Ac weak at highway speeds

[TexasJ]

Hi guys, I have an AC problem I figured I'd submit to the hive mind here lol. I just had my truck into a local shop because my ac wasn't cooling enough. It was still blowing cold(ish), but if I sat at idle the temp at the vent would slowly climb. The shop evac'd the system and pulled the orifice tube, which they claim was full of metal shavings. So they replaced the compressor and drier, orifice tube, and Orings, and (supposedly) flushed the rest of the system before recharching with fresh freon and dye. This was tuesday of this week. Last night I went out of town (3 hrs away) and noticed that the AC, while still blowing cold(ish), is not cold ENOUGH. I also noticed that the fan speed is getting weak at about 70mph and higher, and at that point the temp at the vent will RISE. Below 70mph it's only getting down to about 58°F, and above 70mph the temp will rise to around 65°F or 70°F. If I sit at 70 or 75mph for longer than about 10 mins the fan will speed back up a little bit, then after a few minutes it will slow back down again, and back and forth it goes like that until I bring my speed under 70mph. It's about 100°F every day right now where I'm at in Texas, and I ain't built for this heat so I need to get this fixed ASAP. Any ideas on what could be causing this? I'm trying to decide if I need to take it into a shop where I'm at currently (3 hrs away from home), or if it's something simple I may try and fix it myself in the parking lot.

My possible theories are:
1)The condensor is dirty/clogged, causing it to ice up and cycle temperature like it is.
2) The shop either under charged OR overcharged the system.
3) The blower motor is going out and can't maintain speed with the force of the wind hitting it when at highway speeds.

Do any of those sound likely? Any other thoughts? Please let me know, thanks.

 

[DEEZUZ]

How is your fan clutch

 

[TexasJ]

How is your fan clutch

I'm not sure. How would I go about checking it?

 

[DEEZUZ]

It should have resistance colder, like try to spin it by hand (engine off).. Then when warm/hot that fan should be pretty much locked on, if I know a fan clutch is bad/worn, I'll use a piece of cardboard to try and stop the fan from spinning with engine running.

 

[TexasJ]

It should have resistance colder, like try to spin it by hand (engine off).. Then when warm/hot that fan should be pretty much locked on, if I know a fan clutch is bad/worn, I'll use a piece of cardboard to try and stop the fan from spinning with engine running.

Ok so I just got back from a drive around town. The engine is still pretty warm. I tried spinning it by hand with the engine off, and there was a decent amount of resistance, but I was able to spin it by hand in both directions. Then I turned the engine off and tried to stop it with a piece of very stiff cardboard, and it kicked the cardboard away like it was nothing. I tried 3 or 4 times and couldn't stop the fan from spinning (again, this was right after a drive so the engine was still warm). What does that mean?

 

[Powerstroke Cowboy]

Your fan was spinning?

Sorry, I couldn't resist.
In to see what comes of this. Never delt with this problem.

 

[Tiha]

So hard to troubleshoot over the internet.

But it sounds like under charged.

I would say over charged is possible but I don't think there is a high pressure cut out on these trucks.

Going down the road there is plenty of air going across the condenser, that is not your problem.

It would be really nice to see the pressures, but you can try this.

With the truck sitting still, rev the engine to 2000 rpms and hold it. Watch the ac compressor. If it starts and stops, the system is most likely under charged.

Because, when you rev the engine, everything works better. the compressor pumps and sucks harder. I would imagine it is pulling hard enough to trip the low pressure cutout.

AC systems should be charged with engine at 1100-1500 rpms. Most shops just charge it by weight to what the sticker says. With all the parts they replaced, the factory weights are going to be way off.

 

[Powerstroke Cowboy]

We charge based off ambient air temperature and pressure. Plus with the engine at working speed. Tractors at pto rpm and vehicles would be whatever you normally see at highway speeds. This method never fails as long as all components are working properly.

Another thing. To much oil in the system will not let it cool properly as well.

 

[Tiha]

Easy to remember tip.

The perfect freon charge on any ac system is when the inlet and outlet of the evaporator are the same temperature.

Sometimes it is difficult to reach the evap, but a quick feel with your fingers is a great diagnostic tool.

Needs to be past the expansion valve or orifice tube. Then anywhere on the outlet.

Car, truck, tractor, house, motorhome, refrigerator, window unit, boat. Anything with AC system that applies.

But you need to know your pressures so you are not over charging or to help you troubleshoot other problems.

