V70-XC A/C high pressure switch?

Snarkhunter · Post by **Snarkhunter** » 11 Jun 2011, 14:58

Hi all,

I'm new to the forum, though I've viewed it many times. I own a 1998 V70 XC wagon with 97,000 miles on it and I'm having a devil of a time trying to diagnose why the a/c won't work. The compressor will not come on. I've measured the static low pressure at 90 psi (on a 70 degree F day), so the system seems to be full. If I jumper the compressor, the clutch will engage and it will pull the air temp down to 45 F, so the compressor/clutch seems to be fine. I've measured the resistance across the low pressure switch at zero, so it seems to be working properly. At the high pressure switch, there is 5 V on one of the three terminals. If I jumper the 5 V to BOTH of the other two terminals, the clutch will engage and short-cycle (I'm talking 1 second on, then off). The weird thing is, if I jam voltmeter leads into the 5 V and either of the other two terminals, and set it to measure a low resistance (meaning the meter is supplying a small voltage), the clutch will engage and run constantly. I don't understand that last bit. I'm trying to figure out if the high pressure switch is bad without spending $100 for a new one. Or is there something else going on that I don't get?

Final note: when I do the voltmeter thing, I get the CEL with three codes: P0452, P0107, and P1308, which I THINK are getting thrown because I'm messing up the reference voltage on a common wire to several sensors.

Advice would be greatly appreciated.

Snarkhunter · Post by **Snarkhunter** » 13 Jun 2011, 09:19

UPDATE:
Since I don't know what else to do, I'm planning on replacing the high pressure switch. Note on the info above: when talking about the voltage on the high pressure switch terminals, I was referring to the harness, not the switch. Today I ran the engine, forced the compressor on by manually moving the relay, and tried to see if there was any change in the voltage or resistance across the terminals on the actual switch. There was nothing happening. Maybe I just don't know what I'm doing, but I plan on replacing the switch tonight.

Does anybody know if the switch can be replaced without bleeding the system first?

Post by **Ozark Lee** » 13 Jun 2011, 15:58

The high pressure side switch can't be replaced without draining the system or letting it drain itself. The low pressure side pressostat (switch) can be since it has a schrader valve behind it. If you do drain the system you will need to pull a vacuum on it before your recharge it.

I would do a bit more investigating before your replace the high pressure switch, they fall into the "very rarely fails" category.

EDIT: I was dead wrong, both the high pressure and low pressure switches have Schrader valves under them on the P80 cars. Read on.

...Lee

Post by **holler1** » 13 Jun 2011, 19:13

Snarkhunter wrote: I've measured the static low pressure at 90 psi (on a 70 degree F day), so the system seems to be full.
Advice would be greatly appreciated.

I'm not that familiar with how the switches work, but I do remember from working on mine that a good static pressure reading is not a guarantee that there is sufficient AC fluid in the system. The static pressure is just the vapor pressure of the fluid, and it will be the same if there is a small amount or a large amount, as long as the system has some fluid.

My suggestion would be to get a set of gauges and measure low and high pressures when the compressor is running, and see if it will "take" additional fluid, before deciding the system isn't low on fluid. It's "normal" for the compressor to short cycle if it's low on fluid, and increase the cycle time as the fluid gets added. However, I don't know why the compressor would not cycle on at all. Just thought this might be worth a check.

Post by **Ozark Lee** » 13 Jun 2011, 20:40

We really don't have a good way to measure the high pressure side as there is no official high side service port. That said, it doesn't sound like things are that far off with 90 PSI static pressure @70 degrees.

As much as I hate to start electrical troubleshooting in the middle of the system I would check both terminals for voltage at the low pressure switch. With the system on if you have no voltage you need to be looking inside of the cabin. If you have voltage there then you need to be looking under the hood and you may have a bad compressor thermostat. That part is not critical and you can just wire around it.

Were you able to get any consistently running low side pressure readings?

...Lee

Post by **jimmy57** » 13 Jun 2011, 22:34

There is a rare occurrence on the 98-2000 70 series where the ECC module goes goofy.
You should try disconnecting battery for an hour and then hook back up and see if it healed.
I have also seen the high pressure sensor biased and that is checked with a scan tool that can read the ECM values as that is what has authority over the compressor. The sensor could be reporting 425 psi or 15 psi when the pressure is 90 psi and those values would cancel compressor function. The high side pressure sensor is not cheap.
The signal through pressostat on low side line became a signal input to ECM as of the 98 MY where it had been inline in circuit to clutch 97 and before.

Post by **holler1** » 14 Jun 2011, 06:13

Ozark Lee wrote:We really don't have a good way to measure the high pressure side as there is no official high side service port. That said, it doesn't sound like things are that far off with 90 PSI static pressure @70 degrees.

Were you able to get any consistently running low side pressure readings?

...Lee

Sorry, I have only worked on my "new" Ford f150 AC, and didn't realize the Volvos don't have an accessible high side port (good to know and kinda wierd). However, I am sure as mentioned above that a static pressure reading is not a guarantee that there is sufficient AC fluid in the system, although the fact that you do get cold air when the compressor is running means it has some fluid. The static pressure is just the vapor pressure of the fluid, and it will be the same if there is a small amount or a large amount, as long as the system has some fluid.

As Lee indicated, at least try to get a low side P reading with the compressor running. It should go down to around 20-30 as I recall. That still doesn't mean the system is full, though. About the only way to know that is if the compressor cycle time goes up. It would be easy to try adding some fluid, just in case it's low. Low fluid can cause short cycling. Speaking for myself, I would check the easy things first, and then get into the electronics only if necessary.

Snarkhunter · Post by **Snarkhunter** » 22 Jun 2011, 14:04

Ozark Lee wrote:The high pressure side switch can't be replaced without draining the system or letting it drain itself. The low pressure side pressostat (switch) can be since it has a schrader valve behind it. If you do drain the system you will need to pull a vacuum on it before your recharge it.

I would do a bit more investigating before your replace the high pressure switch, they fall into the "very rarely fails" category.

...Lee

Thanks for all the info and advice. I was told this same thing by my dealer. However, I ordered a new high pressure switch, and it had a spade on the underside for depressing a schrader valve, so I carefully unscrewed the old one. It popped a bit when the valve released, but that was it. So despite what we've been told, the high pressure switch has a schrader valve underneath it and can indeed be replaced without draining the system. I screwed the new one in, and now the A/C works like a champ. 45 degree air out of the vents, and it comes on when the auto climate control calls for it.

Post by **Ozark Lee** » 22 Jun 2011, 16:00

You replaced the low pressure switch, not the high pressure switch. The high pressure switch is located on the right hand side frame rail and it doesn't just unscrew - the lines need to be removed to get it off of the frame and it is kind of a major deal.
The low pressure switch screws into the suction line by the firewall and it is absolutely as you described.

At any rate I'm glad your problem is solved and you have nice cold air again.

...Lee

Post by **jimmy57** » 22 Jun 2011, 18:25

The high pressure sensor on the block valve fitting where the high side lines has that junction just forward of the ECM/TCM box is on a schraeder valve also. This sensor is used on any P80 with Mot 4.3, Mot 4.4, ME7.01, and Denso. On P1,P2,P3, and X40 where it is located on the receiver /drier it is screwed into a threaded hole with no valve.