Well, folks, nobody wanted to tackle my other post about the shifting on my 1981 300D, so I will try another question on another car. Quickly: what is the most common cause of failure for the cruise control system? The switch? or what?
Help will be appreciated!

My cruise control didn't work when I first bought this car (1984 300SD) so I replaced the amplifer unit with one from a 1984? 190E. The cruise worked fine for about 2 months. But I only tested it for about a minute at a time. I even added a third brake light, and the cruise control still worked. If you read postings on this website, you'll read that you need to have OEM lights bulbs in the brake lights to make your cruise work, but now I think that's a myth.

Then I did a real test on the cruise control. I took the car on 1 hour non-stop trip on the cruise control. The car's cruise worked great for 1 hour non-stop, but, it wouldn't engage on the way back.

I did an amp test on the actuator motor (pins 4 and 5). The reading was 271 mA which was over its tolerance of 130 - 180 mA. According to that famous cruise control website (that I cannot remember), this kind of amperage could burn out the amplifier. I took apart the actuator and cleaned the motor amature at the brushes. This reduced mA to 176 mA, now within tolerance. I suppose a better cleaning could reduce the mA, but it works beautifully in short tests. I need to try a 1 hour test now.

I think the actuator motor should have a relay to protect the amplifier from actuator motor overloads. It is possible that my original amplifier was fine. I think these electronics are reasonably realiable, but some people reported cracked solder joints in the amplifier.

Thanks, gentlemen, for your replies, I really do appreciate it.
Now, since the Haynes manual I bought for this car is less than specific on a lot of points, if you could please tell me where this thing is, so I could check it out....
Again, thanks! I really love these cars.(so far...)

This is the website with test information:

http://www.gdl-online.com/begin1.html#contents

The amplifier is located under the dash, left of the steering column. It is an aluminum rectangular box, about 6" X 4" X 1", with an orange plastic end. Hard to get out. I ended up prying the circuit board out.

The actuator is located on the engine's top, front, left side--next to the power steering pump. There is a linkage going from it to the other linkages.

I thank you for your prompt and courteous reply, and wish you a happy New Year. I'm sure I will have other questions, and this forum is a great way to learn about these cars...thanks to folks like yourself who are not afraid to share their knowledge.