I'm surprised I didn't find more posts about cruise control on a 300D. Mine works intermittently, less than 10% of the time. Any suggestions on how to diagnose and fix it?

I have never removed the instrument cluster and am a bit nervous doing so. I'm not a very experienced mechanic.

 

Your cruise amp is likely the cause, very very common problem with MB cruise control!

A new cruise amp will run $300 +, however, many elect to repair corroded solder leads as this is the more common cause for failure.

 

They had a method of wave soldering (during manufacturing of the board) for electronics in those days that lent itself to creating devices with poor solder joints after a while. It didn't only occur with mercedes but with the electonics of many car manufacturers. Anyway BenzBob is correct about this and the solution usually is to remove the cruise amplifier (under dash near parking brake) and disassemble the case and resolder all the connections on the board. This is very tedious work and if not done correctly will render the cruise amp useless. Should only be attempted with correct soldering equiptment and some experience with soldering. I fixed both the cruise and heater controls on my 83 CD Turbo with this method and it was very satisfying.

 

I have a 1987 300TD and have had the same problem with the cruise control. The manual I have says that the problem is in the 16 prong connector behind the instrument panel. This can be removed with a 3/32 dia. wire with a 90 degree bend on one end, about 1/8 long and a wood handle attached to the other end, make two, insert on each side of instrument panel and pull intil unit pops out. You will have to insert the two wires so that the 1/8 bend is longitudinal with the insert space and after passing thru. , turn the wire 90 degrees. I need to due this as soon as the weather breaks and time permit. The manual also states that after diconnecting the connector, clean up all 16 pins and reconnect, if this doesn't solve the problem, replace the connector. Hope this kelps you out, respectfully Steelerman.

 

 

An easier way to access the Cruise amp for the 87 124 is to remove the knee bolster below the steering wheel, it's right behind there!

 

lldanj-I had a 87 with this problem. I went to the dealer and bought an amplefier, installed it and it didn't work. I later found out that this was indeed the problem but some replacement parts have an extra pin hole so this has to be addressed. I forget if the male or female has the extra attaching point but there is a service flyer addressing this. Also, the dash module just sort of wiggles out. It has like a large thick rubber band around the panel and it just pushes in and pulls out. Be careful once it is loose as the connections are short and you could damage the connections.

 

This an old thread! Here it is 2012 and I am still driving my 25 year old 300D Turbo. Sadly electronics have been giving me trouble. Ignition switch wore out causing RPM, ABS, Lights on warning horn and climate control to fail. Replaced the switch and they worked again. However the cruise control still malfunctions intermittently. Replaced the crse cntrl actuator, which made no difference. I have tried to detect a failure pattern. So far I noticed that the cruise control is more prone to fail when the car is still cold and the eengine under a load. Waiting until it is up to speed and engaging the cruise while coasting and holding the switch on until it takes, seems to help.
Installed a replacement amplifier and it would not work at all. Put the old one back in and am back where I was before. Thought of of taking the replacement amplifier apart and doing what was suggested above. Also am replacing the crse cntrl switch, ordered one already - only $20. One thing I learned in the process was that 80% of electric malfunctions are caused by an insufficcient or faulty ground.

 

More about the cruise amplifier. The replacement I got came out of a 1989 300D. It is a basic amplifier for the 1982 to 1989 124 series and has a car specific module attached to it. The original amplifier in my 1987 300D Turbo does not have such a module on it. The cruise amp replacement I got does not have the correct car specific module and it does not work.

 

I thought I had fixed it, when I took the original amplifier appart and checked the pin connections to the motherboard and found everything looked normal. Matter of fact the soldering was exceptional too. I already had bought a cruise actuator. amplifier and control switch, but nothing had made a difference. Changing the amplifiers back and forth in the end had cleaned out the connecting plug suficiently to where cruise control is back and functions normally (for a while.)
(I now have all these spare parts, any one needs them?)
UPDATE:
It's now June 30, 2014, 265,000 miles on my '87 300D Turbo, still in use every day. The cruise control still is intermittent, BUT I am getting warm. The problem is to get the cruise control to engage. Once it has, it works normally, all day if need be, until the engine is shut off. So the problem is not in the amplifier or the control stalk. I noticed that when the weather is hot, the cruise control is more likely to engage. This pins it down to the starting circuit which has to meet specific requirements, like that the car has to be moving above a certain speed (30 mph?), that your foot is not on the brake etc. I suspect the brake pedal. It must have a wire connection to the cruise control outside of the car and it is exposed to the outside temperature variations.