My cruise control will come on.......but it seems to take me on a roller coaster ride. It will speed up, then slow down....all by itself. Any thing to look for/check?
Thanks..

 

Transducer is probably bad but you can look for a vacuum leak,binding speedo cable,bad actuator or bad adjustment of the act. to carb.

 

I don't know exactly what type of cruise control system is in your 1978 but back in the early 1970s the cruise control transducer (the box under the hood where the speedo cables meet) had an orifice that could be "tuned." The orifice is located on the transducer and is the fitting that connects the vacuum line to the vacuum pot that pulls on the throttle cable. The orifice was primarily for adjusting the engagement-cruising speed zeroing. But it could have something to do with erratic speed control.

You loosen the locknut. Screwing the fitting outward caused the engagment speed to increase. Screwing it in caused the engagment speed to be reduced. There was a caution not to completely unscrew the fitting.

The older systems had two switches that pressed against the brake pedal (with the brake pedal at rest). As I remember one was electrical and was a combination brake and cruise control switch. The other switch was purely vacuum. Make sure that you don't have a vacuum leak and that the vacuum switch properly closed and seals with the brake pedal at rest.

Some of the other possible erratic speed control causes:
1). Check for pinched, kinked, or damaged vacuum hoses and fittings.
2). Check speedo cable routings. Check the ends of the speedo cables for distorted or bent tips.
3). Check for binding carburator or throttle linkage.

Since cruise control became available in 1977, that year service manual may have had a more complete description and troubleshooting section.

Jim

 

If you need a transducer, it's not hard to change out. It helps if you have small hands, though.

The '78 I owned had the same problem...replacing the transducer fixed it.

 

It is most likely dirt in the transducer. I have troubleshooted a couple of '77s and they are the same setup with the exception of the servo location. You can take the transducer apart, blow it out and lubricate it. Just make sure when you put it back together that you have the two sides clocked correctly. As Jim pointed out, there is an orifice adjustment where the larger vacuum hose connects. It is adjustable in and out to correct the set speed. I advise you not to remove the orifice from the transducer as it is easily damaged.

 

Thanks for all the info......I will look tonight.