Ultrasonic cleaning to restore the YH-409 HVAC sensor and park brake vacuum switch

[Hillbillycat]

I suspect a vacuum leak somewhere. Vac lines to the servo motor and sensor are tight, same as corrugated hose and connector at firewall and free of any debris.

On my 1984 F250 diesel truck (with itīs simple vacuum circuit compared to the Town Car) the HVAC vacuum doors can be adjusted after the engine is shut off (untill the vacuum source is empty of course). So the system stores vacuum.
Not so on my Town Car.

Thatīs why I suspect a vacuum leak. But since the circuit is way more complex it might be normal for not sotring vacuum after shutoff. But thatīs a thing I donīt know and wasnīt able to read from the shop manuals.

[DerekTheGreat]

It's most likely that thermal switch on the heater core hose. My original was bad, but then I replaced it with a "Motorcraft" unit and it was even worse. Upon acceleration the air would drop out from the vent position and to the defrost position, which is what it does with zero or next to no vacuum. I spent some time rebuilding one thermal switch from three, if I remember right. It's been great ever since. Never tested to see how long the system holds vacuum though.

[sly]

The check valve could also be bad. There's one on the vacuum tree with the fat connection to the tree and 2 tails to the HVAC and soup can vacuum reservoir. That's the usual failure as those only last about 10-15 years. Motorcraft YG-337.

[Hillbillycat]

I also suspect the YG-337 check valve. Have a good one from the donor car so I will swap them out and see.
Thermal Blower Lockout Switch is already deleted and bypassed by a piece of pipe, vacuum lines plugged, connector jumpered.

Air stays on vent or any other position during acceleration. No switching to defrost. I only notice it switches from cool to heat and blows hot when speeding up a grade on the Autobahn?!?!

[gadget73]

thats vacuum loss symptoms. Seems like the temperature shifts before where the air comes out does but with no vacuum it defaults to hot defrost.

[sly]

If it's anything like the 88 MGM I had, it blew out of all of the vents on every setting, but blew the hardest out of whatever it was set to. Air plenum leaked like crazy and you would get a little out the defrost and floor even with panel set. Middle of summer and your feet are freezing because of the air leaks from the ducts.

[gadget73]

thats usually the air doors are broken off the hinge. One end of it moves, the other flops around so it never actually seals. Had that issue on my Towncar, used some teeny hinges to fix the doors. Once you get the plenum out its not that awful of a job to fix but getting the plenum in your hands, and then drilling out the pile of rivets to split the thing open is a real pain in the ass.

[sly]

It wasn't quite that bad yet... but nothing sealed so you could still feel a little puff everywhere else while one was blowing hard. It wasn't a major leak, but enough that on a hour drive somewhere, your feet would be frozen while the sunny bits up top were just right.

[kishy]

Late to the party, but some considerations about the vacuum ATC stuff:

While I only have a sample size of 3 vehicles equipped with this part, I am an aggressive hoarder of spares harvested in junk yards. I do not sell them, that isn't the point here, it's more to establish sample size.

Basic operating theory: vacuum is applied to the servo to change the temperature to be "less hot". In the absence of vacuum, it is hot. In the presence of vacuum, it is cold.
If the temperature drifts from cold to hot, it is because not enough vacuum is being given to the servo. This is likely to be an external leak (e.g. vacuum sucking in outside air through a leak instead of acting on the system).
If the temperature drifts from hot to cold, it is because too much vacuum is being given to the servo. The only way that this is possible to happen is because the ATC sensor/valve is not doing what it's supposed to do, properly.

All of the ones I have observed that do not work properly, appear to work "mostly properly" but very gradually over a long time will leak enough vacuum through that you have to progressively shift the temperature slider hotter and hotter in the winter until eventually it's all the way to the hot side, and yet you're still getting cold air. I have not yet found a single one of this part with an outright broken spring. Others have reported broken springs, so I'll never say they don't break, but it seems that the more commonly observed failure mode is a loss of accurate vacuum regulation.

We do not have concrete evidence to support what actually causes this failure. The obvious thing would be a failure to fully seal the vacuum valve within, and one of the common opinions as to why that might happen was dust or dirt contamination. The ultrasonic cleaning approach clearly is targeted at that, but without a full disassembly, we don't know that it successfully got into the true problem area adequately...it likely did but we don't actually know that.

Accordingly with all that, I would not pay any sort of meaningful money for one of these, would probably not even consider buying New Old Stock (since we don't know that the failure isn't going to come with age), and I'd consider it (in the presence of this knowledge) unfair and unethical to, for example, make a claim with the eBay money back guarantee because one received did not actually work as promised. The seller might know the car behaved properly for 5 minutes. My car behaves properly for 5 minutes, it's after 40 or so that it really shows the symptom.

