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Discussion Starter · #1 ·
I used to have the hot weather worn clutch behavior on my '11 V50 T5, which eventually started short cycling even when the engine is cold, on 7 seconds, off 20-30 with lukewarm air. Using an AutoZone kit I didn't seem to have low freon. Did the reshimming job on my own (had to take all shims out) and cleaned the low pressure sensor connector and no better. Up to my ears in babies now so I took it to a trusted indie shop who insisted I needed a new compressor. I bit, as much to make it their problem as anything, and to get a 3yr warranty on it, since I'd been out of shims anyway.

Now I've got the car back and it is short cycling just the same, and I'm no better off. Before I ask them to try again, what else could it be? It has a full recharge and no leaks, new compressor, drier, and expansion. Wouldn't a faulty pressure sensor or other electrical gremlin just keep it from coming on at all, instead of kicking on for 7 seconds and off for 30?
 

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Did they actually refill the car with the right amount of refrigerant or put the same amount back in that you had? If you were very low on refrigerant it would cause a similar symptom.
 

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Discussion Starter · #4 ·
Did they actually refill the car with the right amount of refrigerant or put the same amount back in that you had? If you were very low on refrigerant it would cause a similar symptom.
Yeah, they did a full evac and recharge with dye. Probably a couple of times during troubleshootin and compressor replacement. It does seem like classic low freon but it just isn't at this point.

Can a relay cause short cycling? I'd always thought they just failed open or closed.
 

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Sounds like a leak to me, maybe there was a leak somewhere else that nobody has found yet.

The gauges should show high side and low side pressure and allow the tech to see what the pressures are when the compressor is kicking on and off.
 

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Discussion Starter · #6 ·
The gauges should show high side and low side pressure and allow the tech to see what the pressures are when the compressor is kicking on and off.
Could it be a faulty low pressure switch? Is there a resistance spec or other way to troubleshoot it without swapping it out? Again I'd guess it to fail open or fail closed but perhaps it can be more subtle.
 

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The idea with the low pressure switch is, if the low side pressure gets too low, the compressor cuts out, right?

I'm assuming you could look at the low-side gauges and if it's got a reasonable amount of pressure (~50 psi or so, varies based on ambient) on the low side and it's still cutting out - and shorting across the switch causes it not to cut out - then the low pressure switch is bad.

More likely, you'll see the low side pressure gets really low, then the switch cuts out, then the pressure builds up, then the compressor kicks on, then the pressure gets low...

I'm assuming the orifice tube was replaced at the same time as the other stuff? I can picture a clogged tube leading to low low-side pressures (and high high-side pressures)...

This is the kind of stuff I think an A/C professional should understand and be able to sort out...
 

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In this case, you would want to use VIDA or a high level scan tool to monitor what the pressure switches are doing when the AC is on.
You also want to pick up the reading from the evaporator temperature sensor: too low Tº will make the compressor cut off.
The evaporator sensor can also be tested with an ohmmeter. VIDA has a table for resistance vs temperature for it.
 

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These are the values for the evaporator temperature sensor. You dismantle the lower dash as per the hvac filter change, and the sensor is mounted a couple inches from the filter door, towards the rear of the car. Twist and pull to get it out.

resistance between terminals 1 and 2

-55 °C = approximately 288.9 kΩ
-50 °C = approximately 201.0 kΩ
-45 °C = approximately 141.5 kΩ
-40 °C = approximately 100.9 kΩ
-35 °C = approximately 72.8 kΩ
-30 °C = approximately 53.1 kΩ
-25 °C = approximately 39.1 kΩ
-20 °C = approximately 29.1 kΩ
-15 °C = approximately 21.9 kΩ
-10 °C = approximately 16.6 kΩ
-5 °C = approximately 12.7 kΩ
0 °C = approximately 9.8 kΩ
5 °C = approximately 7.6 kΩ
10 °C = approximately 6.0 kΩ
15 °C = approximately 4.7 kΩ
20 °C = approximately 3.8 kΩ
25 °C = approximately 3.0 kΩ
30 °C = approximately 2.4 kΩ
35 °C = approximately 2.0 kΩ
40 °C = approximately 1.6 kΩ
45 °C = approximately 1.3 kΩ
50 °C = approximately 1.1 kΩ
55 °C = approx. 895.9 Ω
60 °C = approx. 746.4 Ω
65 °C = approx. 624.9 Ω
70 °C = approx. 525.6 Ω
75 °C = approx. 444.4 Ω
80 °C = approx. 377.4 Ω

These are the values for the interior temperature sensor; you might as well check it too:

terminals 3 and 5

-40 °C = approximately 207.1 kΩ
-30 °C = approximately 118.9 kΩ
-20 °C = approximately 70.8 kΩ
-10 °C = approximately 43.6 kΩ
0 °C = approximately 27.6 kΩ
10 °C = approximately 18.0 kΩ
20 °C = approximately 12.0 kΩ
25 °C = approximately 9.9 kΩ
30 °C = approximately 8.2 kΩ
40 °C = approximately 5.7 kΩ
50 °C = approximately 4.0 kΩ
60 °C = approximately 2.9 kΩ

Ideally, you would use the scan tool to view all the sensors and switches simultaneously. Then you would go AAAHH, this *** part is doing it!!
 

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To be honest I think this is normal behavior on the S40. I've owned mine for almost 6 years now and it has always been doing that - cycling every 10-12 seconds in moderate ambient temperatures but it blows cold. In hot summers it would stay on further and wouldn't cycle as much until the interior gets really cool.
edit: I forgot to mention that when I turned the fan up to higher speeds it'll cycle less often.
 