 

[TexasJ]

Ok thanks for the input guys. I just realized I said I was wondering if the condensor was dirty or freezing up in my original post. I actually meant to say the evaporator. I've heard that the evaporator core fins can get dirty and clogged on these trucks, and that can affect the ac performance. Do you think that's likely? Or is it more likely that it's undercharged? I can check the pressures with my manifold gauge when I get home. I just haven't found a shop in the area I'm currently staying that seems trustworthy, so at this point I think I'm going to have to suffer through it until I can get home next week.

 

[TexasJ]

Update: I just went out to check if the ac compressor clutch was engaging as per Tiha's suggestion. Before I could even do that though, I realized that immediately upon starting the truck, that the fan wasn't blowing at all. The AC compressor clutch was turning initially, but then it stopped. I unplugged the ac compressor and plugged it back in, wiggled the wiring pigtail around, and it wouldn't turn on. I tried putting the AC on different settings, and tried different fan speed settings, still the blower wouldn't turn on. So then I wiggled the wiring pigtails around the blower motor and drier, and finally the compressor kicked on again. I don't know which wire did it, and I wasn't able to replicate the issue. So that left me even more confused than I already was. But since the ac compressor was staying on now, I decided to have my girlfriend rev it up to 2000 rpms and hold it like I initially planned. She held it there for a solid minute and a half, and the ac clutch stayed on the whole time with no issue. I also felt the tubes coming out of the evaporator doghouse, the upper tube and the lower tube, right where the emerge from the plastic. And from what I could tell they felt like they were pretty cold, and close to the same temperature. That said, when I felt that same tube closer to the orifice tube quick connect it was no colder than the ambient temp (aka it was warm). Not sure if any of this helps you guys to understand what's going on any better. Do you still think it's undercharged?

 

[Tiha]

Okay, sounds like the blower motor is not working?

Could just be a bad blower motor. You can hit it with a hammer and see if it starts up.

If the evap is freezing usually it will blow air flow. But if your blower motor is not working that won't matter.

The ac low pressure cutout is kind of in the area of the blower motor wiring as well, but it sounds like you need to get the blower motor working first.

 

[TexasJ]

Okay, sounds like the blower motor is not working?

Could just be a bad blower motor. You can hit it with a hammer and see if it starts up.

If the evap is freezing usually it will blow air flow. But if your blower motor is not working that won't matter.

The ac low pressure cutout is kind of in the area of the blower motor wiring as well, but it sounds like you need to get the blower motor working first.

The blower motor is working now though. I jiggled the wiring and it has been fine since.

Also, what do you mean "if the evap is freezing it will blow air flow"? Did you mean block?

 

[Tiha]

Yes I mean block. Usually if there is water dripping it isn't freezing up too bad. No water, might be freezing.

But seems like when they freeze up and ice over you shut the ac off for 20 minutes, it thaws out and works for 30 minutes again.

 

[TexasJ]

Yes I mean block. Usually if there is water dripping it isn't freezing up too bad. No water, might be freezing.

But seems like when they freeze up and ice over you shut the ac off for 20 minutes, it thaws out and works for 30 minutes again.

Ok, when they ice over like that, is that usually caused by being dirty and/or clogged? Or is it caused by something else?

 

[Tiha]

Ok, when they ice over like that, is that usually caused by being dirty and/or clogged? Or is it caused by something else?

Restricted air flow will cause it to freeze, like dirty evap. Or leaves or mouse nest in front of it.

Also a low charge can make them freeze up. But that doesn't happen very often.

 

[Powerstroke Cowboy]

Overcharged can cause them to freeze as well, right? My brother had that problem on his tractor. It would freeze then start blowing warmer air.

 

[Tiha]

Overcharged can cause them to freeze as well, right? My brother had that problem on his tractor. It would freeze then start blowing warmer air.

I suppose it is possible.

seen some weird stuff over the years. Watched the accumulators freeze up. (on big trucks they are before the evap. Not like cars where it is after.
Seen condensers freeze up half way across.
When you see those things it means there is a restriction in the accumulator/drier or condenser.

That is why I always go with a temp gun or just by hand. Feel the system from compressor pressure to return.

If the line going into the expansion valve or orifice tube isn't hot then you have a restriction upstream somewhere and you have lost a good portion of your cooling capacity.

Same goes for after the evap. If it gets colder between the evap and compressor you are either overcharged or you have a restriction after the evap which would defeat the orifice tube or expansion valve.