Diagnosis And Repair Of Cabin Heating Issues?

Hi Everyone,

I'm a new member, but a long time Mustang owner. I have a 1990 LX 5.0 Convertible that I've owned since new. Over the years I've been doing more work on it myself, but tend to stick to simpler things, and leave the important stuff to the pro's.

My latest issue that I have all of a sudden lost the ability to blow heat into the cabin. The blower is fine, but it's blowing unheated air. A couple of years ago I had the heater core replaced, and the AC upgraded at the same time. So I don't immediately think that the heater core is the source of the problem, since it's fairly new and was working not too long ago. But who knows? I'm hoping that someone on here can talk me through the diagnosis process to identify and then fix the problem.

Off the top of my head the things I tend to think about as a cause for this would be the thermostat, maybe the temperature control door (not sure it's technical name), maybe the connector that goes from the control knob to the control the temperature control door, and of course the heather core itself. There might also be other possible culprits that I don't know about. But I'm not sure where to start, or even where all of these components are, or how to get to them? I'm hoping that someone can tell me what to check in what order? Maybe easy fix to hard, maybe cheap fix to expensive fix?

Any help would be appreciated. Thanks,

madspeed said:

If you dont smell antifreeze and your windshield isnt oily, the heater core is ok. Thermostat wouldnt make a diffetence unless its stuck open, and in that case your temp gauge would be on the low side of normal, but you would still getsome heat. I would tend to think youd need to pull out the hvac control and check the connectors there. Can you change where the air blows to?

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Thanks for the response. Yes, I can change where the heat blows to. But you are right, the temp guague is reading the low side of normal. Anyway to test the thermostat further?

2000xp8 said:

If you are going to take out the thermostat to test it, just change it.
Mr. gasket 180.

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My question was actually much more naive than I think people expected. Seems like the consensus is that it's likely the thermostat, and I was asking (or at least intending to ask) whether there was a way to test that the thermostat is really the cause of the problem, without having to disassemble much? I agree with the post above in that if I'm going to go to the trouble of getting to the thermostat, I might as well just put in a new one while I'm there.