kb3uet Posted January 19, 2019 Share Posted January 19, 2019 Hello All! Wanted to say hi first, I am new to the forum. I have a 1990 MJ 2wd 5spd with a 4.0. My issue I am having is that when the heat mode is selected, the air blows cold. When you select Bi-Level or Defrost, the air is hot as all get out. When BiLev is selected, the dash vents blow hot, but the floor vents still blow cold. Even the vents on the lower parts of the dash are hot, but the black box with holes on the floor are cold. Any help is greatly appreciated. Thanks. Link to comment Share on other sites More sharing options...
Pete M Posted January 19, 2019 Share Posted January 19, 2019 ca you get the upper vents to blow cold? or are they always hot? Link to comment Share on other sites More sharing options...
kb3uet Posted January 19, 2019 Author Share Posted January 19, 2019 The upper vents will change to cold. I forgot to mention that the heater vents on the floor do have some "hot" air and I use hot lightly. When I move the temp control to cold you can feel a slight difference in the floor vent temperature. Link to comment Share on other sites More sharing options...
87MJTIM Posted January 19, 2019 Share Posted January 19, 2019 kb, I am in New Market, MD. Good to see another MJ owner near me in MD. Link to comment Share on other sites More sharing options...
kb3uet Posted January 19, 2019 Author Share Posted January 19, 2019 That's cool you don't see many of these. I actually just got this one out of a local junkyard back in early dec. Link to comment Share on other sites More sharing options...
87MJTIM Posted January 19, 2019 Share Posted January 19, 2019 My wife has an aunt in Smithsburg. I have client just east of there also. You should check the vacuum lines going into/coming out of the HVAC control box. The vent doors are operated by vacuum, so a leak in any of those will affect where the air blows. Also, try back flushing the heater core. If it hasn't already been done, you should remove the heater control valve. Link to comment Share on other sites More sharing options...
kb3uet Posted January 19, 2019 Author Share Posted January 19, 2019 I've checked the vaccum lines I can see and everything looks good. I have already done the heater core but have not removed the valve under the hood. That seems like they are an issue. I'm new to these older jeeps my 2000 xj is a little easier, I'm glad they changed them. Link to comment Share on other sites More sharing options...
Eagle Posted January 19, 2019 Share Posted January 19, 2019 42 minutes ago, kb3uet said: I've checked the vaccum lines I can see and everything looks good. I have already done the heater core but have not removed the valve under the hood. That seems like they are an issue. I'm new to these older jeeps my 2000 xj is a little easier, I'm glad they changed them. Have you checked where the vacuum line runs beneath the battery tray in the engine compartment? That's where it connects to the vacuum reservoir (a.k.a. the "blimp" or the "football") behind the front bumper. The line often deteriorates beneath the battery tray. Link to comment Share on other sites More sharing options...
kb3uet Posted January 19, 2019 Author Share Posted January 19, 2019 From my understanding, and I could be wrong that's why i'm here, if one those lines are broken I should not have control of where to put the air. If the line going to the canister is broken then the air should blow out the defrost not matter where I set the controls. Link to comment Share on other sites More sharing options...
Pete M Posted January 20, 2019 Share Posted January 20, 2019 sorta. there are various levels of vacuum leak and different amounts of vacuum needed to operate different things. a complete failure will do as you mention. a slight leak may do some things but struggle on others. but your issue isn't like any I remember hearing about. personally, I'm just guessing at this point. as mentioned above, it might be that your heater core is partially clogged and so the air going through the top gets hot, but the bottom stays cold. Link to comment Share on other sites More sharing options...
Minuit Posted January 20, 2019 Share Posted January 20, 2019 26 minutes ago, Pete M said: but your issue isn't like any I remember hearing about. personally, I'm just guessing at this point. as mentioned above, it might be that your heater core is partially clogged and so the air going through the top gets hot, but the bottom stays cold. This is the only thing I can think of, or maybe the blend door/HVAC ducting is broken in a very unusual way. There's no other way for hot air to come out of one set of vents and cold out the other. The air passes through the A/C evaporator (if equipped), then the blend door, then the heater core and THEN is distributed to whatever vents are open. I think OP's heater core isn't up to snuff (or has air in it), or there is damage to the HVAC box. Link to comment Share on other sites More sharing options...
kb3uet Posted January 20, 2019 Author Share Posted January 20, 2019 Sounds like everyone has the same thoughts that I have been having, why one hot and the other cold. I'm thinking I'm either going to tape the vents shut on the floor, for temporary, and one day get motivated and pull the dash and look into the heater box. With no A/C the box shouldn't be that bad to deal with. Link to comment Share on other sites More sharing options...
cruiser54 Posted January 20, 2019 Share Posted January 20, 2019 Heat is controlled by a lever. Look up under the passenger side dash and slide the control lever. Link to comment Share on other sites More sharing options...
kb3uet Posted January 20, 2019 Author Share Posted January 20, 2019 Yes, I have checked the cable operated lever. The lever moves and the temperature changes from hot to cold and cold to hot no problems. It's just weird that all the vents will blow hot air except the vents on the floor. The ones supposed to be used for "HEAT" lol, in BI-LEVEL the dash vents are hot as hell but the floor vents are cold, if you switch from BI-LEVEL to HEAT the floor vents stay the same temperature. This is why me and my friends are baffled. Link to comment Share on other sites More sharing options...
gogmorgo Posted January 21, 2019 Share Posted January 21, 2019 Since its been brought up, I did have my parts XJ do the same thing when I drove it out here last winter (at -40 no less). I had marginal heat (well, warmer than ambient) out of the defrosters, and some degree of bent control, but what little warmth there was was gone by the time it hit the floor vents. While changing out the alternator I discovered that vacuum line at the battery was broken off, and a screw had been shoved into the engine side, only partially plugging the line. I found the other end and joined the two with a chunk of heat shrink tube. I can't say I had great heat after that, but it was pretty good after sitting in the thing for a couple days with not much heat to speak of. The vacuum controls not only the vent selector, but also operates and that heater valve. Link to comment Share on other sites More sharing options...
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