Heater Problems in a 1999 Jeep Grand Cherokee WJ Ltd 4.0L.

Is it switched to AUTO climate control mode or manual. If you have your inside temp switch set to 70 and in auto mode it will switch between heater and AC according to the inside temp sensor switch (look like little red lenses on dash or head liner). You can over ride that by taking it out of auto temp mode.

I had a similar problem with my '04. The vents that control where the air comes out of are vacuum actuated(operated). On mine it ended up being a vacuum hose behind the head light assembly. I couldn't say where yours is located but I'm sure that's what it is.

I'd 2nd that... I've seen similar problems with early 90s Mercedes that still used all vacuum hoses for their vent system. Usually the central vacuum regulator would fail and vent would not operate properly. (plastic) Vacuum hoses would also control the locking system. Common was either rear doors would lock slowly or trunk would not release properly when they failed or leaked.

I have found out that the air-con was stuck on so I removed the fuse for the air-con and now I have instant heat, so double check the fuses before doing anything else.

You could replace the fuse with a fuseable link or wire with a toggle switch to where the fuse goes in. That way you can switch it on or off on the fly. I had a similar problem with a 88 Cherokee. The temp switch that was suppose to trigger the aux fan to come on was in a position after the coolant cooled and the fan would come on at higher temps (fixed in later models). I ran a toggle to the relay box so I can turn the fan on when I wanted. It helped in bumper to bumper traffic during the summer heat with AC on. The Jeep never overheated again. ☺

Thanks Matt, I will try it.