After the cruise quiting a few years ago it's now back to working after changing the servo . Before I changed the servo when I engaged the cruise the truck would lose 1 or 2 mph. Now it drops about 5mph or more then slowly gets back to speed. The engine drops about 500/600 rpm as soon as I set the speed and slowly gets back up to the speed I set it at. Also before the servo change the truck wouldn't lose speed going up a hill no matter how steep. Now the truck loses speed till I top the hill. The hill is about 1/4 mile long and the speed drops from 60 to about 45 mph. I haven't gone up any long steep hills but at the rate it's losing speed I"m not sure how low it'll go. Any ideas? I saw there is an adjustment on the cable close to the throttle end but have no idea what that's for.

 

The adjustment is to take up the slack in the cable from the TB to the gas pedal.

IIRC, since you used the term "servo" and not "cruise control module," the servo should be vac operated and not electronic. If I'm right, what's the condition of the vac hoses related to the servo? If there's any question to their integrity, vac hose is cheap enough that you might as well replace it. Weak vacuum will result in poor servo performance.

 

The servo, or at least what I was told was the servo is the round thing mounted on the drivers side almost next to the brake master cylinder. I left the vacuum hoses on that were already on when I got it and the rest of the hoses are in good shape. My truck is a 1987 F250 Diesel N/A and I just replaced the vacuum pump less that a 1000 miles ago. The truck shifts fine so I know the vacuum is ok. If the vacuum was bad I"d have no brakes as I"ve found out in the past.

 

Not necessarily true. Vacuum can be weak and you'll still have brakes and other vac operated systems. If you have any leaks in the supply line or other lines related to the servo, it won't operate properly, but the other systems would.

It's not going to hurt anything to replace the vac hoses that came with the servo, so my advice is to go ahead and do it. At best you'll fix the problem; or at least you will know the vac hoses on the servo are in good condition. Do you have any vacuum reservoirs? They look like a coffee can. If so, what's the condition of those?

Where did you get the servo from? Is it new or from a junkyard? If it's from a junk yard, it's possible the replacement is bad.

I can't find my service manual right now and I don't have time to look for it, but when I get home tonight I will find it & type out the testing procedures for the servo. Unless somebody else chimes in & fixes the problem before then.

 

I have some vacuum hose so I"m going to replace the hose, just in case. I just got back from a test run and it seems to be running better as far as keeping the speed going up hill. On the end that attaches to the throttle cable I noticed a plastic part that was supposed to keep the cruise and the throtle cables close had a broken piece. The piece acted like it was supposed to keep the cables close, almost parellel where they connect to the throttle. Since it was broken off the only thing I saw to do was use a small zip tie to keep them close. I drove down the road aoward the biggest hill around and the truck kept the same speed within 1 or 2 mph.

 

sounds like ya got it.

 

Yes I did and am I glad. After being without cruise for awhile it sure is nice to set it and forget it.

 

That's great. And this thread has inspired me to get mine working again, too.

 

Make sure your truck has the fuse harness added to the cruise control. full time power to a defective switch on the master cylinder.This is the cause of many Ford trucks burning up while parked.Also know that using cruise control on wet or ice covered roads can put you in the ditch.My cruise control works but I dont use it.A dumb A feature.A chevy we has used to run away when the cruise control was on.Dealer wasnt certain they could fix it so I disconnected it.

 

The OP's 1987 truck uses a different cruise control setup than the later models that were part of the recall. There is no Brake Pressure Switch (fire prone) in this vintage truck.