These are able to be disassembled without breaking them. However, you will lose parts, which nets the same result in the end. There is a super tiny little ball-bearing-like thingy that goes in it and likes to immediately disappear.

My vote for a proper long term solution to this is 90s EATC swap, including the unpopular actuator with the gears that melt, and figure out a way to mount it so it isn't a nightmare to change. I will note that it is probably so failure-prone (the EATC actuator) because of the thermal conditions where it is located.

Now, all of THAT being said, I believe from the most recent developments in the thread that your (OP) issues are actually a vacuum leak. The check valve, lines, and reservoir canister are priority suspect items in my experience. The line in the engine bay that snakes around to the reservoir by the fender is my #1 suspect to check first. I've seen multiple splits where heat, time and friction have not been kind to it.

[Lutrova]

In my own recent explorations of this problem, I traced my vacuum leak to the (non-functioning) ATC sensor. I had already capped and bypassed the TBL sensor, replaced the vacuum reservoir, and checked the check valve. The ATC sensor had failed in the traditional way, defaulting to full cold, or providing vacuum to the temp servo. But when tested with a vacuum pump and when driving on long uphill climbs it would lose enough vacuum to send the servo to hot. I confirmed this by bypassing the ATC sensor with a line going straight from vacuum supply to the clear servo line - no issues cruising uphill. From your suggestion to me about the bypass with a 'T' and screw for basic temperature control, it sounds like you've already got that part figured out.

If something else in the system is leaking, a great place to check is right behind the radio. I believe all the HVAC vacuum lines apart from the vacuum reservoir line pass through a connector here. Disconnecting it and testing each line is real easy, and everything should be verifiable against the EVTM.

Something I still don't understand is whether an ATC sensor in perfect working order leaks vacuum when set to hot, or if it caps off the vacuum supply line to keep the rest of the system from running out. If it does just leak in hot instead of pass vacuum to the servo, the check valve and reservoir should mean that there's enough reserve to keep everything else happy. But maybe these hill conditions are long enough that they drain the vacuum reservoir. I can't imagine Ford would've designed the system such that driving on mountain highways would kill your cool air.

Can anyone confirm whether a working ATC sensor holds vacuum in a bench test?

[Hillbillycat]

From my observations the ATC sensor does not hold vacuum on bench test. It vents more or less depending on the setting.
Had no time to upload the pics I had made of the disassembled sensor yet.
It was perfectly clean inside (disassembly after ultrasonic cleaning). Iīll show you as soon as I have more time to upload the pics.
My asumption is the following:

Thereīs this little beveled brass center piece on the diaphragm where the spring hooks up to. The beveled end seals agains the plastic valve stem. This stem is hollow, connects to the smaller black vacuum srouce line and has two tiny, tiny grooves in a star pattern on the sealing surface - looks like some kind of vacuum restrictor to me. The brass "plunger" had deformed the mating surface of the stem with the plastic blocking these tiny grooves and preventing the brass plunger to seat properly.
From inspection of the disassembled unit I can only imagine this being the culprit for the sensor not to work properly.

I also had no time to fully re-assemble the unit and install in my Town Car.

The donor car sensor (dirty with dust, not ultrasonic cleaned) seems to work. I get some changes in heat but not dramatically.
Could this be for the in-car temp is pertty high now during summer? Or should a fully working ATC give you full heat when desired even in the hottest weater?

And yes, I also suspect a vacuum leak somewhere. Iīll have to dig into this and follow the hints you guys gave me.

To be continued.....

[Hillbillycat]

OK, here are the pics of the sensor:

[Hillbillycat]

Sorry somehow messed up with the pics.
Is there a way to upload a preselected batch?

[Hillbillycat]

[Lutrova]

I was inspired by your pictures to pull apart my own sensor, which was just sitting on my desk. I haven't removed the six-pointed circular piece that holds the orange and brass diaphragm in place. Is it glued to the plastic valve?

At any rate, it looks like the diaphragm pulls 'out' for full vacuum/cold, and 'in' for no vacuum/heat. The flexible orange part seems to want to remain 'in' without any other inputs. The bimetal sensor also seems to push the diaphragm in, at least at room temperature. I haven't fully investigated its range of motion. The temperature selection spring, meanwhile, appears to exert a strong 'out' pull.

So unless I've missed something, there are three forces at play, two of them bias toward in/heat and one toward out/cold. Since these sensors all seem to fail toward full cold, I think the failure lies with one of the inward biased parts. The bimetal seems pretty stable to me. My guess is that over time the orange part of the diaphragm loses some of its stiffness and eventually the force from the spring overcomes it. If that's the case then I'm not sure what could be done to correct it.