A/C does not cool when speeding more than 100kmph

Question

I drive a 1.3 liter 2001 Mitsubishi Lancer. I have this weird problem with the a/c where it stops cooling when I try going above 100kmph. The a/c cools just fine under 100kmph. But when I go over 100kmph, or accelerate above 3000rpm, it stops cooling. And once I go under 100kmph, it starts cooling again. There's also loud hiss that comes from somewhere inside the dashboard whenever this process takes place.

That is: "(accelerates above 100kmph) -> hiss -> cooling stops -> (decelerates) hiss -> cooling starts.

I live in Dubai and it gets pretty hot during the summer, and the roads I drive mostly have an average speed limit of 100kmph, so it's kinda important to me that my a/c cools both above and below 100kmph.

I've refilled the gas and also have replaced the compressor, but I still get this problem.

I saw a post about a Chevy having a similar problem (link below) and found out the issue was with the blower motor. Could my car have the similar issue?

A/C Does Not Continuously Blow Cold

Would appreciate if anyone could come up with a solution. Thanks!

Oh and it's always wet at the foot space of my front passenger seat. What's the deal with that? Could that be the a/c water leaking?

score 8 · Answer 1 · answered Jul 31 '15 at 14:12

Seems like the high-side pressure is too high

The refrigerant needs to be operating at the right pressures in order for the air-con to function properly:

If the pressure is too low, there is risk of compressor damage in addition to lack of cooling
If the pressure is too high, the effectiveness of cooling diminishes at the very least

To avoid hitting issues related to this, modern air-conditioning circuits are equipped with a pressure switch/sensor. Its job is to protect the system from self-destruction by preventing the AC from running under the wrong conditions.

Lancer-specific details

Based on this scrap of information on the ETCG forum, it seems that Mitsubishi utilizes both low-side and high-side pressure switches on the Lancer:

AC specs

While I cannot promise the values will work for your Lancer due to regional variations, the fact that there is a high-side cutoff can explain the AC not working above 3000 RPM.

What's so special about 3000 RPM?

Compressor speed depends on engine speed. The higher the engine speed, the faster the compressor spins. The faster a compressor spins, the higher its discharge pressure.

It looks like around 3000 RPM the compressor is able to nudge the high-side pressure past the allowable limit, causing the switch to intervene and disengage the compressor clutch (hiss...).

Where to look for the smoking gun?

Given the symptoms and maintenance history shared so far (non-OEM replacement compressor, refrigerant recharge), there are two possible culprits:

non-conformant compressor specs

Specifically, the compressor employed is not fit for the Lancer because it is increasing the pressure too much.
too much refrigerant in the system

More refrigerant, more system pressure. Less refrigerant, less system pressure.

Recommendations

Measure the low-side and high-side pressures of the system if possible
If possible, find detailed specs on your Lancer's AC system like in the image above. There is a good chance this information will be presented in the owner's manual.
If information found, have the system evacuated completely and have the right amount of refrigerant (by mass) refilled.
If information not available, have some refrigerant removed try to match the pressures on both high side and low side as best as possible.
If the difference between low-side and high-side is too much regardless of the amount of refrigerant used, chances are that the compressor is not a good match and it needs to be replaced with one more suitable for the Lancer.

All the best.

Shobin P · Answer 2 · 2015-07-29T13:42:43.257

There are four things that might cause this particular issue explaining in order of probability:

Some cars most notably Honda city/civic has an ECM cuts off switch which the ECU/ECM uses to stop the compressor of the AC to feed additional power to the engine for highway overtaking maneuver, If your car was like this since the day you bought it then no need to worry here.
Your AC Fan(condenser) is might be broken , i.e, Problem with the spindle, sometimes when the spindle is broken on travelling at high speeds , the fan is pushed backwards/inwards and this restricts the motion of the fan and it might slow down or stop causing the compressor to heat and causing the AC to shut down. another possible cause is with the electrics of the fan , possibly on high speeds something is coming loose and stopping the fan ultimately stopping the AC. I would suggest Checking the FAN FIRST.

score 1 · Answer 3 · edited Apr 13 '17 at 12:47

The "stops cooling at 3000rpm" is exactly what @Anarach stated, in that the clutch disengages above this to allow more power to the engine. It could also do this so as to not over spin the A/C compressor. If you're engine is hitting the 3000rpm at 100kph, this is probably the reason. I just read that your Lancer may have came equipped with a Constant Velocity Transmission (CVT). If this is true, you may be having issues with it, where it is not adjusting to allow the rpm range down to where it's supposed to be. This, along with the 3000rpm limit, would make for the issues you are talking about. A transmission shop would have to verify this. If this pans out as accurate, it would definitely be an issue and would mean there is nothing wrong with your Lancer's A/C system.

Your drain is plugged is why the passenger side foot well is wet. You'll need to go under the car, find the drain tube (should be off the firewall, on the passenger side, made of flimsy rubber), and stick a long thin rod up it far enough to dislodge the blockage. If you've ran the AC recent enough, you'll probably notice when the blockage is cleared as you'll get a "flush" of water come out of the tube.

The problem will not be like the included link (my question). It is totally different I would bet. The main problem there happens all the time, at any speed (engine or vehicle).

score 1 · Answer 4 · answered Oct 27 '15 at 07:57

Get your TPS (Throttle position sensor) checked. It senses the position of your throttle. If you press more than 70%, the system will disengage the AC to save power for the engine as it assumes that you are trying to overtake or climp up a hill. If your TPS is not not sensing the position of your throttle properly, it will definately turn off your AC at normal cruising RPM. My car does that when I am trying to floor the paddle but in normal mood, it keeps cooling even at 120+ km/hr.