Cruise Control amp's out; now what?
 

As I've mentioned before, my 300D has an intermittant problem with the cruise control.

When attempting to set the speed, it works sometimes, doesn't work sometimes, and, on occasion, appears to work on a delayed reaction. When it does work, it will often briefly cut out, making the car surge slightly as if there was an engine problem (the first time this happened, I was real freaked out!). Also, it will occasionally cut out on its own, although when it usually does this, I can hold the ACCEL and, after a delay, the car will begin accelerating. However, when I release the stalk, the pedal jumps to idle and the car starts slowing down again. In this situation, "RESUME" will occasionally bring the car back to the original speed for a brief period of time, but usually won't do anything. The cruise does not work more often than it does.

Assuming the amp had some bad solders, as seems to be very common around here, I just got done pulling the amp and have it sitting in pieces on my bed right now. However, every solder joint appears to be good. :( Nothing looks like it's not making a connection, and I double-checked; all 14 pins have continuity between the board and the pin itself.

What else can I look at? Ideas would be appreciated...thanks in advance!

cruise control
 

The Warden,

I'm having a little trouble with cruise control on my '83 also. Check out this site, it should help on the trouble shooting. Good luck.
http://www.gdl-online.com/begin.html

Marks88

'88 300e
'83 300cd

cruise control
 

GDL is a great site for troubleshooting tips, if you really think you need a new amplifier here's one on ebay that is a rebuilt unit and its at $31.00 now, the bidding ends 8/09 17:22 PDT


http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=1848570768

My cruise control does not work in my 1983 300SD.
I have a feeling that it is the amp, however the tach doesn’t work either. When I bought the car the previous owner told me that the cruise "should work" and that "all that needs to be done"
to make the cruise and tach work is to plug in a cable.
He then pointed to a gray cable that had been secured to the driver’s side fender well, near the IP. Does anyone know where this "speed sensor" cable plugs into? I really don’t mind not having the cruise control, but I would like to have the tach, is there any relation to these items?

I am not questioning your diagnostic procedures or results... but you might want to look at the Performance Products catalog where they describe symptoms... and suggest an order to trying/replacement of stuff...

They will let you order the actuator, and if that does not cure the problem, return it in good condition...

You could also check to see if Fast Lane will do the same thing..

rob_frick,

I also have a 1983 300SD with a cruise and tach problem. I would also like to know if their functions are intermingled in some way. I suppose it wouldn't be too far fetched to think that the cruise amp somehow relies on RPM info for proper function.

Anyway, your tachometer problem is quite likely a malfunctioning tachometer amp, assuming it's not the plug you mentioned. If you're not familiar with the tach amp, it is contained in a short black tube on the driver's-side front wheel well. The top simply screws off and the tach amp itself can be pulled out. It is simply a plug-in module. Some people have had success simply cleaning the contacts and plugging it back in. Another trick is to place a cigarette butt or something of similar size in the cap as you screw it back in. This, of course, snugs the plug in a little more securely when the lid is screwed down.

If I were you, I think I'd start by trying to find out where that plug goes that the guy pointed out to you. Then, if things still don't work, I'd clean tach amp contacts and possibly try the cigarette butt fix. If it then works, check the functionality of the cruise. Perhaps it will work. Again, I don't know if there is any relationship between them. Perhaps someone here will let us know. If you can't get the tach to work, you can find them from some of the Mercedes dismantlers for about $30 or so. I believe you can also buy them new from your dealer and even fromt the company that makes them for about $65 or so.

Good luck.

When my cruise became intermittant, the problem turned out to be the linkage. Specifically, the pivot shaft located on the bracket on the valve cover (the one that links the cruise actuator to the throttle linkage) was loose and wobbly. Fixed it with JB Weld.