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Discussion Starter · #11 · (Edited)
Thanks all! They did replace the orifice tube, probably on the same hunch.

I may take my autozone low pressure gauge and a paperclip and multimeter out there to try to assess the low pressure switch. If it's wonky, can it be replaced without discharging the system? I know there's often a check valve there.

Otherwise I may give up on this shop and take it to an AC specialist and/or the local dealer who has a better idea of where to look. I don't think they're jerking me around but if they don't know how to fix it, giving it back isn't going to get me anywhere.
 

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To be honest I think this is normal behavior on the S40. I've owned mine for almost 6 years now and it has always been doing that - cycling every 10-12 seconds in moderate ambient temperatures but it blows cold. In hot summers it would stay on further and wouldn't cycle as much until the interior gets really cool.
edit: I forgot to mention that when I turned the fan up to higher speeds it'll cycle less often.
I'm inclined to agree, but haven't been in OP's car. Let's take July-August in Tampa. My C30's AC is interesting in that it works fabulous if the car's been in the shade--I'll usually notch up 4 on the temp and leave the fan at one notch for the ride. If it's been in the sun it struggles vice where it would be if it'd been in the shade. Got super dark tint on the windows and use a THICK console/windscreen shade. There were times when I was ready to take it into the dealer, but after cooling off it works great. Has been like that since day 1. I don't require super cold air to survive, so I can deal with it. I'd think someone who prefers ice cold air would have gotten rid of the car years ago. The V60 is completely different--no issues cooling, just like a 'normal' car.
 

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My C30's AC is interesting in that it works fabulous if the car's been in the shade
There might be an issue with the "sun sensor". Inside the little bump in the center of the dash where the red alarm light sits, there are also two light sensors - one for sunlight and one for twilight. The twilight one is used to switch the dashboard and DRLs to nighttime mode. But the sunlight sensor feeds into the A/C algorithm, along with things like outdoor temperature, windows-open, etc.

It'd be difficult to test. However, if you see any kind of physical issue up there, you might want to address it.
 

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I'm inclined to agree, but haven't been in OP's car. Let's take July-August in Tampa. My C30's AC is interesting in that it works fabulous if the car's been in the shade--I'll usually notch up 4 on the temp and leave the fan at one notch for the ride. If it's been in the sun it struggles vice where it would be if it'd been in the shade. Got super dark tint on the windows and use a THICK console/windscreen shade. There were times when I was ready to take it into the dealer, but after cooling off it works great. Has been like that since day 1. I don't require super cold air to survive, so I can deal with it. I'd think someone who prefers ice cold air would have gotten rid of the car years ago. The V60 is completely different--no issues cooling, just like a 'normal' car.
Funny that's exactly how I dial my temp - about 4 notches, sometimes even 5 or 6 but mostly at 4 with the fan down to 1.
 

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Discussion Starter · #15 ·
Well, I checked the low side pressure and it was quite high with the compressor on, and I jumpered out the low pressure cutoff switch and it kept short cycling, so that's off the list. I don't have VIDA and my Bluetooth ODBII device couldn't get to any of the AC sensors using my usual Android apps like Torque. So, I asked the shop to take another look since they've got me for four figures already, will post if they find anything.

As to the normal behavior, I was used to it cycling but when demand is high it stays on for minutes at a time. I'm an Auto AC mode guy and it won't hold temp on a warm day anymore, and the air is muggy from being underconditoned even when set to Lo / max cool recirc.

The car is a 2011 that I got in 2015 and the air has always been ice cold until like last year when the clutch gap cycling got bad. I corrected it and this other short cycle was there (distinct since it's always short even on a cold engine, where the clutch gap problem was only after the car gets hot), likewise still there with the new compressor/clutch. Wonder if the few years of clutch gap cycling got something else to wear out?
 

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Well, I checked the low side pressure and it was quite high with the compressor on, and I jumpered out the low pressure cutoff switch and it kept short cycling, so that's off the list. I don't have VIDA and my Bluetooth ODBII device couldn't get to any of the AC sensors using my usual Android apps like Torque. So, I asked the shop to take another look since they've got me for four figures already, will post if they find anything.

As to the normal behavior, I was used to it cycling but when demand is high it stays on for minutes at a time. I'm an Auto AC mode guy and it won't hold temp on a warm day anymore, and the air is muggy from being underconditoned even when set to Lo / max cool recirc.

The car is a 2011 that I got in 2015 and the air has always been ice cold until like last year when the clutch gap cycling got bad. I corrected it and this other short cycle was there (distinct since it's always short even on a cold engine, where the clutch gap problem was only after the car gets hot), likewise still there with the new compressor/clutch. Wonder if the few years of clutch gap cycling got something else to wear out?
I know this question might seem a little off.....but....when's the last time you replace the pollen filter underneath your dash?
 

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Discussion Starter · #17 ·
I know this question might seem a little off.....but....when's the last time you replace the pollen filter underneath your dash?
Less than a year ago...I had a shop do it during an oil change since it's a pain. I guess there's an evap temperature sensor down there somewhere, do you think maybe they could have knocked something out of whack? Never seen its location myself.
 

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It is very easy to check it. Just grip the cover by the sides behind the waterfall, I believe you either slide it up or down to remove and cover exposing the 'sensor' on that slot seen here (top above the torx screw). My car does not have Auto Temp therefore it is empty. Try cleaning it as it can get pretty dirty.

 

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Discussion Starter · #20 ·
To close this out, the shop replaced the evaporator temperature sensor (not the interior one) and it corrected the problem. Wish they had tried that first! Even so, I'd had to remove all of the shims from the compressor clutch to get the gap back to spec so it was only a matter of time. Thanks for all the tips.
 
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