2001 Mack Truck

[Mamrak76]

Back again! So after my much needed vacation and my truck having transmission repairs it is back in my hands. To fill anyone else in, I was trying to get my ac system up on my truck. Weird pressures which were caused by holes. So I ended up needing a condenser. I also replaced the txv and reciver dryer. And a couple of busted up hoses. So now we are in business, well sort of. I pulled about an hour and a half of vacuum. My truck calls for exactly 2 pounds. So I added exactly 2 pounds. That put me at 15 low and 150 high at 65 vent temp. Also I have a very nice fieldpiece thermometer and I trust it. I am not one to over charge but that seemed low at 80 degrees ambient. So I added 10 ounces extra and that brought me to 20 low and 170 high with a 55 degree vent temp. Only problem now is my Evaporator is freezing up. The vent temp stays around 50 but then after a while it goes up. It seems to be freezing when i am idiling. Any suggestions?

[Cornbinder89]

For what it is worth, My IHC 9670 calls for 4.5 lbs of R-12, 2 lbs seams light to me. I can't remember, does yours have a sleeper unit or is it a day cab? Are those pressures with the engine fan on manual?
If you hadn't supplied the spec, I would have thought you are low on refrigerant I am thinking there should be a frost switch to shut off the compressor when the evaporator starts to frost over, Are you just seeing frost on the outlet line or is there a reduction in air flow and frost on the evaporator proper?
since a Tx valve system with a receiver has the ability to store a little excess as high pressure liquid, the system is less sensitive to exact amounts of refrigerant, un like a CCOT system. I would try and find another reference for the amount required, 2 lbs just doesn't seam enough and the pressures seam to back that up.
Too late now, but with the new site, we have a category for Heavy Trucks.
Almost wonder if it is 2 kg (4.4 lbs) for a spec?

[CJB]

Too late now, but with the new site, we have a category for Heavy Trucks.

Good Catch! I didn't even notice Well, thankfully, with the new site it's a breeze to move things. I'll add this to the more appropriate channel and place a two day redirect on the post.

P.S. I too think it calls for more refrigerant. Not certain though. Since the exact model wasn't listed, I'll check on various 2001 models tomorrow.

[Mamrak76]

It is a 2001 Mack mr688s daycab. It is totally freezing up and I lose air discharge. I will check for a frost switch. Have you ever heard of checking for a sight glass on the reciver dryer?

[AC_Chris]

The Mack MR & LE series R134a capacity specification for that year is in fact 2 Lb. Most others are 3 pounds with the exception of the CL CV, CX and CH with sleepers are 3.5 pounds. Is the evaporator surface clean? Almost every truck I work on has a dirty evaporator. Still, there is a thermostat probe going into the evaporator which should prevent a freeze-up.

Sight glass can be helpful, but mostly put in place these days as a moisture indicator. A sight glass too far away from the expansion valve, under certain conditions can show flash gas bubbles or oil globules, which can be misleading. Systems with fixed metering and accumulators will have no sight glass. Peterbilt is one that comes to mind that uses accumulators and orifice tubes for cab a/c.

[Tom Greenleaf]

Just some notes still learning this format as well: CJB thanks for moving it I don't see the option to? For now doesn't matter as I would be reading all new posts anywhere.

These trucks ARE NOT my thing the principles are. Yes if OE designed a sight glass and system is OE with proper setting while being used it should be the best. If you don't see it look for it. Can be small, even tiny on some cars. Wipe clean and use a light if needed to watch.

It's important to know if amount of charge did mean Kgs or pounds?

Not my machine (this truck) I've owned and worked on many R-12 cars (land yacht types) that held 4 pounds so you must be low and pressures, freeze up would also happen.

Will be watching otherwise out of this as Cornbinder at least would have a clue or full know how with this - I don't,

[Mamrak76]

I wanted to add that my truck uses a viscous fan clutch so it is controlled internally. As for the evaporator it is very clean from about a month ago when I replaced the TXV. So now I re hooked up the evaporator temperature sensor. Someone unhooked it. With it hooked up I am only getting about 55.4 to 58.7 degree vent temps. Which is disheartening. The freezing condition is gone now but I feel the pressures are still off and the performance is poor. As this truck is a cab over engine, the system has an enormous heat load. Also I have an enormous heat load to sit in. Any suggestions on perfecting the performance?

[Cornbinder89]

If the outlet of the evaporator is cold enough to frost, than I would look for air bypassing the evaporator, or heat leaking in from heater being the cause for warm outlet.
On my Cabovers I find the long ductwork takes time to cool down and accurate reading can't be read until the system has run long enough to cool the whole duct system, or about 15 mins of run time.
On my Cabover, I increased the condenser capacity over the OEM condenser. But I was also going from R-12 to R134a also.

[Tom Greenleaf]

OK - Kinda wanted to stay out of this mostly but some thoughts on those vent temps being too high. I'm back up to basics - A/C is about "transferring heat" not making cold is a mind warp of it's own.

The air coming into evaporator can't be way too hot or after passing thru leak out of it's path to vents or temps can only drop so much. This means I'd be looking for why incoming air or recirculated if cab is cooling doesn't get you to a mid 40s at least even when hot outside ambient temps.

