I have seen several similar posts on here over the years, but I have yet to see one with the answer.
The posts generally start out like this:
"My cruise works ok, but it doesn't want to go up hill, and in addition, my engine seems more slugghish then it should."
I have also read other posts that say "why does my engine seem more powerful in 4WD then in 2WD?" and then later on generlly found another post by the same user saying "Why doens't my cruise work?"
If any or part of this sounds like your truck, then you may be as surprized as I was to find out what the problem was (at least in my case).
Most of the GW's I have seen have had the NP-229 (or 228) transfer case in them. As most of us are aware, these cases are vaccuum actuated.
However, most of us (inculding me until now) are NOT aware that the vaccuum for that switch comes from the exact same line as the cruise control!! The distrubutor vaccuum advance also tees off this line as well.
The idea is, you have a 3/8" vacuum line comeing off of the engine. The first T breaks off a line to the distributor. The idea here is that the vaccuum will be strong at idle, and all but gone when you floor it.
So, you ask, how is the cruise supposed to drive you up a hill? Well, after the first T, the vaccuum is routed into a black plastic ball (vaccuum chamber). There is (or should be) a check valve (one way valve) on the line between the first T and the black ball. This means that the vaccuum in the ball does NOT go away when you floor it. From the black ball, the vaccuum line goes into another T. One side goes to the 2WD/4WD switch. The other goes to a metal can (another vaccuum chamber) and then to the cruise. The idea is that there should be enough vaccuum between the black ball and the metal can for the cruise servo to easily pull the throttle wide open. Then, when the throttle is released, the vacuum chambers re(dis)charge.
What about the 2/4WD system? It requires a significant flow of vacuum to switch -- AMC figured anyone who wanted to switch to 4wd with the gas on the floor was retarded anyway.
So, there are two possibilites here. If you are just having problems with cruise, your check valve is either installed backwards (no cruise, and possibly no 2/4WD switching either) or is shot (bleeds down when you step on the gas = weak cruise, but 4wd works decently).
If you have noticed your engine runs better in 4wd, or seems like it, you are not going crazy. Most likely your cruise is weak -- if it even works -- and it takes your case longer than it should to return to 2WD (or you are stuck in 4WD and you don't know why), the vacuum cylinder on the case that is used to shift between 2 and 4WD is partially shot. These cylinders have 2(TWO) diaphrams. One can be broken while the other still works.
That second possibility was my problem. The diaphram on the 2WD side was shot. Would not hold vacuum to save it life! The 4WD side was good, though.
To check, just unplug the connector by the firewall (three tube connector with only 2 tubes. You want the one that is connected to the metal lines going down by the tranny, NOT the one going under the dash. Connect a vaccum source / gauge (hand pump ones are great) And see if it will hold at least 15 inches of vacuum for at least 2 min. Then try the other one. If either fails, you have a failed vacuum motor. BJ's offroad sells new ones.
To those with NP229(8) 1985 model year and before. I wrote this based on my 1986 which does NOT have the vacuum axle. You need to test your vacuum axle seperatly from the vacuum motor on the t-case since it could also be causing the problem.
Anyway,, I just thought I'd put this out there since I've been banging my head on this issue for years now, and I just recently found the problem while the tcase is out waiting for me to finish the transmission rebuild.
Hope this helps