88 4.0 Cruise Control Problems

I recently picked up a new-to-me 88 4.0 Cherokee. I've been trying to get the cruise control working. I found that the connector at the brake light switch was disconnected, so I plugged that back in. I also checked the fuse while I was down there and it is fine. The next thing I checked was vacuum, and I have none at the servo. Is there supposed to be vacuum there all the time, or is it controlled by a valve somewhere externally? I read in the service manual that there is a valve to break the vacuum to disengage cruise control, but I thought that was internal to the servo. My mutlifunction switch is a little screwy so it may be an electrical issue with it, but I want to check the easier things first. Where does that vac line run to?

i believe it runs to the vac canister behind the passenger side of the front bumper. its the black egg shaped thing.

The valve your referring to is mounted higher up on the brake pedal.

Thanks for the info, guys. I found a basic diagram for the cruise vac system and it showed that there are 2 valves to break the vacuum; an electronic one in the servo and the manual one on the brake pedal. I had a feeling that there was a vac canister in that area somewhere. The diagram showed the cruise connecting directly to a vac canister. I traced all the lines and found that a large line from the back of the intake runs in to that area along with the lines for the cruise servo and HVAC controls. I imagine the hoses that run to other things like the transfer case and front axle come from there too since that large hose is the only one coming from the intake I couldn't see end at some component.

It looks like I definitely should have vacuum at that line on the servo though, so I've likely found my problem. How would I go about getting in there to have a look?

Would you be willing to share that basic diagram with us. I'm chasing this problem as well on my 90. There is also a cable that runs from the servo on the firewall to the bottom side of the throttle control. The connector on that cable is messed up and keeps popping off.

When it's connected the cruise works, but when the cruise stops working I find that cable popped off again. The connector seems to be "part" of the cable and I have yet to investigate replacing it. does anyone know if that connector can be replaced or is it replaced as a unit from the servo.

Since I live in the hills I don't use the cruise much (or at all when it's not working) and so don't miss it that much, but when I go on a long trip I'd like to have it working.

ParadiseXJ said:

Would you be willing to share that basic diagram with us. I'm chasing this problem as well on my 90. There is also a cable that runs from the servo on the firewall to the bottom side of the throttle control. The connector on that cable is messed up and keeps popping off.

When it's connected the cruise works, but when the cruise stops working I find that cable popped off again. The connector seems to be "part" of the cable and I have yet to investigate replacing it. does anyone know if that connector can be replaced or is it replaced as a unit from the servo.

Since I live in the hills I don't use the cruise much (or at all when it's not working) and so don't miss it that much, but when I go on a long trip I'd like to have it working.

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That cable is what takes the signal from the cruise module and actually controls the throttle using the servo. I'm not sure if you can replace the connector separately. The vacuum diagram probably won't help you because you have a purely mechanical problem it seems, but here it is. I found it while browsing the web. I live in the hills too, but I occasionally take the Jeep to school and bought it to do just that when the roads are sloppy in the winter, and I really miss having that feature when my car stays home. Surprisingly I can get by without the heated leather seats, auto climate control, power windows, satellite radio and even keyless entry, but the cruise does me in. lol

Thanks for the diagram. That clears things up considerably.

I'll research the cable connection and maybe even have cruise operational by Sunday.

Thanks again

What does the servo look like and location? I have a 98 with the cruise not working.

Great diagram. I've got problems with my 88 cruise and don't even know where to start.

jeepster_259 said:

What does the servo look like and location? I have a 98 with the cruise not working.

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The servo looks kind of like a spray can on steroids, mounted to the firewall with a wiring harness, cable an vac line(s) attached.

I've got the FSM files for the 88 Cruise Control, if you want them send me a PM