Hi All: I'm a retired Volvo tech, now volunteering with a local non profit (Wheels4Hope) to repair donated cars and give them to folks who can't otherwise buy a vehicle. We have a '96 960--the radiator fan quits intermittently, causing the A/C pressure to go high and dump "freon".
We tried a new fan relay, with the same problem. Personally, I haven't been able to make it act up. I suspect the high pressure switch at the condenser.
But, when I unplug the switch, the fan keeps running--I thought it would shut down. (During this test the engine temp wasn't high enough to turn on the fan). I have the Volvo diagram, but no explanation of how the engine ecm controls the fan. Apparently the ECT is ok since there are no ck. eng. codes and engine runs well. Is there a way to test the A/C high pressure switch--it has three wires. I'm thinking one is signal from the ecm, and the others are high pressure and very high pressure signals back to the ecm? Why wouldn't the fan shut off when I unplug that switch? And, is the fan single speed, or dual speed? I never noticed...
Thanks!

 

HP Sensor's job is to cut-out (and cut-in), so that the compressor safety valve wouldn't have to "dump". Dump is supposed to be a last-ditch feature. I would be extremely nervous if I had more than 2 dumps -- what else could bust? Some compressors' dump valves are single-use only. Even if not, I would not push them.

960's spec pressures are really high to begin with. You can get the specifics at the stickies at the top. From memory, high could be low 400s, whereas the Dump IIRC is like 530 psi.


Fan is dual speed. HP Sensor bumps fan from Low to High. But as you know, it is rarely one input only. You can research more at the literature at the stickies.

I don't recall seeing the HP Sensor's Values anywhere.

I would diagnose the HP Sensor very simply based on cut-in and cut-out behavior. Check out the links at the stickies. Look for a document "Pocket Data".

 

Thinking about this -- HP sensor is not a sensor, electrically speaking. It is a switch, a pressurestat. So testing would be different from a sensor.

 

Thanks Henry10! That's good info and it might help me fix this. I'm new to the site so I don't know about "stickies" but I'll figure it out if it's in the FAQ section. (I'm still confused about why the fan doesn't shut off when i unplug the pressostat..._). I'll let you know what i find.
thanks, Paul Smith, Raleigh, NC

 

Like I said, pressostat does not turn off or on the fan. Based on refrigerant pressure it bumps the fan from Low to High speed setting.

Fan is controlled by ECU. Based on other inputs, e.g. coolant temp, vehicle speed, barometric pressure/ air temperature, engine load, etc ECU activates the fan. I believe fan is turned on Low when compressor starts too.

Regardless of the fan, pressostat should have shut down the compressor. Fact that it didnt, that means pressostat is bad or wiring is bad.

 

Thanks again Henry 10: There are a lot of strategies the mfg can use here, and your info helps me sort it out. With a proper functioning pressostat, what should be the result of unplugging it?
Also, how do I get to "stickies" and "pocket book"?
Thanks again,
--Paul S

 

Unplugging the pressostat should cut out the compressor. Stickies are at the top of the threads. Docs are linked in the stickies.

 

Hi Henry10,
OK. Thanks again.