Hello Group,
The cruise control in my 79 300SD works very intermittantly. On another webpage I downloaded the instructions on how to test the various componants. I tested the ohms on the servo, it was fine. I can't find the speed sensor! According to my repair manual it should be right under the dash, connected in line of the speedo cable. I have followed the cable from the speedo head to where it passes thru the floor and it isn't there. This is a very early 79 (12/78). When it isn't working it won't do anything. Sometimes you hit the switch and off it goes, and when it works it works perfectly. Next trick is to re solder all the board connections in the control box. Anyone have a clue to what componant is most likely the culprit? Even when not working, when you hit the engage lever you hear a slight click under the dash..Big thing that is annoying me is I can't find the sensor! Thanks for your help! John Winn

There should be a sensor where the 2 speedo cables connect together. The problem we see the most is the control unit. You can get a remanufactured unit from us in partsshop.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Thanks, I sorta thought it might be that but as expensive as that part is I will be really upset if a new or rebuilt one doesn't fix it..That's why I want to try to rule out the other stuff. But I guess the fact that when it feels like working it works great, might be a good indication that the servo/switch/speed sensor are working. Is there any way to test the amp? Thanks a lot! John.

Hi John.

You probably have a type 2 system on your 79 SE (my 78 450SE is type 2). In type 2, the sensor is a black block, about 1" x 1.25" x 0.5" right on the back of the speedo. It has a pair of contact pins sticking out that are normally connected to a 2 pin socket. (If you're looking down near the floor, you're looking for a type 1 sensor.)

Anyway, the sensor is just a magnetic pickup coil, and there are no moving parts. There's not much to fail there, so I suspect your problems are likely to be with the amplifier. By the sounds of it, a dry joint or two. Another possibility, given the age of the car, is dirty connections on the CANCEL or STOP inputs (from the cancel switch and stop lights respectively). Try unplugging the amp and plugging it back in a couple of times, and make sure your stop lights are OK.

Tony