OT What does a temp switch change?

[saidyene]

I am a mechanic in the U.S.Army. The other day I was working on an A/C system in one of our trucks. I was trying to figure out why the condenser fans wouldn't come on. I found that a $10,500.00 computer wasn't putting out a ground to the relay. At that point my only choice was to replace the computer and hope that was it. We didn't have the schematics for the inside of the computer or i would have done more. I started thinking about it and there doesn't seam like there is anything that the computer does that i cant do with some relays. So I started building a replacement while I was waiting on the part. I now have everything working except the temp selector switch( the one with the red bar on one side and the blue on the other). I have no idea what it would control for the A/C. I know that on the heat side it would open the water valve in varying degrees until it was fully open but no idea on the A/C side. Im looking to provide a design to TACOM which is the group responsible for reducing the cost/ waste of the military. So far im up to about $20 in parts using the shell from the old computer. That is a far cry from the $10,500 that the computer costs. If one of you can provide me with some schematics or pics or even a description of what is changed I would appreciate it. I would provide the schematics but im not sure if im allowed to so if you need more info i might be able to describe it to you. I will check on the classification of the schematics though.

[Wes]

yeah my commuter Honda has the same design a cpu decides when the compressor clutch should be engaged. So the temp control will normally open/close a blend door in the ac heater plenum assembly. This allows a blend of the heated and cooled air to meet desired temperature. Depending on how complicated the system is the computer may monitor the selected temp and the evaporator temp to adjust the cycle rate of the compressor. I hope this helps. I find it hard to believe that the ac module is 10k our tax dollars at work

[saidyene]

I thought the same thing. Most of the part we use are about what I would think to be average price but this was just a big surprise. The AC dosent have a door it just dumps out into the cab. Im thinking with my design that the AC will be on full all the time just to simplify the design and ill just wire the temp control into the heater.

[mr_josh]

If the temp selector is on full hot, it's still operating the AC, it's just blowing hot air through it. The idea is, the heater core can heat up whatever temperature air is blown through it, so if you are operating the system on an air conditioner setting -like Defrost-, it still cools the air (and also dries it), but then it heats it back up with the heater core. So basically, the heat intensity is adjusted with the temperature slider, and if the air conditioner is on, it's on full blast. It's up to the heater core to decide how warm the air will be.

Par exemple:

Mode Selector: Vent
Heat Selector: Full heat
System status: Blend door allows full heat in to the cabin, AC compressor does not cycle. Vent temperature is as hot as the heater core can make it.

Mode Selector: Vent
Heat Selector: Full cold
System status: Blend door allows no heat in to the cabin, AC compressor does not cycle. Vent temperature is the same as the outside air.

Mode Selector: Max AC
Heat Selector: Full cold
System status: Blend door allows no heat in to the cabin, AC compressor cycles. Vent temperature is as cold as the AC can make it.

Mode Selector: Defrost
Heat selector: Full hot
System status: Blend door allows full heat in to the cabin, AC compressor CYCLES. Vent temperature is as warm as the heater core can make it.

Things to keep in mind: are you implementing a full-throttle compressor cutout relay? Is the AC pressure control system a part of the computer that you're bypassing, or is it controlled by switches in the system? If you have electric cooling fans, are you commanding them to engage while the AC system is in use?

Stuff like that is really important, it's not usually just a matter of making it AC on / AC off.

[saidyene]

I have the Fans come on with the AC and the pressure and freeze switches are wired into the box im making. I have never heard of a full trottle cutout but ill find some info to read up on. I didnt think of warming the air with the AC on to control the output temp. I think thats what ill do. I still have to figure out some other things. Such as there was a wire to let the AC computer know when the engine was running. I need to figure out if that is a digital signal or just a power/ground coming in. I also have a high idle trigger for when the vehicle is in neutral and parking brake set. Again i need to find out if this is a digital signal or power/ground. If they are power/ground I will include them into the design if not ill likely leave them out. I just dont see those functions as worth the cost of the AC computer. The plan at this point is to finish this design and present it to the higher ups that will either accept the design and ill get a nice bonus or they will turn it down and ill have a nice troubleshooting tool. Ill be able to put this box in and isolate each circuit so i know why the system isnt working. When i get a chance ill post the schematics of what I have made so far. Thanks for the help so far.

[Wes]

If by running the a/c full, you mean the compressor, you'll find that the evaporator will freeze up. This can/will block the gas/lube flow back to the compressor. The compressor will over heat and fail. there must be some way the system controls the cycle rate of the compressor clutch. Possibly pressure either low pressure cut out after the evap or Hi pressure cut out. Some systems use a fixed orifice to meter the gas, this requires the compressor to cycle on and off. Others use a thermal expansion valve, which monitor the temp of the evap core. This valve opens and closes controlling the flow of gas. These are two of the more common a/c system used in the automotive industry. I hope you get it figured out.
Did you get clearance to tell us what kind of equipment your working on ? An Abrams ? F15 ? AH64 ?

[saidyene]

I guess I never did tell you what Im working on. Its the new truck the MRAP. Mine Resistant Ambush Protected from international. That part I know has no classification Im just not sure on the manuals and schematics. The system Im working with has a freeze switch on the evaporator which cuts power to the compressor if it gets too cold. The system also has a low pressure switch near the compressed that wont let the Compressor come on if the system dosent have enough pressure. As well as a trinary switch that will shut it down if the pressure is too high or low near the evaporator. All of that is already wired in except the trinary switch. So I dont have protection for high pressure conditions. I need to figure out how to wire the trinary switch. Im now having issues with getting access to the truck. I have other trucks to work on so I dont get much time to work on this as well as it is assigned to someone that has to be with me when Im working on it. The truck has been fixed with the standard part Im just continuing on with this in hopes that the military will pick it up and use it. Ill figure out the trinary switch next time I get on the truck which might be in the next few days.

[Wes]

Good deal, I have some newer International cab and chassis, in our fleet. I haven't run across this issue yet. I hope to here how your switch box works.

[saidyene]

This AC system was built just for these vehicles so you wouldnt have seen it. The box that contains the blower, evaporator, and heater core is 3 ft tall. Its able to cool the vehicle to 50* when its 120* outside. Its kind of nice its so big cause its a bit more spaced out then the typical system but at the same time it takes forever to find leaks cause there is so much more area to cover. Honestly I have learned alot from working on these. About 7 months ago I knew the basics of how an AC works and now Im one of 2 that know how to fix them at all in a fleet of about 80 vehicles(all of the vehicles not just the MRAP). I love learning new things.

[Wes]

get the neon dye kit to find leaks. It works bitchen. Add 1/4 oz of dye to a charged system, run it for a few mins, get out the black light led probe and any leaking area is day glow green. Well worth the money, esp since r134 can leak from much smaller gaps than r12.