85 300TD cruise control inop

The cruise control on my 85 300TD used to work intemittently, but has not worked at all for a few months. What are the common problems which cause it not to work?

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Diesel Engine Specific Aid

Chassis Model Number

W123

cruise control

The cruise control on my 85 300TD used to work intemittently, but has not worked at all for a few months. What are the common problems which cause it not to work?

More detail...would like to get into electrical stuff on these

My background is in electronics/computers and complex systems. I'd like to figure out/reverse engineer the electrical systems on these cars.

I would need to know the symptoms of when it worked. For example, did it initially hold the speed and gradually slow down? Or is it just plain intermittent?

In these cars there are two analog "computers". One is the cruise control (old and new) and one is the climate control (the old one and the new one are both analog). One could extend that to say that the glow plug relay is a "computer" as well, with both old and new revisions.

I don't mean to sound didactic...but the function of a computer is to take a set of inputs and create a set of outputs. In the case of the cruise control, the input is a voltage reading generated by the speedometer and an input that says the cruise control is on (and a reference to the speed you want) and the brake pedal is not pressed. The output is a voltage/vacuum level to feed the servo that controls the throttle position.

My understanding of the system is that when you press the "on/set" lever, your speed is stored in a set of capacitors...kinda like batteries (only they have no capacity to store on their own) to hold an electrical charge. The system then switches to a "reference capacitor" which gives the current speed. If the two do not match, an output is generated (increasing or decreasing the throttle) to make them match. The same is true of the climate control system and the glow plug relay.

After 25+ years, capacitors break down. They either short or go open after a period of gradual decay for ones made in the 60s-90s. (After that time, they fail much faster because of bad formulas used to cheapen their manufacture. If you had a computer by HP, Dell, Apple or any number of manufacturers from 2001-2006, the capacitors actually exploded leading to numerous recalls.)

In any case, information is useful...any that I can get will be helpful....certainly something I shall share with everyone.

1985 300D

Where are these capacitors, and can I order new ones? Your post addresses two problems with my car that I can't seem to isolate, especially since the other vacuum-driven systems like brakes/doors locks work as advertised. But my cruise control soetimes works great, other times not so well. Seems temperature-dependent. And the climate control seems to have a mind of its own and only sometimes works according to what I have set on the panel.