ShiftRX7

Hi guys, just decided to try my A/C system and have a weird issue:

- everything works fine at idle (A/C blows cold air, fans run at the correct speed, stable idle); same goes if i rev on neutral
- if i start cruising the car stumbles, it feels (and sounds) like the compressor is rithmically engaging/disengaging

havent used the the A/C for a looong while (car has been sitting while engine was replaced and some other work done); i currently run on the PFC.

Any suggestion on what could be wrong it's highly appreciated.

Thanks!

XLR8

iTrader: (52)

Though rhythmically engaging, does the A/C still blow cold? The system could be overcharged causing the high pressure switch to constantly kick off the compressor. You could also have a clog causing the same issue. The stumble is likely just the compressor rapidly engaging and not the car's "tune" itself.

I would consider evacuating the system completely, and starting with a fresh and proper charge. With A/C, it's often better to start from ground zero. Put the system under a vacuum for a few hours and get all the old stuff out of there.

I'm not sure if you have switched to R134, but I highly recommend ES-12.

Enviro-Safe Refrigerants

ShiftRX7

it's surely the compressor cos i hear the noise it makes when it engages...i haven't realized if still blowing cold as i was too concentrated on the stumbling, i'll check on that. Actually the charge is brand new, since everything was pulled off the car for the engine rebuild and the system evacuated. btw, i'm still using the old R12. so you think it could be too much charge?

JM1FD

iTrader: (1)

At higher vehicle speeds and in cooler weather it is perfectly normal for the compressor to turn on and off. There's a switch to shut the compressor off for a little while when the evaporator gets near freezing. When cruising, there's plenty of air coming through the condenser so you can reject tons of heat. The compressor is turning faster than normal, so it is able to move a ton of BTUs. This means the evaporator can get really cold....which is fine until it starts forming ice.

A low cabin fan speed and recirculate turned on on a somewhat cool day will make it cycle more. A high cabin fan speed on a hot day will make it cycle less, if at all. Again, this is at cruise. At idle it shouldn't really cycle unless it is fairly cool outside since the compressor isn't turning very fast.

How long are these cycles? 30 seconds on, 20 seconds off? 2 seconds on, 3 seconds off?

When they recharged it, how did they determine how much to put in? On my car, the amount that the under-hood sticker calls for is a couple ounces too much. I clear the sight glass then add another ounce or two.

JM1FD

iTrader: (1)

It should be noted that ES-12 contains a large amount of Propane. A substance that is quite popular due to its flammability.

ShiftRX7

the cycles are probably 2 seconds long: goes on for a fraction of a second then disengages and after 2 seconds tries to engage again...it keeps blowing cold air by the way; i don't have a sticker with the amount info, is there a reference in the manual on how much refrigerant needs to be inputed in the system?

finally, weather it's not pretty cold right here right now, temps are in the low 70s.

JM1FD

iTrader: (1)

Yep....a split second on and two seconds off is waaaay too short of a run time. When this happens, can you hear a faint click under the dash at the same time the compressor comes on and off? If so, that's your thermoswitch turning on and off too fast.

I believe the manual does state a charge amount....somewhere around 20 ounces.

ShiftRX7

I've been reading the manual a bit and the behaviour sounds like the thermoswitch disengaging the clutch (as suggested).

I have also checked charging specs but i'm not sure how much refrigerant the guy put in the system, i'll check back with him this saturday.

Question is now, what are the possible causes of this improper behaviour?

- is the thermoswitch known to fail?
- wrong gas charge can cause excessive load on the compressor thus raising temp so quickly that the thermoswitch blocks the compressor? if that is the case i will assume that too much refrigerant was put in the system (less gas to compress, less power required, less heat produced?)

thanks again for all the help provided so far.

jm2fd

-

JM1FD

iTrader: (1)

Can you hear the faint click that I mentioned previously coming from under the dash? In my car, I can hear the thermoswitch turning on and off.



It isn't a common failure, but it is an electromechanical device, so it is possible.

No.

There are actually two thermoswitches that can cut the compressor off:

1. If the temperature of the vane housing of the compressor gets too HIGH (I don't think this one is in any of the wiring diagrams, but nevertheless, it is there).
2. If the evaporator temperature gets too LOW.

There is also one pressure switch that can cut the compressor off if the high side pressure is too high or too low. This is the one that would come into play if you have a massive silly huge overcharge. But this switch would definitely cut the compressor off if you hold the engine RPMs at cruising speed while sitting still in the driveway for a minute or two. You said that isn't happening.

ShiftRX7

There have been a couple of stormy days here and i've not been able to check on the a/c yet; will get back as it gets better and i can do some additional testing.

