My cruise control has started surging when engaged. It's not bad, it just feels like it engages and disengages the throttle too hard so that on flat ground it doesn't run smoothly, it constantly accelerates and decelerates pretty hard every 5 seconds or so. I could not find anything in a search. Does anyone have an idea what is happening here?

86 F250 4x4, 6.9 w/early Banks turbo. About 96K miles.

Thanks.

 

My cruise control has been “hunting” off and on for years and if I wiggle the electrical plug that goes into the cruise control unit it will work fine for a while. I’ve even pulled the plug apart and cleaned it real good but after a while it’ll start acting up again. Since it’s a minor annoyance, I just need to remember to wiggle the plug before any long trips, I haven’t gone into it any further.

 

I would check the linkage or cable the cruise uses. If it or the carb linkage is sticky, it will pull on it till it finally gives, and then that is too much throttle and it lets loose, and when it finally drops back that is too low so it starts all over and tries to pull it open, it finally gives, it's too fast, etc, etc. A little wd40 may do wonders.

 

Check your vacuum lines and brake booster. I had the same issue on my 1984 6.9 F250 4x4. It totally disappeard after I had to replace the master cylinder and brake booster due to leakage at the MC. The vacuum fitting and rubber seal on the new brake booster was a way tighter fit than the old one, so I suspect that was the reason for losing slight vacuum and the CC not working properly.

 

I had an 86 K5 with a 6.2 diesel and also installed a Banks turbo system.
Mine also hunted and if you watched the boost gauge you would see when the CC gave more throttle the boost went up, the truck would then be going to fast and cut back on CC to the point it was to slow and it would start all over again.

Where I lived at the time you really could not use CC so never looked deeper into it but it was the turbo boost that would make it hunt.
I think in my case if I could have turned down how fast the CC reacted it would have been good.

BTW the K5 had hydro boost power brakes so that was not a cause.

If what was posted by others does not pan out try calling Banks, I never did, to see what they have to say.
I also don't remember anything in the install book that came with the kit about CC adjustments.
Dave - - - -

 

Little off topic, but can someone give a very brief explanation on how the cruise control works on these carb trucks and what controls it? I've always wondered that, mine works great but the system is a mystery to me.

 

The system is quite simple really. It senses the frequency of the pulses from the hall effect speed sensor on the speedometer cable and adjusts the throttle to maintain that frequency. When the frequency drops it opens the throttle when it increases it closes the throttle. That is all it does in a nutshell.

 

RE: wiggling the electrical connector: I will give that a try. Thanks.

 

I will check the throttle linkages for sticking.

 

RE: Turbo as the cause. This seems plausible, except I have had this truck for 10 years and it just started doing this within the last 12 months. Thanks.

 

Has the truck had the turbo on it the whole time?
On mine I did not use CC that much but before the turbo was installed, by me, it worked fine it was after the turbo was added the CC hunted.
Dave ----

 

A few suggestions, in no particular order:

As.others have mentioned, the vacuum supply could be the culprit. Does your diesel use a mechanical vacuum pump? If so, the output may be low or erratic. I'd tee in a test gauge you can run into the cab. Go for a test drive and make sure the vacuum supply is within specs and steady.

Clean and lube your speedometer cables. If binding a little, it might be just enough to throw off the speed control sensor, but not quite enough to make the needle bounce around. I say cables (plural) because the speed sensor is in the middle of the run, between the upper and lower cables. The location can be seen in figure 138 at the link below.

Wiring diagrams and troubleshooting tips here:

Speed Control - Gary's Garagemahal (the Bullnose bible)


Pay particular attention to the three slip rings in the steering column. The control switches communicate with the amplifier by inserting certain resistance values into a single wire control circuit. If the slip rings get dirty, the wrong signal can be received. You can check the resistance from one of the wires at the amplifier under the dash, just to the left of the ashtray. Details at the link above. Also note the horn uses two of the same brushes. If your horn is also erratic, that's the first place to look.

 

By this era the Vehicle speed sensor was on the trans or transfer case in most applications.

 

I don't know if the mid-cable mounted VSS is available anymore, but the trans mounted one is (aftermarket) as well as the pigtail and cable for it. I purchased one myself but have not installed it yet. I also purchased a spare 1989 M5OD and it has a trans mounted VSS installed.

I plan to install an aftermarket Rostra CC before next summer. (I've used these before with great success, and there's no pesky vacuum issues with them).

By the way...13 posts in this thread, and nobody has been correcting you guys on the proper Ford terminology (Speed Control)...LOL. The crowd here is slipping. J/K.

 

Ya it has gone by many names. Cruise-Command, Speed Control, Auto-pilot ,Speedostat, Cruise-Master Speed Control, etc, etc. But colloquially known as cruise control as that is what is what called in many of the original patents. Ford did patent the mounting the switches on the steering wheel and never let other manufacturers in on it till it's expiration. The other one they never let other manufacturers in on was their Keyless Entry System (now called SecuriCode) it is far and away one of the best options ever I wish the Mercedes had it to be honest.