Strange cruise control issue
 

The cruise on my 81 300D just started to behave a bit weird recently.

When I first noticed the issue (at highway speeds), it would surge, accelerate and drop out, repeating. Sometimes after setting it, it will just continue to accelerate. With that I assumed, well a bad amplifier.

But then I noticed that when I set it at speed ~50mph or below it works just fine. I suppose it still could be the amplifier, but I wonder now. Anone ever seen anything similar?

The cruise control going out on my car happened weeks before my whole vacuum system went out.

Whether it was related, I can't tell you.

I don't notice any other vacuum issues.

I guess that there are only two main components, in the cruise control system, the actuator and the amplifier (other than the control switch).

I have the older vacuum controlled actuator.

Contact JamesDean on this forum

you might just need to adjust the cable from the actuator to the linkage. shorten it!

The 1981 300D uses a vacuum type cruise control system. I've only ever worked on the electro-servo type that followed it.

Here's some ideas though:
1. check your vacuum actuator. make sure it is not leaking and is getting appropriate vacuum.

2. if that all checks out I would suspect your amplifier. It most likely needs some new capacitors and might even have some cold solder joints.

-Kris

Vacuum seems OK. That is, everything else working under vacuum seems to be functioning fine.

The odd thing is that the cruise worked OK this morning.

You make a good point about solder joints in the AMP.

Looks like there are two versions of the AMP from what i can tell one for the vacuum actuator (10 pin?), and one for the electrical actuator (14 pin?).

Is the AMP located under the driveers knee panel?

Thanks for the feedback.

Yes i do believe the amp is located under the drivers knee panel. I believe its left of the brake pedal, near the parking brake.

You are correct. The 10 pin = vacuum. The 14 pin = servo electrical actuator.

I do notice that if I move the actuator cable, there is perhaps a half to 3/4 inch of free movement in the cable.

Should the cable be without any play? If so how do you adjust the play out? Thanks.

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!

So the cruise actuator cable is adjusted with the throttle closed?

Yes, Throttle fully closed, including the idle-riser knob on the dash....

Maybe worth checking all your throttle linkages and lube them up first before re-setting the actuator cable.