Vehicle is a 1989 300E with M103 engine. It has 195,000 km (say 120,000 miles) and is regularly and gently used (about 5,000 km/yr or 3,000 mi/yr)

The cruise control has operated flawlessly since I've owned it over the past 5 years. On a recent drive the cruise control cut out after about 20-30 seconds of engagement. Clicked "Resume" and the speed was restored to the set speed but then kicked out again after 20-30 seconds. There was no previous pattern of instability with the cruise control - it had always worked flawlessly.

I read through the various threads of 124/300E cruise control problems and all indicated that the most likely problem is with amplifier. I accessed the amp (part number 005 545 05 32) but could not get any readings with my tester (I must be doing something wrong?) although the reference resistor met the specs listed in the MB 30-506 procedure.

I lightly sanded/cleaned the connections at the amp and put it all back together. I sprayed some WD-40 on the throttle linkage even though they appeared to be moving freely and without any slop at the linkage connections.

I was not able to do anything with the actuator as that part is well hidden under the intake manifold. I was not able to check the speed sensor.

A more accurate road test indicated the following:

• still kicks off after 20 - 40 seconds
• using "Accelerate" works but delays in stopping and therefore oversets by about 2 km/hr (say 2 mph)
• using "Decelerate" works but delays in stopping and therefore undersets by about 2 km/hr (say 2 mph)
• testing speeds were 50 to 90 km/hr (30 - 55 mph) and it appeared to be more reliable at higher speeds 110 km/hr (70 mph) but did not have enough time and space to evaluate properly and safely
• "Resume " feature worked perfectly
• there was no surging or hunting for a certain speed (ie the short time it is working it seemed to work just fine)
• both brake lights are working

I'm not that familiar with auto electrics so I think I need help with using the multi-meter. What was I doing wrong in testing the amplifier? The multi-meter worked when testing the reference resistor but I couldn't get anything at the amp.

Based on the above is this the amplifier that is causing the problems or should I continue to try to test the other components? Or, considering the vehicle mileage and age, should I simply get a re-built actuator and amplifier (GDL-online)?

Thanks in advance,
Paul

Have you tried following the GDL troubleshooting procedures:

http://www.gdl-online.com/testproc.html

You haven´t changed any of the tyres?

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First) Yes, I did go through the GDL website. It is extremely informative but I wasn't able to get any response while checking the CC amplifier therefore I assumed I was doing something wrong. I have emailed GCL for assistance.

Second) No tires have been changed recently.

Thanks,
Paul

If it makes you feel any better, I am 90% sure it's the amp. I'm all for testing and making sure you have the problem nailed, but comapre it to a bad headlight bulb I guess, you gonna take the time to make sure everything else is working, or just bite the bullet and get a bulb?? Wish I could say "100%", but it COULD be something else. Sounds like you have all the bases covered, the actuator is CAPABLE of working, you tested the brake switch (make sure it isn't set too sensitively and is making contact over bumps or something weird). Way out in left field is a possiblity of a bad wire in the harness I suppose, and the switch seems to be responding OK, so low probablity there. I say 90% chance it's the amp as 90% of the time, by my experience, it's the amp. If you already have the newer style with the ref resistor, then make sure they don't try selling you another one of those.
Gilly

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I called Peter at GDL and found him to be very informative. After reviewing the symptoms he also felt that the problem was most likely the amp and added that it was probably not worthwhile to do anything with the actuator at this time.

Although not likely, I'll try the to check the brake light circuit to ensure that this is not the culprit. If this proves OK then I'll probably have GDL rebuild the CC amp.

Thanks all,
Paul