Cruise Control Question

[Kilian]

For the 83-87 cruise control systems (cruise master III?), what differences are there in control units? There are several different PNs listed in the 79-84 and 85-87 parts catalogs. Just wondering if you can swap between years. Thanks.

 

[SirRobyn0]

I don't know how to answer your question with specific years and changes but I can give you some general ideas of changes that occurred. If we go back to the 70's for a moment, or look at most of the trucks that were carbureted, there is a vacuum unit under the hood with the chain thing that jerks on the throttle lever. The speedometer cable is often routed up under the hood and connected to a box with a couple of wires on it, and then another cable leads off of that and runs to the speedometer in the cab. The controls changed in the cab when the wiper switch went from the dash to being on the turn signal stalk. When trucks got computerization the speed sensor went down on the tranny.

Now to your question about control units. I really have no idea, but if the electrical and vacuum connections are the same from one unit to another I think there is good likely hood that they will interchange. Changes might be a simple as internal changes for smoother operation or different minimum speed of engagement, but that doesn't mean that they won't interchange.

Hopefully someone will come along with greater knowledge than I, but that is my take and hopefully at least somewhat helpful.

 

[Big Ray]

Double post

 

[Big Ray]

my 79 has the speedo cable coming into a sensing unit, then another speedo cable going to the speedo inside.
The sensing unit (my term) works like a speedo in how it senses speed. When activated, it controls a calibrated leak in the vacuum system that controls the bellows.
Also, my 79 does not have a VSS.
Older system...note the 2 speedo cable ports.

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The newer ones have a vss (green box behind the instrument cluster).
Has more electronic parts...it has a module that goes with it.

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There's a couple of decent videos on YouTube for the older system. Amazing stuff, all mechanical.