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HOT or COLD only on my Honda
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monkeyman
New User
Mar 24, 2007, 9:58 AM
Post #1 of 3
(1710 views)
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HOT or COLD only on my Honda
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I have a 99 Honda Accord whose heating and A/C is acting a bit bipolar. It will either be HOT or Cold. Turning the system off doesnt stop this. I first noticed on a sweaty commute to work, despite the cold Wisconsin conditions outside. After a few days it was only putting out cold air. Finally I had the opportunity to check it out, thinking it was the heater water control valve. What I noticed was that with the car not running but the key in the on position I could change the position of the valve. When it was running I was stuck with blasting heat or frigid temps and could change from one to the other by shutting off the engine and making the change. My best guess is my thermostat is fine, the valve is fine, the coil is also ok, but maybe my controls aren't working. Any thoughts on this??? What do one of these panels cost?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Mar 26, 2007, 6:06 AM
Post #2 of 3
(1705 views)
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Re: HOT or COLD only on my Honda
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OK, I'VE PULLED ALL MY HAIR OUT ON THIS ONE   ,
Have thought about all the items in the car that are sensitive to whether the engine is running or not in the same key position.
The problem is almost certainly a vacuum and maybe electrical fault. I would hold off on the control panel as the problem right now as replacing that will be expensive and difficult and may not solve the problem. So far all the items show that they can work??
Electrically the car knows RPMs are missing but what items know that.... The car may try to adjust idle up and possibly fuel delivery changes and the RPM gauge would read zero. What esle is different: There is an increase in voltage from about 12.2 to 13V when when the engine is not running and those numbers would reflect how long it's been since engine was running. When running voltage will rise to about 14.5V if charging system is working properly. The MVAC controls chould operate at all those close voltages but something might be too sensitive??
An out there thought is that the key/ignition switch just returns by itself to run position when you have started thet engine and when just turning the key you haven't released it from start and it may not be the exact same spot and electrical flow thru it could be not getting any voltage or altered power to controls. I would think giggling key while running might show if there is a fault there. If that changes for fixes it the ign switch or sticky lock cylinder need checking out or replacing.
The vacuum side: The controls to many or all heater control valve(s) and diverter doors are actuated by vacuum diaphragms but told what to do thru electrical reuests. Vacuum is almost always from the intake manifold. There are cars that use a belt driven pump to make vacuum but that shouldn't make it act differently. There are vacuum reservoir bulbs of metal and plastic to allow for vacuum. Engine vaccum is highest at idle and when you let off the gas and reservoir will take advantage of that and save it for use for short time when vacuum is not replenished like when you have a load on engine -- accelleration, going up hills etc. It's very important as the controls would go nuts if supply wasn't steady. Power brakes are usually vacuum boosted with vacuum held in check for an assisted stop or two if vacuum was not replenished. Test that with engine off -- hit brakes a few times with engine off and they will feel norman for the first couple hits to service brake and it will firm up a lot when assist is depleted. The other way of brake assist would be hydraulic but still must hold a reserve of power for a couple brakings or they could sell that car in the USA or elsewhere no doubt. If hydraulic you will see hoses that will get the power from the PS pump.
Hydraulics are NOT used for MVAC, TMK.
The source of this problem is the control of that heater control valve, or the controls of it. If you can move it with engine running when the problem happens it may be sticky but I would think that would me more erratic.
Your description of the problem seems clear which helps a lot.
One last thing to chase down is the vacuum line to heater control valve -- does it go thru a vacuum tree? You may want to test the flow of vacuum thru that tree especially if a vacuum tree is sensing direct manifold vacuum.
We both live in temperate climates with heat and A/C used in the same day for a while Spring and Fall. There are places that don't do much of the switching around.
Rec'd your PM and will see if I can hook you up to some folks who may deal with this more often from hot climates.
Sorry for the disertation. Just my ramlbings of the possibilities. Please post the fix when you find it for others to search out and get the answer if they have the same problem, Good luck, T
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monkeyman
New User
Apr 4, 2007, 3:46 AM
Post #3 of 3
(1689 views)
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Re: HOT or COLD only on my Honda
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I havent had the opportunity to address this any further. Fortunately I can switch between hot and cold by pulling over and shutting off the motor. Ive been trying to observe the behavior further. All the controls seem to work fine except for the temp control which is all the way hot or all the way cold. What I need to do is shut the motor off, and turn the key to the on position. Then with the control panel on if I change from hot to cold or back the heater control valve responds. Then I can start the motor and will either heat or cool from there. When the engine is running the control valve does not respond at all. A/C will defrost the windshield so it works ok. I can change the vents just fine. When the system is off the residual unforced air will continue as hot or cold. Im still stumped... you?
Thanks again for your input. Its good to know there are resouces like this website for us doityourselfers.
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