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heater control problem, HELP PLEASE!!

M

mitsumotors

NAXJA Forum User
Location
Winnipeg
My heater control(that select vent, defrost, floor, etc.) does not stay were i put it when driving at highway speeds. it usually defaults to the defroster and if i let off the gas for a second it goes to what i have selected but when i get back on the gas it usually goes back to defrost. it seems that the vacuum is not holding the vacuum diaphram open to hold the heating passage door open. its kinda random though. sometimes it is not a problem at all. i'm assuming that the little vacuum diaphram thingy is weak or leaking or something. what do you guys think?
 
i don't think i have a vacuum leak though because after having my jeep parked for like 4 hours i unhooked the vac canister and it made a suction noise so it still had vac in it. if there was a leak it probably would have all leaked out right?
 
mitsumotors said:
My heater control(that select vent, defrost, floor, etc.) does not stay were i put it when driving at highway speeds. it usually defaults to the defroster and if i let off the gas for a second it goes to what i have selected but when i get back on the gas it usually goes back to defrost. it seems that the vacuum is not holding the vacuum diaphram open to hold the heating passage door open. its kinda random though. sometimes it is not a problem at all. i'm assuming that the little vacuum diaphram thingy is weak or leaking or something. what do you guys think?
Click to expand...


you have a vacuum leak in the vacuum system.

engine vacuum is used for many of the controls. since engine vacuum drops at highway cruising or under heavy load, the vacuum system uses a vacuum reserve. the vacuum reserve(looks like a small black bottle) is located in the front passenger side of the bumper. you need to make sure the reserve is not cracked and the lines leading to it are not leaking or disconnected. common failure of those lines are under the battery tray.
 
well, my vac reserve hold vac for a long time after i shut the vehicle off so it can't be that. but i just thought of something. when i bought the jeep i put an aftermarket stereo in and all the wires are mashed up in there with the heater control wires, maybe there is a vac line in there or something?
 
From the symptoms that you are describing, you definitely have a problem with vacuum. As much as a pain as it can be, you're pretty much just gonna have to check all the related lines and components for leaks. Look carefully though because the tiniest little nick in a line can cause problems.
 
also, could it be that there is no leak in vac but i just don't have enough vac at highway speeds because of the load. my jeep seems to have very little vac at highway speeds( like 5-7 on a vaac gauge). maybe this is the problem?
 
thats why they put the reserve in the system. Unless you have a huge cam in the motor or some other modification that would cause extremely low to no vacuum, your problem most likely lies in the heater control vacuum system. Either the diapragm has a very small leak or one of the lines do. If you have a vacuum guage you should be able to hook it up to the source vacuum line that goes through the firewall and see if it holds vacuum, then try the vacuum lines off the back of the selector. It should help to narrow it down.
 
the engine runs good. It idles at about 17 vac and seems to run pretty good, the only problem is that it runs at low vac on the highway, especially if i'm going into the wind, or if i try to accelerate a bit to pass someone the vac drops to like 0. just doesn't seem right to me. also, '91 4.0 aw4 242 4" lift, 35x12.5" tires and 4.56 gears.
 
it is normal for it to have low vacuum on highway and under load (not sure if 0 is right or not but I don't think its that far off). This is why they have to put vacuum reservoirs into vehicles. Either way though, you have to start somewhere and if I were you, I'd be checking for vacuum leaks everywhere. Even though it seems like it hold vacuum, there could be a crack or pinhole somewhere that only leaks intermittantly. If this is the case though, you're gonna be pulling your hair out. Not sure if its an option for you or not but if you were to take it to a shop that has a smoke machine for testing evap. system leaks, they should be able to check your vacuum system with it as well. Shouldn't cost too much if you get them to mark where the leaks are and fix it yourself.
 
can i just fill the sytem with smoke myself? there has got to be a way to do this? also if it is just a pin hole will the smoke ven be forced out of it with no pressure really forcing it through?
 
it seems to me that having a vac reserve doesn't make sense. if the system has alot of vacuum then the cannister has vac too but as soon as the system looses its vac(under heavy load) wouldn't the cannister loose its vac too?
 
mitsumotors said:
it seems to me that having a vac reserve doesn't make sense. if the system has alot of vacuum then the cannister has vac too but as soon as the system looses its vac(under heavy load) wouldn't the cannister loose its vac too?
Click to expand...

im assuming since its a reserve, it has a back flow or check valve inside. so when vacuum drops on the input/feed line, it has no affect on the outfeed/output. same goes for the brake booster(which has a check valve). do you loose brake pressure when vacuum drops, no.
 
89xj said:
im assuming since its a reserve, it has a back flow or check valve inside. so when vacuum drops on the input/feed line, it has no affect on the outfeed/output. same goes for the brake booster(which has a check valve). do you loose brake pressure when vacuum drops, no.
Click to expand...
this statement may solve all my problems if it is true!! you say there are two vac lines going into the vac cannister:doh:

mine only has one line going to the vac cannister. the previouse owner moved it from behind the bumper to under the hood and replaced it with cannister from an older f150. it has two nipples on the cannister but one is blocked off and the ther has a vac line.

where do both of these lines come from? i'll have to figure out which one of mine is missing.
 
mitsumotors said:
can i just fill the sytem with smoke myself? there has got to be a way to do this? also if it is just a pin hole will the smoke ven be forced out of it with no pressure really forcing it through?
Click to expand...

I always rely on the Stogie Test. Low tech, but works like a charm.

1. Get yourself a nice cigar (or whatever), fire it up, toke it up. Don't inhale.
2. Remove vac line from firewall.
3. Place lips around vac line and blow.
4. If you have a leak (pinhole or otherwise), that's where the smoke will go.

I'll betcha you have a vac leak in the lines running along the fender and under the battery. "The Stogie Knows".
 
k, today it took a look at it and there seems to be now leaks under the hood. the only thing that seems strange is that the vac cannister doesn't hold any vac once i turn the vehicle off. but tracing back the vac lines it seems that the cannister would just pressurize through the line that goes to the manifold as soon as the vehicle is off? there is a one way valve at the "T" that splits the line from the manifold and goes to the cannister and to the interior of the vehicle, the valve is on the line that goes into the vehicle. anyone know if it is supposed to be like this, the previous owner might have done it.
 
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