Defrost is stuck on - what where do I check vacuum lines?

[swc69]

Greetings All!

I haven't posted in quite a while. I'm coming up on 60k miles on my MM with only a few minor problems to date.

The biggest of which is my Defrost is stuck on, regardless of the settings on the HVAC unit. I did a little searching and it seems most likely that I've lost vacuum so the door isn't opening/closing right.

I have a big road trip tomorrow (about 1k miles), so I'm hoping to get this fixed after work tonight.

Can someone save me some time and give me some guidance of what vacuum lines to check and where they are located? I found a post that said "under the hood and under the dash", but could anyone be a little more specific?

Also - is there one that tends to fail more often so I would know where to look first?

Thanks in advance!

Steve C.

[magindat]

A general vac leak will cause this. The problem is likely in the engine bay and not under the dash.
The quick shadetree way is to get a length of hose and stick one end to your ear and use the other to listen around the top portions of the motor. This will help pinpoint the vac leak sound.
Of course, visual inspection is the place to start.

[MMM2003]

Greetings All!

Can someone save me some time and give me some guidance of what vacuum lines to check and where they are located? I found a post that said "under the hood and under the dash", but could anyone be a little more specific?

Also - is there one that tends to fail more often so I would know where to look first?

Thanks in advance!

Steve C.

Steve:

I had the same problem. Please check this thread with some detailed steps on how to check and replace the HVAC unit. Most likely you won't be able to fix anything short-term. Failure is typically inside the HVAC control unit, not one of the lines or hoses.
http://www.mercurymarauder.net/forums/showthread.php?t=31184&highlight=hvac+unit

Good luck!

Peter aka MMM2003

[swc69]

So - this looks like it will be real easy or mildly annoying but expensive.

Hopefully its not a complete failure of the unit - it sure sounds like a simple vacuum leak.

Otherwise - I might have to tie a peice a string around the floor vent actuator so I can pull on it when my feet get cold.

Thanks for your help!

Steve C.

[MMM2003]

So - this looks like it will be real easy or mildly annoying but expensive.

Hopefully its not a complete failure of the unit - it sure sounds like a simple vacuum leak.

Otherwise - I might have to tie a peice a string around the floor vent actuator so I can pull on it when my feet get cold.

Thanks for your help!

Steve C.

That's what I thought. It's simple to check and look at. There is another vacuum manifold accessible from the passenger side's foot compartment. Make sure that all the connections are good on it. It's rubber with a plastic manifold and about 7 different colored hose connections.

Good luck!

Peter

[JMan]

Peter,
Nice write-up! I thought about doing a write-up when I changed mine. I'm glad I didn't. It would be pale compared to that!

Great job,

J

[johnjamis]

When my grand marquis did this it was the HVAC control head; the control box in the dash. Easy to replace. Get a used one from your local junk yard.

[MMM2003]

Peter,
Nice write-up! I thought about doing a write-up when I changed mine. I'm glad I didn't. It would be pale compared to that!

Great job,

J

Thanks for nice words. The least I can do in return for all the help I received from our board.

Peter

[MMM2003]

When my grand marquis did this it was the HVAC control head; the control box in the dash. Easy to replace. Get a used one from your local junk yard.

True. You can use HVAC units from GM or CV (I believe 2003 and up); only thing you will loose is the automatic temp control feature. Not sure about the outside temp.

Good luck,

Peter

[swc69]

I agree the write-up was great. I originally asked for more specific detail on what to do and that was pretty darn specific!

I poked around last night and found:

Decent vacuum at the vacuum box (could't figure out how to open it, I was worried about breaking something by prying it open).

The passenger compartment vacuum connect always had vacuum on only ONE connection. I tried to cheat and apply vacuum with a vacuum pump to other connections, but that didn't seem to make a difference. (Pump wouldn't hold a consistent vacuum). I had some 1/4" hose laying around and tried to "re-route" the existing vacuum to other inputs but that didn't make a difference either.

But - I was doing all this at the last minute trying to fix it before a big roadtrip where its really cold.

I was hoping it was a simple vacuum leak, but based on my initial tests I think its the HVAC unit.

Quick question re: the passenger compartment vacuum hoses: (i.e. - the connecter with all the colored hoses in the passenger footwell on the left side that comes apart). I assume this is coming FROM the HVAC unit and then going TO the actuators, Right?

300 miles down, 700 miles to go - My face is warm but my feet are cold.....

Steve C.

[edit] I stumbled across the Coolent Burp thread and did the burp - at least the Defrost is now warm when I'm at idle......

[Drock96Marquis]

True. You can use HVAC units from GM or CV (I believe 2003 and up); only thing you will loose is the automatic temp control feature. Not sure about the outside temp.

Good luck,

Peter

You don't loose anything - MM uses the same HVAC panel as the CV/GM, automatic and all. It has remained virtually unchanged since 1995.
2003+ CV/GM is a direct swap for you guys.

The 3 rotary knob style HVAC is not swappable.

[ndmoorpwr]

I had the same problem with mine and it was the HVAC control unit. I just went to boneyard got another from lincoln town car and just changed the little vacuum valves out, just had to bend the little tabs out that held them in place and put the other back in, took about 30min and cost me $10. Really simple.