A/C works - some of the time

The A/C in my 98 Volvo V70 has been acting up and the dealer hasn’t been able to diagnose the problem. It cools just fine when first started - it puts out 40 degree air for about 10 minutes, then the air temp goes up to ambient. Turn the A/C off for 15-20 minutes (driving all the while) then its good for another 10 minutes of cooling. The golden cooling time has been getting gradually shorter - was 20 minutes two years ago. Two years ago, the dealer fixed a leaking evaporator (had to pull the dash, don’t ask how much $$). The system was leak tight after that repair but the problem described above persists (and gets worse). I’m reluctant to let the dealer have another crack at it, I have no confidence that they will do anything other than replace the next most expensive component. Any ideas?

Does the amount of air decrease also when this happens? That would indicate that the evaporator is freezing up.

Does the compressor continue to run when the problem occurs?

If the evaporator isn’t icing up as suggested by an earlier post and the compressor is running, here is a thought. I had a similar problem on a Ford Aerostar minivan. It turned out that the fan (the main fan that pulls air through the radiator and the condenser coils) had a bad fan clutch. My mechanic traced it down by putting his gauges on the air conditioner and when the air conditioning quit functioning, the pressures went way up. He took his shop fan and directed the air through the radiator and condenser coils and the pressure went back down and the air conditioning started functioning. Replacement of the fan clutch solved the problem.

I don’t know if your Volvo has a clutch controlled fan or an electric fan on the radiator, but it’s worth checking if all else fails.

NyBo - the air flow does not decrease, I’m not sure about the compressor - I’ll have to check under the hood and see if I can tell.

Triedag - that’s a possibility. One symptom I left out is that the problem does not occur unless its over 80 degrees or so outside. If its in the mid 70’s, the A/C continues to cool. This is one reason the dealer couldn’t replicate the problem - they ran the A/C inside their shop (which is air conditioned itself) rather than do the road test I asked them to perform. If the flow is restricted across the condensor it would make sense that it have some threshold. It might also be why the problem is getting progressively worse. I’ll check it out - Thanks to all.

Usually the first thing that should be done with an A/C problem is put the gauges on it and see what’s going on pressure-wise.
Since this could be caused by evaporator icing you can test this by looking underneath the passenger side of the car at the firewall when the A/C has become ambient. You should see a slow condensation drip. If not, turn the A/C off and wait about 5 minutes or so. If a drip starts then this will mean the evaporator is icing up.

The cause of evaporator icing could be one of a number of things so this falls back on the checking the pressures first as a slight undercharge could do it. The undercharge can allow the low side pressure to fall too much and since the low side pressure is roughly equivalent to the evaporator temperature you can see what would happen if the low side pressure is about 30 degrees/30 PSI. Hope that helps.

I also have had exactly the same problem with my Volvo but it is a V70XC, model yr 2000. I would be curious to know if you have resolved the problem and what turned out to be the issue.