the IP shutoff or a small leak in the vacuum pump. There is a MB part, a transparent plastic filter, that you can put at the IP line and the service lines from the big main supply to the brake booster and they will show oil leaks before it gets in the system. I had the IP leak and the little can with the shut off level was full of oil and working (when it did work) on fluid rather than air. Oil in the vacuum lines will deteriorate the vacuum servos as I understand it. I've heard you are to clean the system when this happens, but I've never heard how. All my stuff is working now. I blew out the lines with compressed air. I'm waiting (and I will have this car long enough to find out) for any of the rubber parts to go.
Thanks for details on your cruise control fix. I will be going there soon.
I enjoy the 300D turbo that I drive over the road, but once you are cruising at 70 mph I sort of miss my daugter's '77 240D automatic. Cheap, smooth, simple. I think I wrote you before that the only problem is the hills. She is in the habit of racing down the hills to get up the other side. Sort of runs with the trucks.
Good luck and thanks again for sharing your MB experience.

__________________
Dan Stadt
'85 300D turbo
'79 300SD
'73 220D