1990 Town Car - no a/c only heat?

[drhill]

Hi Gents,

Don't know if this is the correct sub forum, but I just had an issue pop up on my 90 Town Car. It's stuck on heat. AC system is working as normal (compressor kicks on, lines and dryer are cold) but no cold air from the vents.

After digging around a bit, I started looking for the blend door actuator - found it way up in the dash and it is not moving when I slide the temperature lever on the dash. I am thinking this is either a bad blend door actuator(appears to be an electric motor) or maybe the inside temp sensor for the ATC? does anyone know how to test that sensor before I start to replace the BDA?

Or maybe there is something I'm not thinking of?


Thanks!

[DerekTheGreat]

I think you're right on the money with that. VCV or someone who knows those systems will probably chime in soon to give you solid advice. I know with Box TC's if everything else is working like normal the ATC sensor under the dashpad on the passenger side is most likely bad or there's gunk blocking the one white tube thing.

[VicCrownVic]

I don't actually know that much, but it does sound like a bad BDA.
Here's a pic of the BDA in my blue '91 MGM:


(ignore the red highlights, this pic is from my heater core replacement)
The square peg in the upper right of the BDA in that pic is the shaft that moves the arm that moves the blend door. If that thing is not moving when you change the temperature setting I can see what the voltages are and what pinout looks like for the connector, but that will be later this evening when I can look through my EVTM. Once you have that info, check voltages at that connector to make sure the right voltages are being sent, and if that checks out replace the BDA.

[drhill]

Thanks guys, I tried to reach the BDA going through the glove box opening and moving a bracket and some other stuff out of the way....no dice. Looks like you have to pull the whole dash back to even get that plug off so I'm just going to replace it if I have to go through all that work - it appears to be a 26 year old original so it's peace of mind even if it turns out to be another issue.

I actually got a flethead into the groove on the top of the BDA that moves - couldn't turn it either way and it doesnt move at all no matter what I do with the controls.

I may replace the thermostat piece up under the top of the dash as well - I can't tell how to test it? It has a two wire plug going into it and it has a white air tube connected on the other side that routes to the hvac box on the lower passenger side. I assume this is to compare the actual interior cabin temp with the temp of the air coming through the HVAC box and vents?

I would still like to have the voltage info if you have the time...thanks!

[VicCrownVic]

Dang, I was hoping there would be an easy way to test. You could test at the back of the control head, I'll look at the EVTM later this evening when I get home.

If you're certain the BDA is not moving at least you can rule out a broken blend door (unless you are just that unlucky ).

[drhill]

BDA is definitely not moving for sure!

[gadget73]

Change the heater core while you're in there. You're basically right next door and you know perfectly well it'll fail next month if you don't.

[drhill]

Change the heater core while you're in there. You're basically right next door and you know perfectly well it'll fail next month if you don't.

ha ha - too true. the previous owner already did it, but, dang, i may do it again!

[gadget73]

If its not old, it might be OK. Just thinking that if its original there is no better time to deal with it than when you've already got access to it.

[VicCrownVic]

Here's what I was able to come up with for testing.

Since testing at the connector at the BDA is not possible without removing the dash, testing at the ATC connector on the back of the control head might be a viable alternative and would be as follows:
BDA pin 1 = ATC pin 14
BDA pin 5 = ATC pin 7
BDA pin 6 = ATC pin 13
BDA pin 7 = ATC pin 4
BDA pin 8 = ATC pin 3

So, ATC pin 14 to ground will measure 0-5 volts and will traverse that range as you move the temperature lever.
ATC pin 7 to ground will measure 5 volts.
ATC pin 13 to ground will measure 0.
I'm not entirely sure on the next two.
ATC pin 4 to ground will measure battery voltage, but I think only when the BDA is moving CCW?
Likewise, ATC pin 3 to ground will measure battery voltage, but I think only when the BDA is moving CW?

Corresponding diagrams and charts to go with the above info:

BDA C2004
Pin	Circuit	Color	Circuit Description	Volts
1	351	BR/W	Blend Door Actuator Potentiometer Wiper	0-5
2	---	---	Not Used	---
3	---	---	Not Used	---
4	---	---	Not Used	---
5	243	LG/O	Blend Door Actuator Potentiometer Reference	5
6	470	PK/BK	Blend Door Actuator Potentiometer Ground	0
7	247	W/Y	Blend Door Actuator Motor Drive CCW (+)	Bat.
8	246	P	Blend Door Actuator Motor Drive CW (-)	Bat.

ATC C2006
Pin	Circuit	Color	Circuit Description	Volts
1	---	---	Not Used	---
2	790	W/O	In-Car Temperature Sensor Reference	5
3	246	P	Blend Actuator Motor Drive CW (-)	Bat.
4	247	W/Y	Blend Door Actuator Motor Drive CCW (+)	Bat.
5	754	LG/W	Variable Blower Speed Controller (VBSC) High Blower Input	Bat. in high blower
6	752	Y/R	VBSC Signal from ATC	0-2
7	243	LG/O	Blend Door Actuator Potentiometer Reference	5
8	767	LB/O	Ambient Air Temperature Sensor Reference	5
9	359	GY/R	Ground	0
10	54	LG/Y	Power (Hot at all times)	Bat.
11	250	O	ATC Mode Switch Input
12	759	DG/W	VBSC Feedback
13	470	PK/BK	Sensor Ground	0
14	351	BR/W	Blend Door Actuator Potentiometer Wiper	0-5

[drhill]

VCV, you're the man! I will put this info to good use in the morning and report back!

[DerekTheGreat]

And he said he didn't know much!

[drhill]

ok, did the tests and here's what happened:

- ATC pin 14 - was initially no consistient - about 1.5 to 3.5 volts as a moved it, then all of a sudden it settled to 3.3 volts and stayed there no matter what I did with the temp control lever. I checked it one more time and it stayed at 3.6 volt no matter what. wierd.

- ATC pin 7 - 5 volts yep all good here

- ATC pin 13 - 0 volts , yep, ground is working!

- ATC pin 3 and 4, wierd results, it changed as I moved it hovering around 7 volts plus or minus.

But then, through all of this monkeying around, the BDA had moved to full cold - so now I have AC and no heat. I looked up through the glove box and the piece that turns on the BDA was in a different spot.

So what do you all think? Faulty ATC or BDA? Or both?

ATC i think is a junkyard only item, BDA is still available I see. I read somewhere that a worn out bda can cause a short in the ATC?

Right now it looks like one of the circuits in the ATC is bad and I have the atc accessible and ready to change but do I need to do the BDA as well? The BDA does not make any funny noises etc...doesn't seem like the mechanism is loose either.

[sly]

maybe some bad contacts on the ATC controls? Might just need to unplug and plug in a couple of times to re-seat everything. May also be broken solder joints and just need a soldering refresh on that connector on the board in the ATC.

[gadget73]

Sounds like the door is not moving. The usual failure mode is the little plastic gear inside the actuator breaks and it stops moving the door. No door movement means no position sensor change. I'd open that thing up and have a look at it.