With throttle closed, there should be just around 1mm free play in the cable--ie, enough to insure the complete free throttle movement to closed.

Check the cable is not freyed in the outer cable covering all the way to the actuator. Its possible to use a push-bike brake-cable to replace the inner if needed.

The 'Amplifier' Controller--

There's two elect. capacitors in the controller that fail as well as 'dry-joints' of the soldering. As I recall, These caps are 47uF 25V. There's only those two electrolytics in the whole controller.

The double reed-relay also can electrically 'leak' causing drifting speed. (Usually downwards)

--The set speed relies on setting a voltage on a 'special' ultra low leakage 0.1uF cap which is attached to a MOSFET gate, as the stored/set reference--leaking voltage into or out of the cap via the reed-relays and resistors to earth and supply to set-speed reference, via the user-control on steering-column.

This cap seldom gives issues--Best never touch it--Or the MOSFET--its an old and obsolete type these days, and easily prone to static damage--You'll see its covered in some hard varnish stuff and attached above the board in 'free air' to reduce leakage paths.....

This voltage from the MOSFET is fed to a VCO and the O/P from this compared to the frequency generated by the taco-generator on the back of the speedo. An error voltage is derived which is amplified etc depending on speed error and used to alter the M/S ratio of an oscillator driving the solenoid controlling the vacuum in the actuator.

--Very clever and simple 'analogue' system--Who needs digital!....

Seems the electro-servo systems appeared in the USA around 1981 but here in the UK we had the vac servo type till end of production of W123!

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http://i190.photobucket.com/albums/z...0TDnoplate.jpg

Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....