Insulation of parts could be a problem like air getting around evaporator. Air if from outside if only choice really can't drop much more than 40F of what it's getting IMO and experience.

If system is exactly on the spot and correct charge now known it could mean insulating or re-insulating items/parts. You should feel the liquid side into evaporator line as close as you can and outcoming line should be within just a few degrees or charge isn't correct yet. If that right system simply doesn't produce enough BTU of cooling force but it by rights should at the vent despite if it can cool the area intended.

Think of everything. If inadequate is engine temp staying steady? It should stay right where a thermostat's rating is at with a "fan clutch" of any sort that's working and no obstructions to air flow.

Also in some cars that cute air dam under a bumper isn't for good looks. It creates a vacuum behind it to "motivate" air thru condenser and radiator to down and out. If vehicle (any) is just plowing against incoming are with not enough way to also get out it's stalling air flow and should show that as engine temp rise also -- if so.

Do not forget area between engine radiators and the condenser when they are in both in the same air can be either dirty in between or if radiator too hot "radiant" heat is back heating the condenser.

Rule some things out as possible. Lots of touch and feel parts and components what the temps really are,

[AC_Chris]

Tom brought up a lot of good points and Cornbinder89 reminded me of the pesky hot water bleed that I see quite often. You may have done this already, but, you might double check to see if your heater hoses are shut off. Factory heater valves rarely work. They seem to always let hot water bleed through. In many cases I have to add a manual shut-off on the heater hose if there isn't a manual shut off on the engine.

And don't forget that extra 10 ounces of refrigerant you added.

[Mamrak76]

I appreciate the tips. As for the engine temp it is staying around 180 and the fan is cycling as normal. I did my best to clean the radiator when I changed the condenser. Both of the Macks we have on the engine oil cooler have a garden hose looking valve for heater control. I do have that off. I will check for evaporator inlet and discharge temps to determine if the charge is correct. I would say that at work and on the side I have successfully repaired about 25 vehicle air conditioning systems. But this truck, the truck I spend 10+ hours a day in, has stumped me.

1. Is the evaporator temp sensor set to a specific degree Fahrenheit?

2. Could something be problematic because the old condenser had large passageways and the new one has small passageways?

[Cornbinder89]

With 134a the smaller passageways are better (more surface area contacting the refrigerant. If the outlet of the evaporator is cold, then I doubt you have a condenser problem. My rule of thumb is the condenser outlet should feel slightly warm but not hot to the touch with the fan drawing air over it. The more heat you can shed there the better, but it will always be above ambient temp.
The temp sensors job is to prevent iceing of the evaporator, so close to 32 deg F
I looked up the model you are working on, and a lot of glass and very little engine/ radiator room! Even less than my 9670's. But if the evaporator iis getting close to 32 deg, it is doing all it can, if vent temp is above that, then look for heat getting into the ducts , it can be as simple as the cab getting hot right above the radiator and transferring the heat into the duct work.
On one of my trucks I added a Red Dot air controlled water valve to the heater circuit, that is controlled with an air toggle switch on the dash. I was having trouble with the cable operated control bleeding a small amount and adding heat to the cooled air. They also work great for the fuel heater circuit, I can turn on from the dash on the fly.

[Mamrak76]

Ok. Just from my symptoms it seems like a low charge. I do not know much about txv's. Is it possible that the wrong txv was given to me but it still fit the system? They looked identical. Can two expansion valves look the same but be different inside?

[Tom Greenleaf]

Quoting Mamrak76 ">Ok. Just from my symptoms it seems like a low charge.<"

True that it might not be. TXV = spell that out is "Thermal Expansion Valve" that is sensing temp in a bulb with the temperature it creates pressure to open the expansion valve to just the right amount of flow of compressed liquid to be evenly evaporated thru an 'evaporator' in hopes to uniformly change state from a liquid to a gaseous form to return to be compressed.

So, if that "flood" gate isn't right as like if way too open it just pumps the refrigerant around in circles so pressures alone would be deceiving. I leave it to Cornbinder89 to suggest if these TX Valves are adjustable to perfect or pre-set once such that if wrong can't be used.

General on them they must be insulated so the bulb isn't allowed invasion of warm air. A putty used to seal its tube alone can throw them way off if not sealing.

This point just that to try understand the principle of how those operate not all the issues of this thread in total.

Note: I've haven't so much as ridden in these type trucks so can't know how layout of components can also be a factor especially if heat get to the wrong components,

[Cornbinder89]

It is possible but unlikely to be the problem, Is it the block type? That is what my book shows. they are made in two capacities 1.5 ton and 2 ton, according to my book the most common is a 1.5 ton. (MEI 1600) and that is what is called for in the model you have.
The Tx valves job is to meter the refrigerant into the evaporator such that it all boils by the time it get to the outlet, too little and it will all turn to vapor too soon and reduce capacity, too much and you have liquid that can't absorb heat and could make it back to the compressor and damage it. Heat is absorbed when the refrigerant goes thru the phase change from liquid to gas. A too big Tx valve will have trouble controlling the refrigerant, will "hunt", and flood or starve the evaporator, and give poor cooling. The telling thing is the outlet will tend to be warmer then the inlet or the suction line will be frosting as liquid is still boiling as it makes its way back to the compressor. How much refrigerant is needed for any given time is determined by the heat load on the evaporator. As long as the inlet and outlet are about the same temp, the Tx is doing its job.
What all this means is: if the Tx valve is too small or too big it will effect how well it works. Kind of like putting too small or too big a jet in a carburetor, the engine will run but not produce as much power as if the correct one is installed.
Tom and I must have been typing at the same time!

