98 V70: A/C Comp Thermal Switch?

Post by **sleddriver** » 05 Jul 2017, 19:02

Recently I've noticed the sled's A/C will greatly diminish when the ambient temp gets hot enough, about 104°F. It will blow barely cool but that's it. Once up to speed for a bit, it'll suddenly begin to blow ICE COLD again. Though the evap core has a leak, I don't suspect low refrigerant per above.

I have 'zip-tied' the clutch awhile ago to reduce the gap. The A/C compressor is now 19yrs old as is the clutch. Definitely is heat related. While searching the RDB, I noticed the note on 850's having a compressor thermal switch. Does the 98? I'm still waiting for both outdoors and the sled-motor to cool off (hood up). If so, I'll bypass it and see if that was it.

Perhaps the compressor clutch just needs to be replaced, regardless of the zip-tie-trick?

I'm also thinking the compressor relay is tango uniform. It too is now 19yrs old. However, if it was faulty then I'd have NO COLD AIR, rather than 'barely cool'.

Some component is giving up when it's too hot, then suddenly coming back to life once it cools off, with more airflow, like when I finally get out of stop-n-go traffic and am able to drive 60mph for a few minutes.

Any ideas?

Post by **abscate** » 05 Jul 2017, 19:34

Isn't 104 a sign from the Lord to not live somewhere?

That is way too hot for me.

You may well just me down on charge.Wait\ for the civilized 90 F weather and check your temp delta on your system after 15 minutes of equilibrium at 1500 rpm. You want 35-40 F of delta

Post by **sleddriver** » 05 Jul 2017, 19:43

HA! That's just outdoor ambient...in the shade. The solar heat index was a bit over 130°F. We're baking in Fajita Flats. You yankees don't want to come here during the Summer: You'll melt......

Didn't have my vent thermometer with me. It's now back in the sled for temp measurements. I've experienced a low charge condition before. It was verified due to vent temp being too high and no sweating of the suction line. Currently experiencing the former, not the later.

Your sig shows a 99....very different animal.

Post by **sleddriver** » 05 Jul 2017, 20:48

Lo & behold, my A/C compressor indeed does have a thermal switch attached to it. When I read in the '95 850 article 'they'd been eliminated in subsequent years', I thought Volvo might have done so by '98. The sled was born in Aug '97 however though sold as a '98.

Tomorrow morning, first thing, I'm bypassing it.

Post by **cn90** » 05 Jul 2017, 20:55

Do a temp drop check, a drop of 40-50 deg F is healthy.

Post by **jimmy57** » 05 Jul 2017, 21:15

The thermal switch shuts the compressor off, when that happens it quits cooling PERIOD, until compressor cools down. If the switch shut compressor off the vent temp wouldn't feel like barely cooling it would be ambient plus.
The switch, when it does trigger, usually does it above idle when the compressor is pumping 100% and able to produce the heat that opens the switch. What does it do if you run engine at 1200 or more RPM? weak compressors show first as loss of cool at idle when the compressor is already a good bit below 100%.

Post by **sleddriver** » 05 Jul 2017, 22:31

jimmy57 wrote: ↑05 Jul 2017, 21:15 The thermal switch shuts the compressor off, when that happens it quits cooling PERIOD, until compressor cools down. If the switch shut compressor off the vent temp wouldn't feel like barely cooling it would be ambient plus.
The switch, when it does trigger, usually does it above idle when the compressor is pumping 100% and able to produce the heat that opens the switch. What does it do if you run engine at 1200 or more RPM? weak compressors show first as loss of cool at idle when the compressor is already a good bit below 100%.

Excellent points. I do concur. Overheat would cause it to turn OFF, resulting in no cooling period. This doesn't match with my experience.

It does sound like a weak compressor. Makes sense. Struggles at idle/low RPM while in hot traffic, then begins cooling again when RPMs are up to 2400 or so. Also makes sense given its age and frequent use here in the hot/humid South.

So I may be looking at compressor replacement. New? Used? Refurb'd? Sources?

Post by **abscate** » 06 Jul 2017, 04:15

Low charge. Definitely evaluate this before you tangle with compressor. If you know your evap has a leak, bite the bullet and replace that too. It's long but stupid work , but at least in TX you know you will use AC every day so it's worth fixing. The temptation in my temperate zone is to not fix an AC that is only needed for 6 weeks each year

Post by **FlyingVolvo** » 06 Jul 2017, 06:07

+1 for starting with basics. Make sure your fans are working. Some years ago, I was having issues with the AC being crap while at low speeds and the coolant temp needle rising in stop & go. Did a little testing and found out my fan relay was shot. Replaced it cheap and was good to go.

Post by **sleddriver** » 06 Jul 2017, 14:06

Spent quite a bit of time to find this below. It was buried in someone elses thread:

jimmy57 wrote:First, ALL A/C DIAGNOSIS IS DONE WITH ENGINE SPEED AT 1500 RPM +/- 200 RPM. (OK, if you have a car with a variable displacement compressor this is faintly less important. The cars in this forum do not have a VD compressor) I am not making this rule. It is in every factory service manual.
The pressure you get with engine idling is more or less meaningless as compressor is at a speed where its rated displacement is not being reached. This is why a perfectly working system will warm up when you stop and idle on warmer days.

On orifice tube systems "normal" pressure is widely variable. "it is running 35 psi and that seems about right" is far from a true statement. The refrigerant pressure varies with ambient temp, fan speed, temp difference between car and incoming air across evaporator and of course, engine speed if below 1200 RPM.
Since there is no throttling of refrigerant flow on orifice systems, the system works at full capacity all the time. When the heat energy goes down as car cools down and moreso if the heat level is lower due to cooler days, the compressor is turned off to prevent freezing. The pressure and temperature of refrigerant are directly related so the compressor is switched off at 22-25 psi and will be switched on at 42-45 psi. Cycling once the vent temps go down to 50 F range is totally normal and desireable and indicates proper charge. Cycling frequency is energy level dependent. Cycling rapidly with vent temps of 60 F or above is a sign of low refrigerant as an undercharged system goes to the low low side pressure because compressor sucks down the low side, not because the temp of the refrigerant is getting lower causing the pressure drop. Rapid cycling can occur with full charge and the orifice inlet side screen partially blocked with debris (usually from failing compressor).

Procedure for a/c evaluation:
1. run engine at 1500 RPM
2. windows closed
3. recirculated air setting
4. Fan at 75% speed
5. air discharging from dash face vents
6. allow system to run for 8-10 minutes
Now monitor vent temp and system pressures
If you turn down the fan speed and the compressor cycles at the pressures above with vent temps in the 44-52 F range then your system is working as designed. IF you turn up the fan speed and cycling stops with higher vent temps, again this is normal.
If it is a cooler day then the cycling may occur within 2-3 minutes of starting the evaluation. If it is a 100 F + day it may never cycle and vent temps may not get to much below 55F and not cycle.
Car parked in the sun will decrease chance of cycling as heat load into car is greater.

High side pressure is also variable with heat load due to ambient temp and inside car temp.

Note: provide a fan to help airflow across condenser if ambient temps are above 95 F

Posted here for reference for myself & others.