1972 Road Runner "Cruise Control" will not engauge above 65mph.

[race57]

Can someone give me some ideas as to what is the problem with my cruise control? It works good (for a 43 year old car) at any speed below 65. It generally has a 5mph variation. but it will not engauge above 65.
Is ther a place to buy replacement parts?

 

[mopar 3 B]

The point of engagement is adjustable. Can you set it below 65 and bump it over or does it disengage?

 

[73runner]

Check all vacuum connections and diaphragm. I think you have a leak. If it is engaging below 65 it means it is working and just may not be able to hold enough vacuum to pull the throttle back to get you over 65.

 

[race57]

I have tried to engauge it at 60 and bump it up and the best it will do is 65. I will check the vacuum lines today. Might have to look at the vacuum lines to the grabber hood also, it bleeds off when the engine is turned off. Thanks

 

[73runner]

Air grabber.... Cool.

My guess it's a vacuum line. But if that doesn't pan out check the diaphram, cable and linkage for leaks and any binding.

 

[race57]

Checked the vacuum lines, they look good very solid with no tight bends. Think it might be the diaphram... Where can I get one?

 

[73runner]

The unit should be connected to a takeoff fitting on the brake booster if you have power brakes. The rest of the vacuum system is fairly irrelevant to the cruise control operation. I am guessing that the unit in question was original to the car and is the standard mother Mopar unit used in the day.

Did you also check engine vacuum? Poor or insufficient vacuum at the manifold or where it connects into it could also be your problem. You need a steady vacuum signal of 16-18 inches for the unit to work as intended.


Ok.... Let's get to the diaphram.

You may have to remove the throttle control cable to check the diaphram to which it is connected behind the cone shaped cover. Any cracking or obvious leaks and you found your problem. They made that same unit until about 86 or 87 in the F cars (fifth avenues) so finding a good unit and diaphram will not be too big a deal hopefully.

By now your unit is probably over 40 years old so you are way beyond the reasonable service life of the unit. It's a simple device but by now it may just be on its last legs. The unit itself allows for vacuum control via a valve which could also be an issue. It receives a pulsing frequency from a pick up inside driven off the speedo cable which it is connected to. This, in turn, is then matched electronically by the unit and varies the opening and closing of a solenoid in the unit that then varies the vacuum level felt by the diaphram which then controls the throttle.

With me so far?

Often, if the vacuum lines and diaphram are good and you also have a good source vacuum from the engine, you may have a weak or bad control valve. I have had some luck by putting a VERY SMALL squirt of WD 40 to the vacuum line fitting on the unit and let it set overnight. This helps to re-seal and/or free up a the vacuum valve which may be leaking or sticking.

Likewise be sure the linkage and cable are operating freely and not hanging up or binding.

The problem you have should not be very hard to correct and is probably something stupidly simple..... way before its complicated.

As always, start with the FSM to get familiar with how the unit operates and so you don't break anything while fiddling with it. If after all of the above, the problem persists... the problem is in the electronic governor circuit which controls the valve... In which case, it's often just easier to swap it out

Hope this helps.