ShiftRX7

Finally i was able to clear things a little bit; the conclusions i draw in the beginning are only partially true because the main issue appears to be that there is a problem when the A/C is on and fans try to kick in cos water temp reaches the target temperature set with the PFC.

My previous conclusion was only partially true, in fact it was like that since i never reached the 89°C (that is my setting for the fans to kick in) while stayin still at idle and the A/C was working properly (cold air, compressor going on and off at normal rate, can hear the thermoswitch activating, fans running with level 1 speed).

When temp reaches or surpasses 89°C (just realised after a good half an hour in stop&go urban traffic) the A/C seems to go crazy; i can hear a strange noise, like something that is trying to be sucked off of something but with not enough power, so the sucking force is released for half a second and then starts again. The noise is pretty intense (much more than the clicking of the thermoswitch) and it's this thing that make the car stumble, as rpm drop when it's trying to suck and jumping up when it's releasing.

Any idea of weird interaction of this kind is known? thanks in advance for any suggestion on what to look for.

Cheers,

G

JM1FD

iTrader: (1)

If the cabin blower is on, and the A/C button is depressed, you should always have radiator fans, usually LOW unless there is another "request" for fans from somewhere. This is regardless of the state of the compressor clutch.

If the ECU is calling for fans while the above conditions are met then you should get MEDIUM speed fans.

Can you verify that your fans speed up as they should when these conditions are met?

I'm wondering if your MEDIUM speed is broken and the fans are just stopping when the additional request comes through from the ECU. That will **** off both the A/C and the engine....

ShiftRX7

yep i know abount fans control; fans are usually working fine: they are at speed LOW when water temp reaches the desired level (which is 89°C in my case as specified above); they are on at LOW when A/C is depressed and we are below target setting; will need to check on what happens when it start creating problems and the two conditions are both met; btw, never had the opportunity of checking if the third condition works properly (reaching the thermoswitch temp, which is at 95°C) cos i never reached temp higher than 90...

will update tomorrow with the result of the test.

ShiftRX7

Here i am again...today i run some more extensive tests to better understand the issue; a few conclusions:

1. it's not related to fans control: in fact, fans work properly at all differents speeds and responds correctly to each input

2. it's not related to temperature: it happened at a wide range of different temps.

3. it's not related to idle or cruising: again, i was able to replicate the issue in both cases

Trying to clarify things a little bit, i made a short video of the issue presented at idle in which you can see the idle bouncing up and down and can hear the weird sucking/releasing noise i was trying to describe. here it is:


thanks again everyone for your help!

G

JM1FD

iTrader: (1)

Wow. I'm stumped! I'll throw out some more ideas....

So there's only two moving parts in/on the evaporator housing down there....the expansion valve and the thermoswitch. It sounds in the video like it is coming from inside the housing, so that would lead me to think something is amiss with the expansion valve? But the moving part in an expansion valve is of very low mass and doesn't have just a huge range of motion. I can't fathom it making a noise that loud....but anything is possible.

I would be curious to see what the high and low side pressure readings look like while this noise is occurring.

Maybe pull the glove box and see if you can pinpoint the source of the noise better? That should give you access to the thermoswitch.

It could also be that one of the refrigerant lines is moving around and hitting something as the system pressure goes up and down due to the clutch cycling on and off.

ShiftRX7

The weirdest thing is that it doesnt always do that, but after it starts it keeps doing that. Will try to track down the source of the noise but i don't think it's one of the lines, as i checked the bay to see if something abnormal was going on but noticed nothing...moreover i would expect a sharper sound in that case...let's see if i can get to something more specific about the noise.

anybody else who wants to step in the conversation is welcome! ;-)

XLR8

iTrader: (52)

Definitely need to get a set of A/C gauges on it. Monitor your high and low pressures as things are happening.

As mentioned, you can pull the glovebox to get a closer ear of what the noise source may be.

Definitely weird...

ShiftRX7

Hey guys, i finally got the solution for this and i wanted to share it with everyone in case it happens to somebody else.

Basically the issue was only generated by an incorrect amount of filling in the A/C system; too much gas was present and the compressor couldn't quite get its job done, causing pressure to rise quickly and the compressor to disconnect quickly, and then trying to come on again. Simple as that, just a correct charge according to the standard amount and it has been working fine for a hundred miles.

Thanks everybody for your support.

G

IRPerformance

iTrader: (41)

Most likely a pressure problem. Verify the system is not overcharged. Otherwise it is most likely the pressure switch. One off a miata works.

JM1FD

iTrader: (1)

The pressure switches on the FD can be removed?? I thought it was welded to the line...