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Help on understanding CCOT cycle frequency

wheelsup on Wed June 12, 2013 1:32 AM User is offline

Year: 1995
Make: Volvo
Model: 850
Engine Size: 2.4L
Refrigerant Type: R134a
Country of Origin: United States

Hello everyone.

I have a 1995 Volvo 850. I replaced the evaporator (common source of leak), accumulator, and orifice tube. Very little debris on orifice tube. Added roughly 4 oz of PAG 100 oil.

Sucked down system to 29" vacuum. Charge level on car indicated 1.63# of 134a or 26 oz. Added ~26-27oz using 12oz cans and a postage scale.

Volvos of this year do not have a high side hookup. You can to get some sort of an expensive adapter which I'm not sure where you'd get one so all I'm going off of is low side pressures from my manifold gauge set.

The car will, at idle, do about 25 PSI on the low side around 80*. With outside temps the same, when driving on the highway, I am finding the compressor is cycling every 15-17 seconds on. So it will run about 10 secs or so and will shut off for 6 secs or so. Then come back on. So 4x a minute. This seems rather frequent to me. It happens even when the evaporator is not cold and system is just turned on.

At idle the output temp is nice. Idle, doors/windows closed, full fan and rec on the output temp is 38*. This is in my garage while not moving. So performance wise, I am happy.

Hooking the gauges up, at idle like I said the PSI is around 25. Increasing the RPM to 2000+ gives a rapid suck down of the pressure and the low pressure switch cycles the compressor off somewhere around 20-22 IIRC. It will come back on at 40-ish PSI. Rinse and repeat.

So I believe the low pressure switch is working as required. When running, the clutch does not slip on the compressor and the unit itself is nice and quiet for an 18 year old compressor.

I'm wondering, is this how the CCOT cycle is supposed to work? It's going to be 90* tomorrow so I'm going to try it out as well to see how it performs in the warmer weather. But what I'm concerned about is how quickly the compressor is cycling even with the evaporator not cold - it was my understanding that as the evaporator gets colder the unit will cycle off on its own, but when first turned on the compressor should run constantly.

Presently it makes me think the system is still under charged.


iceman2555 on Wed June 12, 2013 10:16 AM User is offlineView users profile

It appears the system is undercharged. The method of recharge is not a sure fire approach to a correct recharge amount. Also the amount added does not allow for the amount of refrigerant to pre charge the manifold/hose assembly. The LPCO is probably cycling due to the drop in low side pressure due to the lack of refrigerant. Also the low side pressure at idle tends to support this.
Check the inlet and outlet temps of the evap. The temps should be the same or within a couple degrees of each other. If the outlet is warmer the system is undercharged and more refrigerant will be necessary to insure proper operation and lubricant migration.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

wheelsup on Wed June 12, 2013 2:50 PM User is offline

That's interesting. I had thought the same as you. OK. So I had about half the can left over. Added refrigerant and idle pressure is now around 30-32 psi. At 1500 RPM it indicates 25 PSI.

Outlet air temp at 84F ambient is roughly 41*F @ ~1500 RPM.

Driving the car compressor is still cycling as you get to above 2000 RPM and pressure will drop to the low end of the cut out switch and cycle. It happens less frequently but it still happening 3x a minute +/-.

I have added I would think roughly 30 oz even accounting for bleeding the lines and priming the lines I would think. Maybe not. It still feels like I am a little low. Adding the refrigerant this time around definitely made things perform better (temp was higher, but I believe only because the pressure was higher as lower temp = lower pressure).

It's also less noticeable in the cycling, not as much drag it seems.

I used aftermarket parts (accumulator and evaporator) so it's possible the items have a larger capacity than the original OEM ones.

