Do any of you here know how to wire the cruise control pump to test it (see below)?

The cruise control in my truck works, but is very weak compared to what it once was and compared to the cruise control on my other car.

When I set the speed on the highway, the speedometer drifts down a few mph before slooooowly coming back up to the speed I set. It will then maintain speed on flat terrain, but the truck slows down going up even a moderate grade.

That indicated the electronics are working as they should, and that the problem is either a weak pump or a leaking actuator. (The hose connecting the pump to the actuator is new.)

I next did something I preach against – I threw parts at the problem. A part, actually. I bought a new actuator. Didn’t help.

I know, I know – I always say diagnose, diagnose, diagnose.

I didn’t find a way to test the system so I decided to try the new actuator. I’m usually pretty good at figuring out wiring using the RAVE Electrical Circuits and Electrical Library documents, but figuring out how to test the pump eluded me and I didn’t find any clear instructions online. I know that the white/blue wire supplies power to the pump, but it wasn’t clear to me how to connect the control valve and dump valve solenoids. Please post if you know how to wire the pump for testing.

I was leery of buying used parts. I figured that if the old parts were failing, replacement old parts might be failing too. I started with the new actuator partly because it costs a LOT less than a new pump, and because of its large rubber (silicone) surface area.

An ’04 showed up at the local LKQ a couple days ago. I’ll probably try to get the pump from it tomorrow. If that doesn’t work it will be easier to return it locally vs. buying one off eBay or elsewhere.

I’ll report back. Stay tuned.

 

Maybe try checking for leakage of the tubing. See if it holds a vacuum.

 

Thank you. As stated, the hose is new.

 

Oh man, you got me on this one. The symptoms as you’ve said seem like a leak. Did you make sure the actuator is holding vacuum? Is the seal to the hose to the pump good? Those darn things are expensive too. I guess my only thoughts are to check the other hoses in there because the electrical sounds good.

As I think about it, you might be correct in the pump failing. Can you do a pin out with a meter and rig a harness straight to the battery to test? The pin out at a minimum should be in the electrical library.

 

The brake switch is another potential leak point. There are three sections of vacuum tubing.

 

No it's not. The brake switch is electrical. The cruise control system is entirely electronically controlled by the ECU.
It could be possible for the control and dump valves to develop leaks though.

 

Not on D2. D1, you’re correct.

 

Whenever I've had a D2 CC seem to surge/wander it was due to slack in the CC cable. You can check the vacuum part actuator easily.

 

Success.

It appears a weak cruise control vacuum pump was the problem. I replaced the pump with one I pulled yesterday from an '04 DII at the local LKQ. I took the truck for a test drive this evening and the cruise control is back to working normally and is no longer weak.

I say it appears the problem was a weak pump, but I can't say that for certain because there are three hoses that branch off from the primary pump hose connection. One hose runs to the actuator, and is the one most often cited here as the problem. I had replaced that hose, but when I looked closely I found there are two other hoses that run from the same "manifold" to other ports on the pump. I could have tried replacing the smaller, L-shaped hose, but I couldn't figure out how to disconnect the other end of the third hose.

I therefore went ahead and replaced the pump, and happily the cruise control is now working as it should. And yes, the cruise control cable has been adjusted correctly for as long as I have owned the truck.