[Cornbinder89]

I don't believe ANY of the block or "conventional" Tx valves used in trucks have an adjustable "super heat" like some do in industrial applications, Mainly due to the fact they are located very close to the evaporator, so have a accurate "read" on the evaporator.
The "block type" don't have an external sensing bulb, both the liquid and suction line are connected as is the inlet and outlet of the evaporator, all sensing, equalizing, and metering are done within the block. Furthermore, the size oof the fittings are such that it can not be put on backwards.

[Mamrak76]

It is a block type. Let me rethink this. I have poor cooling. The pressures are low. I added that 10 ounces and it didn't change the pressures at all. I am going to recheck the pressures again. I kind of want to add more until I get up to 30 low and 200 high. Where I want to see it at.

[Cornbinder89]

DO NOT charge to pressure! Pressures ONLY tell you the temp at the point of measuring. Example: today was 90, the outlet of my condenser was only warm to the touch, inlet was hot! I would guess by "feel" my outlet wasn't warmer than 110 deg, which would be 140PSI. It might have been cooler. This is the reason my A/C works so well, it sheds almost all the heat from the refrigerant. If I added until I saw 200 psi it would be way overcharged!
You've already proved that the system will get cold enough to frost over the evaporator. It can't get colder than that! Either you don't have enough air flow over the evaporator , or you have heat leaking into the system (duct work).
Over charging a system will reduce the effective area of the condenser, and raise the pressure causing more heat in the refrigerant that can't be shed by the reduced effective area of the condenser! To put it short and sweet, you'll make the problem worse!
It would be helpful to get a temp reading right at the evaporator with the fans running and the system operational.
The real telling thing, is that with the frost switch disconnected it got cold enough to ice. I don't remember if that was with the correct 2lbs or with 2lbs 10oz overcharge?
Many trucks have an air filter on the inlet air to the heater/A/C, does this truck? is it clear? Some trucks have dual blowers, are both functioning? It seams like either not enough air flow, or too much leakage in the air distributing duct system.
I have seen both heater and evaporator coated with dirt, dog fur, plant material such that air flow is reduced to the point it doesn't cool.
As to the low side, if you don't absorb heat in the evaporator, (i.E if there isn't sufficient warm air flowing over the fins) the outlet will cool, and the Tx will throttle back, and you will see low pressure. It just means there isn't sufficient heat load in the evaporator.
The long and short of it is: Pressures will tell you what the temp of the refrigerant is but not if the charge is correct or why the refrigerant is at the temp it is. You must use other means to figure out why.

[Cornbinder89]

I guess what I'm trying to say is: Your problem is not with the refrigeration cycle, but with the air handling, either air is not flowing over the evaporator (or not enough flow) or much of the air is bypassing and not being cooled. My advice is to restore the correct amount of refrigerant and look hard at the air flow thru the HVAC system, that is where I think the problem lies.
Sometimes worn bearings/bushings in the fan motor can slow the blower. Also most squirrel cage blowers are directional, turning them backwards will not reverse the flow but will reduce the flow by 1/3 to 1/2. So that is one place to look. Another is seals and air control doors. The Evaporator must be sealed so not air can flow around it rather than thru it. Doors must direct the air and shut off alternative paths for the air to flow. It may take pulling the dash apart or the heater box.

[worzel gummage]

Back again! So after my much needed vacation and my truck having transmission repairs it is back in my hands. To fill anyone else in, I was trying to get my ac system up on my truck. Weird pressures which were caused by holes. So I ended up needing a condenser. I also replaced the txv and reciver dryer. And a couple of busted up hoses. So now we are in business, well sort of. I pulled about an hour and a half of vacuum. My truck calls for exactly 2 pounds. So I added exactly 2 pounds. That put me at 15 low and 150 high at 65 vent temp. Also I have a very nice fieldpiece thermometer and I trust it. I am not one to over charge but that seemed low at 80 degrees ambient. So I added 10 ounces extra and that brought me to 20 low and 170 high with a 55 degree vent temp. Only problem now is my Evaporator is freezing up. The vent temp stays around 50 but then after a while it goes up. It seems to be freezing when i am idiling. Any suggestions?

Just a thought, - you did end up with the same spec condenser?? I know you have questioned the TXV you replaced, just wondered if the new condenser was 100% kosher?

[Cornbinder89]

Just a thought, - you did end up with the same spec condenser?? I know you have questioned the TXV you replaced, just wondered if the new condenser was 100% kosher?

You missed the part at the end where the Evap was freezing, Right or wrong the condenser must be doing well to cool the evap that much, so not the problem.