Reasons for car heater to not blow hot air?

September 24, 2009 12:02 PM

Need help from mechanics! Asking for a friend: On a 2005 Mercury Marquis, the air temperature control does not work. What could be the cause and possible ways to fix it?

The air conditioner works fine -- it blows cold air. But, when we adjust the temperature to heat, the temperature stays cold and the heater does not get warm.

This has nothing to do with being in a super-cold environment (today it's in the 80s Fahrenheit).

posted by Houstonian to Travel & Transportation (11 answers total)

Its probably the temperature control knob (or slider) itself. There is usually an electrical harness that goes from the knob or slider to a fuse box to control the heating and cooling. If you take off the plastic and get to the guts of the knob then you can address the problem. I've fixed the issue before by putting a paperclip in the harness to make the connection but I don't recommend doing that !
posted by premortem at 12:07 PM on September 24, 2009

Response by poster: Some more information I forgot to add...

We believe that the knob is not connected by a cable to the slide box (or whatever the box is called that provides the heat). We think this is done electrically. But, the heater coil is under the firewall, so we can't see it.

We checked fuses, but couldn't find any that were blown.
posted by Houstonian at 12:13 PM on September 24, 2009

It could be the thermostat too. It it is stuck the hot water won't be able to circulate the the heater coil.
posted by COD at 12:31 PM on September 24, 2009

If the fan runs, but there's no heat, it's probably the thermostat (there's one here). It should be inside a coolant hose at the front of the engine near the water pump. As the coolant heats up, the spring opens and allows the hot coolant into the heat exchanger, which is where the cabin heat comes from. If the spring doesn't open, you get no heat. It's a relatively cheap fix.
posted by kuujjuarapik at 12:31 PM on September 24, 2009

If the fan runs, but there's no heat, it's probably the thermostat (there's one here). It should be inside a coolant hose at the front of the engine near the water pump. As the coolant heats up, the spring opens and allows the hot coolant into the heat exchanger, which is where the cabin heat comes from.

Uh, no. The thermostat controls whether coolant circulates through the radiator, not the heater core. However, if the engine is overheating then the thermostat is a likely culprit...
posted by jon1270 at 12:53 PM on September 24, 2009

Also,

when we adjust the temperature to heat, the temperature stays cold

Are you saying that the AC keeps cooling the air even when the AC is supposed to have turned off, or are you saying the air is simply at the ambient temp instead of being heated or cooled?
posted by jon1270 at 1:12 PM on September 24, 2009

Response by poster: It is still cooling the air, even when we turn the temperature up. It's not going to the ambient temperature; it's cold air.
posted by Houstonian at 1:15 PM on September 24, 2009

Uh, no. The thermostat controls whether coolant circulates through the radiator, not the heater core.

In my experience with 2 GM cars and 1 Ford truck, the radiator and heater core were both downstream of the thermostat.
posted by kuujjuarapik at 3:09 PM on September 24, 2009

In my experience with 2 GM cars and 1 Ford truck, the radiator and heater core were both downstream of the thermostat.

I haven't seen such a configuration, but most of my experience has been with imports so I could be wrong. Even so, a thermostat stuck closed would cause the engine to overheat, and that doesn't seem to be an issue here.
posted by jon1270 at 4:11 PM on September 24, 2009

Response by poster: The engine is definitely not overheating.
posted by Houstonian at 4:51 PM on September 24, 2009

Unfortunately, this is not the sort of problem that's easily pinned down at a distance. It sounds as if the temp control knob isn't doing anything at all, but whether the failure is in the switch, the harness, or a relay or control board can't be known unless someone starts physically poking around and testing things.
posted by jon1270 at 4:12 AM on September 25, 2009