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Old 02-13-2020, 06:34 PM   #1
Fresh Air Inspector
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Default Cooling Fan Staying On

Hello All,

The electric cooling fan on my 1994 940 is staying on after the car is shut down. I've traced the problem to a bad low pressure switch (bottom of the AC condenser). Pity as it's less than 2 years old.

As a stop gap I've unplugged the low pressure switch. Being winter here in the Great White North (aka Canada), I don't believe it will adversely affect the AC performance (only kicks in during Defrost cycle) or the cooling system temperature.

My understanding of how the cooling fan operates is as follows;

Engine Cooling
If the temp sensor in the cylinder head is triggered the ECU switches on the cooling fan. I assume this is high speed mode.

AC Cooling
The cooling fan is triggered by the pressure in the AC system. This is linked to the AC compressor engaging and/or the rise in R134a pressure in the system. Where I draw a blank is the purpose of the low and high pressure switches in the condenser.

So, hopefully someone can help me to understand how the low and high pressure switches function in relation to the cooling system as well as the AC system.

As always, any insight will be appreciated.
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Old 02-13-2020, 07:38 PM   #2
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https://www.myvolvolibrary.info/Tech...ction-3-39.pdf
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Old 02-13-2020, 09:47 PM   #3
Fresh Air Inspector
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Hello PCH,

Appreciate the quick response. I have all the Mitchell wiring diagrams but having the factory diagrams is always a benefit.

What I'm missing is the explanation of how the cooling fan circuits operate. The diagrams show how everything is connected but not what triggers the sensors and how the various components react.

For example, what causes the low pressure switch to close and what happens next.

Thanks again.
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Old 02-13-2020, 10:57 PM   #4
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I have a '85 240 that's been swapped to the '93 R134a A/C, and I know how it works. For the 940, I'm guessing based on the diagrams/wiring.

There are 2 fan circuits, low and high, plus a row of colored pressure sensors (pressostats) on the condenser (violet, gray, brown), and another pressure sensor (black) on the accumulator.

The LH2.4 ECU can turn on the low or high fan based on coolant temperature (if it's regina, the ECU can only turn on the low fan). 2 of the condenser pressure sensors are also wired to the low and high fan circuits. I'm guessing that, as pressure increases, the fan and sensors go from off, to low, to high.

For the A/C, the low pressure sensor on the accumulator turns on and off the compressor - it goes on when pressure goes low, off when accumulator is refilled and pressure has gone up.

The third highest-pressure sensor on condenser is wired in series with the compressor. It's an overpressure safety. If the pressure gets too high, the compressor is disconnected until the refrigerant cools off and pressure drops.

https://www.skandix.de/en/search/?q=...20switch&k=668

You should be fine unplugging the fan pressure sensors during the Winter. You want to keep the A/C overpressure sensor plugged in or else your A/C Compressor will never engage (and it will still disengage if pressure ever gets too high).
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Old 02-14-2020, 07:21 PM   #5
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Hello Bobxyz,

Many thanks for the detailed explanation as well as the link to Skandix. I lived in the Netherlands for 16 years and ordered from them frequently (I had a SAAB 99 and a Mercedes W126 at the time). Great company to deal with.

I did an R12 to R134a conversion on my Mercedes. It was a complex one including replacing the original AC Delco compressor (yes, Mercedes used GM hardware in their vehicles), with a Sanden compressor. Worked well when completed. :-)

I had replaced the condenser and all three pressostats in May of 2018 so I'm not pleased that the grey one packed it in so soon.

If I recall correctly the pressostats can be removed / replaced without losing AC pressure?? The threaded fittings in the condenser have Schrader valves built in. Could you confirm?

Volvo still stocks the grey pressostat (p/n 684532), it is also available from eEuroparts (link below)

https://www.eeuroparts.com/Parts/754...-Gray-6848532/

All the best.
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Old 02-14-2020, 09:14 PM   #6
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Quote:
Originally Posted by Fresh Air Inspector View Post
If I recall correctly the pressostats can be removed / replaced without losing AC pressure?? The threaded fittings in the condenser have Schrader valves built in. Could you confirm?
I _think_ you're right, but I've never tried it. I'm not sure if there's any way to clean a misbehaving sensor. Seems like it would be worth a try. If you're R&R'ing an old crusty sensor (not your case), make sure you have a nice soft new o-ring for the re-install -- o-ring $0.50, full refrigerant re-charge $$$.
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Old 02-15-2020, 04:16 PM   #7
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I've had the same problem. Twice. I've replaced the pressostat only to have it happen again. Both times the fan has run until the battery is dead. Finding a new pressostat switch has proved difficult, so I have a few spares I've pulled from junkyard cars.

