Automotive Air Conditioning Information Forum (Archives)

bobinyelm on Wed July 03, 2013 11:23 AM

Year: 1989
Make: Volvo
Model: 740

1989 Volvo 740

Just converted to R-134 w/ all new components (except evaporator), and wired elec. pusher cooling fan w/ a relay to run when compressor is running. Installed a Variable Orifice Valve (VOV) as well.

System blows cold, but Compressor will NOT shut off, and runs continuously (though if the plug on the accumulator switch is pulled, it shuts off, so the wiring is correct).

I charged the system w/ 20% less R-134 than the R-12 specs.

Center dash outlet temp is 36-38 deg on the highway, and 45 degrees at idle engine RPM.

I'm not sure how to adjust the R-134 pressure switch I installed in the new accumulator, but it does have a center adjustment screw (painted over with glyptal or some other paint).

My concern is that the evaporator will possibly freeze over if it gets too cold.

My WAG is that the VOV is causing different pressures than the switch is factory set for for use with a fixed orifice valve, so the switch is not opening and shutting the compressor off?

The system is not equipped w/ a High Side Service port (that's how the factory built it), and the Low Side stays around 35psi running.

I have no idea HOW the system is supposed to act with a VOV, unfortunately.

Edited: Wed July 03, 2013 at 11:27 AM by bobinyelm

94RX-7 on Wed July 03, 2013 11:56 AM

Quote
Originally posted by: bobinyelm

I'm not sure how to adjust the R-134 pressure switch I installed in the new accumulator, but it does have a center adjustment screw (painted over with glyptal or some other paint).


My concern is that the evaporator will possibly freeze over if it gets too cold.


My concern is that you're concerned with a concern that isn't even occurring. If it freezes over, then worry about it. Chances are, if it is a quality switch that was calibrated correctly for R-134a, you won't have any issues.

bobinyelm on Wed July 03, 2013 12:54 PM

OK-Just never had a system where the compressor didn't cycle, and just stayed running for 30min of driving (that's as long as I've yet driven it), and I was told that the output air temp should never go below 45 degrees, and it's at 36 degrees and very close to 32 deg freeze point.

Since I knew the VOV runs different pressures than a fixed orifice, I considered the idea that maybe the switch was set for the pressures developed by a fixed orifice.

So 36 degrees is OK for A/C center air outlet temperature?

Cosmos1960 on Wed July 03, 2013 1:19 PM

Just curious, what are your ambient temps and how big of a pusher fan are you using?

bobinyelm on Wed July 03, 2013 2:15 PM

This morning for the test it's about 85-90 degrees only, and I wired the OEM aux cooling fan to run beside running when the radiator temp exceeds 210 deg (the engine has a viscous mechanical fan as well).
The electric fan is about 14" in diameter, and was not part of the A/C system with R-12.

Bob

94RX-7 on Wed July 03, 2013 2:18 PM

Quote
Originally posted by: bobinyelm
OK-Just never had a system where the compressor didn't cycle, and just stayed running for 30min of driving (that's as long as I've yet driven it), and I was told that the output air temp should never go below 45 degrees, and it's at 36 degrees and very close to 32 deg freeze point.


Here in Houston at this time of year it would be perfectly normal for a system to not cycle off, especially if recirculate mode is off. Cycling frequency depends on heat load.

Output air temp will vary based on heat load. Certainly, if you're at freezing or a degree or two above, then you may have an issue. But if I was seeing 36 degree vent temps (and no ice) I'd rejoice and give thanks.

bobinyelm on Wed July 03, 2013 6:19 PM

Thanks-

All of my other vehicles do cycle, so I assumed something was wrong, and none of my others get that cold.

I was thinking if I measured 36/38 at the vent, the evaporator must be several degrees colder.

BTW, I went out and put the gauges on both my daily drivers (a Mercedes C320 with 70k miles since new and a Mercedes Sprinter Van w/ 36k mi since new) and with the Low/High side pressures at about 40/300 at 92 degrees F, the Sprinter A/C was putting out 56 degree air, and the C320 was making 52 degree air using teh SAME thermometer I used in the Volvo vent.

I have not yet put it in ice, but at 94 degrees it was 1 degree different from an infrared temp gauge I have.

The '89 Volvo is making almost 20 degree COLDER air. Should it be able to do that under normal circumstances? The Low Side Pressure on the Volvo is 35psi BTW and there is no approved High Side Service Port.

Edited: Wed July 03, 2013 at 8:49 PM by bobinyelm

94RX-7 on Thu July 04, 2013 12:37 PM

Quote
Originally posted by: bobinyelm

The '89 Volvo is making almost 20 degree COLDER air. Should it be able to do that under normal circumstances?

No. Your system is possessed by Satan and needs to be exorcised. (Don't ask why a Satan infestation results in colder air, rather than hotter air.) Exorcism is best performed by jamming a screwdriver (a Phillips head since the end is shaped like a cross) into the condenser repeatedly until the demons come out as a white smoky mist.

Kidding. Don't do that.

Your circumstances are no longer normal. R-12 and a fixed orifice would be normal circumstances as far as your system is concerned. You've got R-134a, magic-in-a-can, and a VOV on board.

Also, your Sprinter and C320 were designed in a different political climate where environmental concerns are at the forefront. That means the systems hold less refrigerant than the Volvo. The materials and parts used in the system were designed to be as light as possible, and use as little horsepower as possible to maximize fuel economy. Cool but not frigid air was likely deemed an acceptable tradeoff.

If you want to test and see if the switch will protect against freeze ups, unplug the blower motor and turn the A/C on. The evaporator should get quite cold with no air moving across it, and the switch should cut the compressor.

Edited: Thu July 04, 2013 at 12:45 PM by 94RX-7