Originally Posted by Bonesbrown
I see there are several heat issues out there. Just got my 98 gc on Sunday. It is the simple single system not the dual. I can only get heat from the defrost and it seems to fade in and out. Was messing with it this morning and giggling the key seems to kick it in but not always. It blows hot when it does, cant get it to come from bottom or vents.
Congratulations on your new ZJ!
The defrost only problem is symptomatic of a vacuum system leak, which sounds a whole lot worse than it really is.
Follow the rubber/plastic lines (black tubes) from your brake booster along the firewall. Somewhere between there and about 3/4 of the way over to the passenger side where they pass through the firewall there could be a leak or disconnected fitting.
If one of the lines itself is broken you can repair with a bit of shrink tubing or a piece of tubing that is slightly larger by sliding the broken ends into the new tubing.Even some tape will do in a pinch.
As for the fading in and out, you can check underneath the glove box, look for a white plastic wiring connector.
Should look like this-
Sometimes the black wire overheats and gets a bit loose which makes your high fan setting not work. You can just try to tighten it by squeezing or something or cut and splice the black wire. Jiggling it first with the blower on high will let you know.
Only high blower and no low setting means the blower resistor is fried, but that's not horrible to replace.
Heck, almost nothing on these things is hard to replace!
No heat can also be caused by bad water pump, stuck thermostat, clogged heater lines/core, or no coolant.