My solution: I wired up a relay in between the pressostat and the cooling fan relay. I used the low pressure switch on the receiver/drier as the trigger. That way, the relay is only closed and potentially supplying power to the cooling fan when the ac is switched on and operating.
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Old 02-17-2020, 06:18 PM   #8
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Hello Bobxyz,

I checked an old condenser and the pressostat sensors can be removed without loss of R134a. All three sensor fittings on the condenser contain a Schrader valve (car tire valve), so all you will lose is a small puff of R134a upon removal. :-)

Hello Nyalex,

Not fun when you have 'new' parts fail so quickly. I assume they were aftermarket and not Volvo items? Lots of reports surfacing about the 'questionable quality' of aftermarket parts. If you are interested Volvo still stocks these switches, the grey one is p/n 684532 and around $45 USD / $60 CAD (prices as of Feb 2020).

The 'grey' pressostat switch that failed on my car was a Santech item purchased from FCPeuro some 18 months ago. Santech typically makes decent products so I will chalk this up as a one off failure but I will replace it with a Volvo part (almost the same price) and keep an eye on the other two switches (also Santech and replaced at the same time as the grey one).

Your bypass solution is one that I was considering. The only downside I can see is that the cooling fan is operating at highway speeds when you don't really need it. Although I'm not a fan of electronics, the use of pressostats does allow for more accurate management of the cooling fan cycles as it relates to AC use.
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Old 02-18-2020, 08:21 AM   #9
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Quote:
Originally Posted by bobxyz View Post
I have a '85 240 that's been swapped to the '93 R134a A/C, and I know how it works. For the 940, I'm guessing based on the diagrams/wiring.

There are 2 fan circuits, low and high, plus a row of colored pressure sensors (pressostats) on the condenser (violet, gray, brown), and another pressure sensor (black) on the accumulator.

The LH2.4 ECU can turn on the low or high fan based on coolant temperature (if it's regina, the ECU can only turn on the low fan). 2 of the condenser pressure sensors are also wired to the low and high fan circuits. I'm guessing that, as pressure increases, the fan and sensors go from off, to low, to high.

For the A/C, the low pressure sensor on the accumulator turns on and off the compressor - it goes on when pressure goes low, off when accumulator is refilled and pressure has gone up.

The third highest-pressure sensor on condenser is wired in series with the compressor. It's an overpressure safety. If the pressure gets too high, the compressor is disconnected until the refrigerant cools off and pressure drops.

https://www.skandix.de/en/search/?q=...20switch&k=668

You should be fine unplugging the fan pressure sensors during the Winter. You want to keep the A/C overpressure sensor plugged in or else your A/C Compressor will never engage (and it will still disengage if pressure ever gets too high).
The short version of what Bob said: all these condenser and dryer switches are GROUND paths which enable the fan relay to run. When all the ground paths in the condenser are removed, the only ground left to engage (in the 9 series) comes from the ECU.
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Old 02-18-2020, 10:10 AM   #10
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Quote:
Originally Posted by DET17 View Post
The short version of what Bob said: all these condenser and dryer switches are GROUND paths which enable the fan relay to run. When all the ground paths in the condenser are removed, the only ground left to engage (in the 9 series) comes from the ECU.
Unless it's Regina powered (not applicable here, but I'm posting for future readers). The Regina cars simply use the temp switch in the radiator end tank to turn on the high speed fan. They also use the pressure sensors in the condenser to turn on low speed (and maybe high speed depending on pressure) with AC on.
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Old 02-18-2020, 03:14 PM   #11
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On the (2) 940's that I've owned, the A/C system also played a part in the windshield defrost system. If the defrost is turned on, so is the A/C system.
No dry A/C air........less windshield defrost......or de-icing.
Something to consider.
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Old 02-19-2020, 01:47 PM   #12
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Quote:
Originally Posted by iHateVolvoPeople View Post
Unless it's Regina powered (not applicable here, but I'm posting for future readers). The Regina cars simply use the temp switch in the radiator end tank to turn on the high speed fan. They also use the pressure sensors in the condenser to turn on low speed (and maybe high speed depending on pressure) with AC on.
That could very well be true; I've got zero experience with Regina cars... thus far!

THEREFORE, my comments apply to the 90+ 7 series, and 92+ 9 series LH2.4 vehicles.
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