FSM states:


1) Ensure compressor drive belt is okay, fan motor runs at all speeds, and that temperature vents shut completely with temperature switch in full cooling position.
2) Ensure that all air comes from panel vents with mode control on vent position, recirculation motor is working, and condenser fan and cooling fan are working.
3) Start and warm engine to normal operating temperature. Ensure compressor clutch engages when A/C is turned on. Set temperature switch to full cold position, place mode control to vent position, turn on recirculate air switch and blower fan switch high
4) Close engine hood, doors and windows. Run engine at 1500-
1600 RPM. Place thermometer in one of the center panel vents. Allow system to stabilize for at least 8 minutes. Check temperature in center panel vent. See A/C SYSTEM PERFORMANCE SPECIFICATIONS table.
Ambient Temp. F ( C) Outlet Air Temp. F ( C)
68 (20) ………………………. 41-48 (5-8)
86 (30) ………………………. 41-48 (5-8)
104 (40) …………………….. 46-54 (8-12)
(1) – Based on a relative humidity of 40-60%.


Application Specification
Compressor Type ……………….. Zexel DKS-15CH 6-cyl. Compressor Belt Tension (1)
Compressor Oil Capacity ………………… (2) 6.8 ozs. Refrigerant Capacity (R-134a) ……………… 26.4 ozs.
System Operating Pressures (3)
High Side …………… 406-450 psi (28.5-31.6 kg/cm )
Low Side ……………….. 25-33 psi (1.8-2.3 kg/cm )
(1) – Belt tension is maintained by automatic belt tensioner.
(2) – Use PAG Oil (Part No. 11 61 407-0)
(3) – Pressure switch cut-out points.

iceman2555 on Wed June 12, 2013 4:46 PM User is offlineView users profile

Adding and subtracting refrigerant to a assumed operational pressure is not a valid recharge method. Pressures are not an indication of a properly charged system, they are a diagnostic tool for a properly charged system. It can not be said enough that the proper recharge can save a drastic amount of labor time in diagnosis of a performance concern.
Since we are working with an unknown variable, it is necessary to determine if the system is charged properly. Try this, MAX COOL, HIGH BLOWER, BOTH FRONT DOORS OPEN, ENGINE AT IDLE. Determine the inlet and outlet temps of the evap. They should be equal or a 2-3 degrees different. A warmer outlet typically indicates an undercharged system. Could be a restriction in the system, this is why it is important to know the correct charge amount.
This could also be as simple as a LPCO switch malfunction. But once more, the system charge amount needs to be determined or take a chance an purchase a new LPCO. There is a specific time sequence for cycling, check your OE spec's for this info. However, rapid cycling tends to be a charge issue...or a malfunctioning switch.
You can also measure the temp of the discharge line and utilize a pressure/temp chart to determine the high side pressures. Always good to know.
Good luck

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

wheelsup on Sat June 15, 2013 12:58 PM User is offline

OK. So I thought I had a firm grasp of the a/c system. Really, I did. What I realized is I know enough to be dangerous (famous last words).

I understand the importantance of weighing in the charge more so than ever, and why pressure readings really means squat. The next day temps were in the 90's and the pressures were MUCH higher. 50-55 PSI on the low side w/ the recirc off. With the recirc on I was seeing cooler temps and lower pressures.

I measured the evap temp. It corresponded with the pressures I was seeing using a PT chart. Both inlet and outlet were exactly same temp.

I'm really confused here. I might take it to my local shop. He does good work. I did the brunt of the work replacing the evaporator on this car. He normally charges about $100 to suck it up and refill it, so he is very reasonable.

mk378 on Sat June 15, 2013 1:41 PM User is offline

Inlet and outlet the same temp means you're real close to a proper charge, or possibly over charged. Don't add any more. Was your 55 psi at idle or revved up? Measure pressures at 1500 rpm. Idle performance on many cars is never very good.

wheelsup on Sat June 15, 2013 6:50 PM User is offline

The 55 PSI was at idle. Rev'd up it was roughly 40-45 IIRC.

If I put recirc on the pressure drops as the temp is much lower as well.

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