jerkycruise control
 

I have an 87 300E. The cruise control is very jerky. It started out doing it just part of the time and now it does it about all the time. I replaced the modulator pt#003 545 09 32 and the overvoltage protection relay, but it still does it. Is there a way to check the modulator to see if it is faulty or something else to look for?

try this
 

Richard:

I am not into Mercs as much as some other cars, but had a 1986 190E 2.3 gas and just got rid of it @ 230K miles. This may not apply, so I apologize ahead of time.

I had a similar problem, and couldn't figure it out. I can't hear too well, but my wife was in the car with me and heard a hissing. I let it go for a while. It got a little louder to the point where I could hear it. The car idled a little rougher. Turned out I had a vacuum leak at the back of the brake booster (inside the car under the dash). I replaced the booster and the cruise control worked fine after that. I haven't figured the connecton between the two, but there might be one.

Metrikos

First thing to check for a "jerky" cruise control is the speedometer cable. Does your speedometer needle vibrate or fluctuate? If it does, this is likely the cause of the jerky cruise control. The speedometer cable is a relatively easy replacement.

Just make sure you buy an OE quality one, for our 86 190e I replaced the speedo cable and got a no-name brand one and it had cheap connections and shotty quality.

Check the accelerator and acc. servo linkages for binding; lightly clean/oil them with (weird, but true) transmission fluid.