Finished the install of my returned parts, racing against the ever encroaching darkness. (DST frustrates me to no end - I think they do it backwards) Initial testing put my cruise performance at about 95% in my opinion. (Understand that I am a perfectionist as you read) As far as general operation goes, it is at 100%: sets, accels, decels, turns off as it should. Sticks to the speed you set it at as well. Remaining problem is very slight surging at the set speed, 100-150rpm tops. I can barely feel it, but I feel it - less and less the higher the set speed. I double checked the all the linkage - still very tight from my adjusting for initial diagnosis. I called Peter at GDL to report the progress and get ideas for the next step. He believes, and I agree, that the actuator is probably not at fault, and the amp is certainly not. I believe the remaining 'fine tuning' lies in replacing the speed sensor, which I will do as soon as the dealership is open next. I am confident that the system will be 100% after this next operation.
Going through this cruise diagnostic experience, I have learned a few things that I want to share for the record.
1) The tightness of the linkage at all points is critical to proper cruise operation, as well as proper operation of the speedometer cable. If your speedo needle doesn't move smoothly you better start there. Also check the female connectors at the amplifier and actuator and recrimp them slightly (per GDL). I used a 1.4mm flathead screwdriver (smallest in the common set of minis) and lightly pushed it in between the 'tube' of the connector and the wall of the 'holder', resulting in a slight d-shape on the 'tube'. Be careful not to overdo it! It will take more pressure to plug it back in, but use steady pressure - don't beat on it - and make sure it goes all the way on. Incidentally, I found out from Peter at GDL that some 124 and 126 cars do not have a female #14 amplifier socket. It can cause the same slight surging I am experiencing. It is a ground and making it common to #12 can fix the problem. Call him for details if this applies to you.
2) If after the above, you still have the same symptoms you see on all these posts, do the GDL troubleshooting to the letter except for one thing: testing the speed sensor is difficult as it has to be done with the vehicle in motion with test leads held at two different positions around the driver's feet. Forget about it - just replace it. It's a $30 part, easy to replace, and pulling the instrument cluster out is a PITA - I am dreading doing it again. Go ahead and eliminate the speed sensor from the equation from the get go.
3) The GDL troubleshooting completed, you have now eliminated the speed sensor, switch, linkage, and connections from the problem.
4) If the problem still exists with all the aforementioned items OK, pull your amp and actuator and send them to GDL for testing. For every 1 successful resolder there are who knows how many unsuccessful. Not to mention, there are a limited number of these original boards and once you waste yours-it's gone (and so is your core)
5) Your amp has a 90% chance of being at fault now, but unless you send your actuator in as well, you risk blowing a replacement amp, as it is not possible to determine the exact state of the actuator unless you have the equipment (which costs far more than the testing)
6) Don't open the actuator yourself. There's really nothing you can determine from doing so and there is a tricky spring in there - waste of your time.
7) Pay the extra $60 for the internal inspection on your actuator. Just because it tests well doesn't mean it is A-OK inside, and to hell with going through all this to do it all again (if it fails it will more than likely blow your amp)
8) When your parts get back, you should be at 100%. If I had replaced my speed sensor I would be I'm sure.
Although many of my recommendations cost money (~$300), they are based on my own cost/benefit/time spent/frustration analysis. Yours may be different
I will repost once the new speed sensor is installed and give you another update in this thread. One thing I noticed about all of the cruise control posts is a total lack of followup as to how/if the problems were resolved. You'll hear from me in this same thread until mine is 100%. I sincerely appreciate your collective wealth of knowledge as well as the opportunity to contribute my own experiences. Have a Happy Thanksgiving and see you 'post speed sensor'...