90 MGM Ac

[87GrandMarq]

Does anyone know how the 90-91 MGM auto ac systems work? I’m assuming is uses electric blend doors unlike the earlier units. Is there a cabin temp sensor? While working on the car, I’ve noticed no matter how much I mess with the head pressures, the discharge temp will be about 60* unless the ambient temp is under 80* and the car is moving. I know the stock condensor isn’t helping but the discharge temp being almost always the same makes me feel like it’s there’s something else at play keeping the temp constant.

[sly]

yeah... 90+ have electric blend door. They are known to break at the top of the door where the square (maybe D shape) rod goes into the motor module. The auto version has a temp sensor, but that should be bypassed when at full cold. I don't remember where that temp sensor is. I know on whales, it's behind the fake wood belt trim on the dash. I think it's in the same place as the mechanical one from older boxes, under the dash pad where the slot above the glove box is.

[gadget73]

VicCrownVic probably knows. I think he did a later style EATC conversion on a 90s box.

[87GrandMarq]

I’ll take a look. The only reason I mentioned the blend door and temp sensor is because at night when the ambient temp is low and I turn the auto fan speed off, you can hear what I’m assuming is the blend door move. The other thing I forgot to mention is that the ac sweats profusely too. After just a couple of minutes of idling and there’s a good size puddle under the car.

[87GrandMarq]

This is after maybe 15 minutes of idling.

[VicCrownVic]

Yes, electric blend door actuator (BDA) on '90+ which is physically the same as the aero/whale blend door actuator but there were a couple more wires on the later ones. The same electrical connector but on the later models the two extra pins occupy terminals that were unused on the earlier years and the pin positions changed a bit. (I installed a later BDA, from the same car that donated the control head, and wired it directly to the control head bypassing the stock BDA wiring.)

I didn't touch the cabin air temp sensor since the later control head ('97 LTC) was happy with the '91 MGM sensor. I'll have to dig out my books to see where that lives. Upper center or right of center area of the dash would be my guess. I'm not familiar with how the temp sensor controls things, so if Sly says it should be bypassed when set to full cold (or I assume anything other than the auto setting) you can probably eliminate the sensor as being the issue.

Until you had mentioned the sweating (I assume accumulator) I was thinking that perhaps your condenser was clogged with fuzzies not allowing air flow or your fan clutch could be not working well. With the sweating I think that would eliminate the fan clutch and condenser is probably not clogged. So I'm then with Sly on the blend door itself perhaps has broken if the BDA is still working.

[gadget73]

at the extremes it should push the blend door full hot or full cold. Not so much bypassing the sensor, just ignoring it. If there is something preventing the door from going all the way its not going to be very cold, same if the foam seal is shot and leaking a bunch of hot air around it.

[87GrandMarq]

Anyone have a diagram of the hvac box? Or can tell me how I’d get eyes on the blend door with the dash together? One part of my issue, which I forgot to mention, is that the recirc door will not open. I’ll have to check vacuum one of these days. If I manually open the recirc door it’ll stay open and the discharge temp will drop by almost 10*. My guess is probably a combination of a leaking blend door and not enough airflow.

[gadget73]

If its gonna be visible anywhere, pull the glovebox all the way open past the stops and its basically straight back. The BDA sits on top of it, its a rectangular thing about the size of a deck of cards, and it sits on top of the blend door to rotate it. Not sure if you can actually see it or not though.

[VicCrownVic]

I'm not sure you can see the blend door without taking things apart.

I stole the heater box out of The Ice Car since one of the doors in the heater box in The Scab is seized.

There is duct work that would need to be disconnected to see into the box then there is a mode door that defaults to blocking your view of the blend door.

[87GrandMarq]

Ah I see where it is now. So pretty much right behind the control head. I have slipped my fingers between the hvac box and the plenum that goes to the vents and by feel, it doesn’t seem like much heat is leaking out but that’s not a very scientific way to test temperature. I think a bore scope is in my future.

[87GrandMarq]

What is this valve on this heater line?

[VicCrownVic]

Thermal Blower Lockout (TBL). If the temperature of the coolant passing through it is not high enough it prevents heat from being used on the floor setting. Pretty useless feature and was done away with on the later version of ATC. Would not affect AC. Can be removed; plug the vac lines and I think jumper the connector.

[gadget73]

also a common source of vacuum problems and usually why the blower doesn't work in 'floor' mode.

Running the full current of the blower through a thing that very slowly opens and closes is a guarantee for roasted contacts, and the sliding rubber seals that force the fresh air door closed are not durable.

[kishy]

also a common source of vacuum problems and usually why the blower doesn't work in 'floor' mode.

Running the full current of the blower through a thing that very slowly opens and closes is a guarantee for roasted contacts, and the sliding rubber seals that force the fresh air door closed are not durable.

Hang on, those internally use the sliding rubber ported block thing? Like the way the auto park brake release valve works?

No wonder they get stuck. The sliding rubber things always turn into goo and get locked in place, leak vacuum, or both.

It's not clear to me that the contacts would see a very large arc (relatively speaking) since the blower is not fully locked out, only held at its slowest speed, IIRC. But either way that's some nonsense.