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

[Hillbillycat]

....and here´s a picture of the plunger with retainer.

[Compeek]

Hi, everyone. Sorry to revive a 5-year-old thread, but it's the best source of info on the internet I've found about these sensors, and I have some additional findings to share that I think fully explain why this thing is designed the way that it is. I also want to say thank you for the efforts you all made previously--I was able to get mine functioning again after studying the info here.

Here's the most important thing I have to add to the discussion: The brass piece is also the vent, and the plunger also acts as a valve for the vent.

The third hole in between where the two vacuum lines connect to the sensor was previously suggested as a permanent vent, but I think it's actually just an artifact from manufacturing. If you look at it carefully with a flashlight, you can tell that it bottoms out and isn't actually connected to the internals. I also tested with a vacuum pump by sealing up hole in the brass piece without the plunger in place, capping off the supply vacuum line, and pulling a vacuum through the servo vacuum line. The vacuum holds indefinitely, which confirms there is no permanent internal vent.

Fortunately I still have the original black plastic triangle piece that sits behind the plunger in the brass piece, and I noticed that with it in place, the plunger still has room to move in and out a little bit (maybe 1-2mm). The black plastic piece bottoms out in the brass piece, leaving a little extra space behind the plunger. That is intentional because it allows the plunger to retract enough to allow air to pass through the brass piece. That's why the brass piece and plunger are not one solid piece and why the plunger is not fixed in place with glue. It also explains why the black plastic piece is triangular. It allows air to pass by it. If it were round, it would seal up the hole. Likewise it explains why there was foam behind the black piece to hold everything in place and not glue or neoprene. Foam allows air to pass through.

In cold mode, the brass piece and plunger are pulled away from the valve in main body, which allows two things to happen. One is that the vacuum is allowed to pass from the supply line to the servo line, but the other is that the vacuum keeps the plunger fully extended, thereby sealing off the center of the brass piece so that the vacuum doesn't leak through it. In hot mode, the opposite happens when the brass piece and plunger are pressed against the valve in the main body. The vacuum supply is blocked off from the servo line, but also the plunger is forced to retract into the brass piece a little bit, thereby unsealing the center of the brass piece and allowing vacuum in the servo line to escape. (It's a little less clear to me what happens at intermediate temperatures, but I think it probably has to be venting in all cases except max cold. Otherwise the vacuum in the servo would continue building until it reached the level of the vacuum supply, and the main valve would only be controlling how fast that happens. I think venting is required to actually control the amount of vacuum reaching the servo in order to get any sort of partial blend door setting.)

I discovered this somewhat accidentally. I first tried to fix the sensor by jamming a piece of rubber o-ring into the brass piece to hold the plunger and black plastic piece in place, which seemed to work initially on the bench, but when I tested it in the car, I found that it would go from hot to cold but not back to hot because the vacuum couldn't escape. After poking around some more and finding that the third hold is not actually a vent, I realized that the only other possibility is that the brass piece is the vent, and then suddenly everything made sense. For those of who you used a piece of wood to keep the plunger in place, I'm guessing it was not perfectly round and still allowed air to by pass by, although you may not have realized that was important.

I ended up using a piece of light, spongy foam from a Briggs and Straton carburetor repair kit that I had lying around. A lot of small engine carbs have a circular foam gasket thing on the throttle shaft. I cut off about 1/8 of the foam ring, rolled it tight in my fingers to compress it, and then fed it into the end of the brass piece. (So the foam was behind the black plastic piece, which was behind the plunger.) That fixed the problem. It keeps the plunger and black plastic piece from falling out, but it still allows air to pass through. I bench tested it to confirm that the vacuum from the servo line can vent when the main valve is closed, and when I put it back in the car, it worked as expected.

I've annotated Lutrova's excellent diagram with a few corrections to show what I mean. I moved the plunger (yellow) out so it's only partially retracted. The black plastic piece and foam (blue) are drawn in. The green lines represent the vacuum escaping through this "vent". (This is all in hot mode.)



Here are some photos of the foam piece I used.




And here is a demonstration of how much the plunger extends and retracts even with the black plastic piece and foam behind it.

[friskyfrankie]

Nice work and many thanks for the in-depth explanation!

[Compeek]

Here's some additional info in case it helps somebody else.

Besides the issues with the plunger, my bimetal piece seemed to have lost its arch. I bent it back into a curve, but I had no idea what the spec was. Initially I tried about 2mm, but it wasn't enough, and the cooling came way too early. I then tried a hair under 3mm, and I think that's a little too much because the cooling came in too late, but it was close enough to let me fine tune it with the star dial after reassembly. To be clear, when I say 3mm, I mean with it sitting on the desk with the arch upward, I measure 3mm from the surface of the desk to the very top of the arch.



If you're missing the black plastic piece and need to find an alternative, it's a hair over 1/8" inch long. I don't think the length is that important. What's important is that it be thick enough to seat properly in the brass piece and leave room for the plunger to retract a little (shouldn't force the plunger to be fully extended always), but it must be shaped such that it does not seal the hole and air can still pass by it. A piece of wood could work as others have done, or you could could get creative with something else. (The piece of wood in my photo is a part of a toothpick that I was experimenting initially, so ignore that.)



The plunger would be hard to recreate, but it's about 5/32" long.



The brass piece is about 13/32" long.



The dimensions of the foam piece are also not critical. Again you want something that has enough material to hold things in place, but not so dense that it seals off the hole. I included a photo in my previous post showing the piece I used.

I'm including a picture of the spring as seen through the side opening with the sensor fully assembled. I took this before I disassembled it so I could use it as a reference. It helped me with reshaping the curve of the spring after hooking it back on to the brass piece with needle nose pliers through the side opening since doing so tends to deform it. I don't think it's super critical since you can fine tune it with the star wheel, but I wanted to get it as close to how it was before as I could.



I also have a photo of where my cable was clamped down originally on the sensor before I removed it from the car. It's easy to get it right though. Just set the temperature lever to 75 degrees and then make sure the black thing where the cable attaches is lined up with the arrow on the side of the body (in other words, perfectly in the middle of the range) when you clamp it down. I read that somewhere, but I can't remember where. It's logical though because you're just setting the lever and the sensor to the same position when you secure the cable.



Trying to recalibrate the sensor feels rather ambigious, and I think I'll probably just have to play with it over time as I drive around and get a sense of whether it seems to hold the temp I have set. However, there are two possible clues I found helpful. One is that the fan speed (on auto) is lowest presumably when the system thinks it's at the desired temperature. The other is that the recirculation door seems to open/close at about the same point, which makes sense because you generally want recirculation when cooling (for most effective cooling) and not when heating (for fresh air). My method was to set the temp to 75 (middle position), wait until the air temperature in the sensor was measuring about 75 with a meat thermometer, and then adjust the star wheel in or out until it seemed to be hovering around two reference points (lowest fan speed and recirculation door on the verge of opening/closing). Turning the star wheel clockwise reduces tension on the spring (bias toward heat), and turning it counterclockwise increases tension on the spring (bias toward cold).