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Air Flow going away and warm air

Gerald K4NHN on Fri June 21, 2013 9:55 PM User is offlineView users profile

Year: 1983
Make: Isuzu
Model: PUP
Engine Size: 2.2
Refrigerant Type: R-12
Ambient Temp: 85
Pressure Low: 30
Pressure High: 160
Country of Origin: United States

I've had this Pup for about three years and it had a bad clutch on the compressor when I got it. First off with a 30 year old truck made in Japan you have a hard time finding parts and you have to live off of a lot of used parts to keep the old diesel trucks going.. anyway. I was able to get a clutch and a new bearing and put that on the compressor. I was able to get a new drier from the good folks here. All of the fitting on this system are the flare type and they can be a problem getting them to seal well. Back then the system worked well other than the air flow out of the vents started to get less and less after a few minutes. The air would be in the 38 to 40 degree range, but the air flow was sick. this has been an on going problem and I finally decide to really tackle it...

These came into this country like most of the trucks from Japan, with no AC. They were made with the blower over on the passenger side, the heater core and damper doors close to the steering wheel and the two unites were joined by a plastic duct that was sort of like a accordion that could be removed with ease. This left the space for the evaporator to be slipped in it's place and the two line went though the fire wall and was held in place with three nuts. It could be installed in just a couple of minutes once the glove box was removed.

So I reclaimed the R-12 with my recovery unit. I pulled the evaporator and capped off the lines to it as soon as I opened them. I put in the plastic duct work back between the blower and the heater box and drove it around for several miles and the air flow out of the vents were great. No reduction in air flow.. So I've tried to see if there was something going on that I've over looked... In all of this testing I've even run a 12 volt line straight to the blower motor to make sure it wasn't a voltage drop and the fan was slowing down. While the evaporator was out I was able to check to see if all the doors were in their correct place and they were. They are all hard controlled and no vacuum operated doors other that the one that controls the one on the blower for inside and outside air and with the glove box out I can see that it's in the correct place.

The expansion valve is inside the evaporator box so I can't do any cooling or heating of it to sort of check it out.. So I decided to replace the evaporator unit with another one that I had. It had a replacement expansion valve in it because I could see that the lines from it were not coiled up like the factory did. I've seen several units before and know how the factory did them. So I put the spare unit in. it was dry of oil and I thought that the unit that I removed would still have some oil in it so I added 2 oz of oil into the evaporator.. I pulled a vacuum down to about 900 microns for 45 minutes and it held that for a hour. I put in the required 30 oz of fresh R-12 by weight.. The air flow started out good but lasted only a few minutes and started to get less like it did in the past. The water was flowing out of the drain really great, the suction line was sweating great all the way back to the compressor, but the little bit of air coming out of the vents was now 60 degrees. Air flow was like it use to be, poor, water pouring out of the drain, at idle the low side was 32 and the high side was 165. Idling up to 1500 RPM's the low was 130 and the high was 160.. all the doors are in the correct place and none of the sealant cushion on them are loose. To me the readings look real good for 85 degrees outside air, water condensing real well off of the evaporator but it's stopping off my air flow and the cool air isn't getting to my vents. These systems were made as sort of a plug and play when they came into this country.. I do have a new expansion valve I could put in if needed. I didn't replace the drier this time since I sealed off the lines as soon as I disconnected each of the lines. I do have a new one if needed. Did me adding the 2 oz of oil to the system add two much. It holds 5 oz. of oil and 30 oz. of R-12.

In know that it sounds like there is a door in the wrong place but I've checked it and doubled checked it. The sight glass looks good. The evaporator is right thick, 4.5" X 8.5" X 6" .. Both units were clean and I could see light through them they were so clean.. I can post pictures of all of the units if I need to.. Can a condenser get so laden with water droplets that the air won't flow through it and what air does come through it be warm... I know that this is a long post, but it's about whipped my butt..

Gerald K4NHN
Cayce, SC

fix_it on Fri June 21, 2013 10:59 PM User is offline

It it possibly freezing up? I looked quickly at an online PT chart for R12. At 30 psi suction pressure, the evaporator is probably close to freezing. That would explain the gradual reduction in air flow. Is there any way to physically look at the evaporator while the system is operating? Someone here posted about a motorhome where the blower blew straight up through the evaporator and kept the water from draining. The water blocked the airflow enough to cause it to freeze.

Gerald K4NHN on Sat June 22, 2013 12:14 AM User is offlineView users profile

Don't think it's freezing up, the water is coming out of the water drain with a steady drip, drip, drip making a nice puddle under the truck. After turning the truck off, not an unusual amount of water comes out of the drain as if it's defrosting. I can feel good cold air coming out of the water drain tube when the truck is running. I wonder if I drilled a small hole before and after the coil in the evaporator housing and used a thermocouple would tell me anything of value. ..

Dougflas on Sat June 22, 2013 12:47 AM User is offline

If the low side hose is sweating back to the compressor, I would tend to think refrigeration system is working. Do you have the equipment to measure and monitor the current draw and voltage of the blower motor? Havr you tried to disconnect the compressor lead when the air flow reduces and see if the airflow returns after a few minutes? Got to start thinking out of the box.

94RX-7 on Sat June 22, 2013 9:28 AM User is offline

I'm on board with the freeze-up theory too. How much trouble is it to pull the thermoswitch and test it?

Gerald K4NHN on Sat June 22, 2013 9:59 AM User is offlineView users profile

I've changed out the blower motor with a known good one and it had the same problem. I'll do the disconnect the compressor and see what happens.. I can measuer the current to see if the current changes like they do if the air flow is shut off.

To check the thermo switch I"ll have to remove the evaporator unit to pull out the probe. but I'm not beyound doing that again... I'll keep you all posted.

Gerald K4NHN
Cayce, SC

94RX-7 on Sat June 22, 2013 11:34 AM User is offline

Originally posted by: Gerald K4NHN

To check the thermo switch I"ll have to remove the evaporator unit to pull out the probe. but I'm not beyound doing that again... I'll keep you all posted.

Actually, it just occurred to me that you should be able to test the thermoswitch by simply unplugging the blower motor with the system running. That will allow the evaporator to get cold enough to where it should trip the thermoswitch and turn off the compressor.

Gerald K4NHN on Sat June 22, 2013 2:53 PM User is offlineView users profile

Sorry, Double post

Edited: Sat June 22, 2013 at 2:55 PM by Gerald K4NHN

Gerald K4NHN on Mon June 24, 2013 1:39 PM User is offlineView users profile

Well I have some good news. I had a couple of other guys come over that also owned Pups and the others have great AC skills. But after a lot of looking, testing and looking at gauges they all went home saying that I wasn't crazy in what I was telling them.. All agreed that the AC was working and no air flow after a few minutes. I even had all of the spare parts laid out on the tailgate to show how all of the boxes fit together..

Later on in the day I looked at both my Pup and Trooper shop manuals and found a chart that showed where all of the doors were to be when the controls on the dash are selected. So on about dark I went back out to try and see better under the dash with a flash light and I used the spare junk heater box that has the doors in it and tried to see where everything under the dash was and positioned. I got into positions that I hadn't been in the pat 30 years trying to look at everything under that dash.

I was wrong that the only door operated with a vacuum switch and actuator was the inside, outside air door, there was another one. I had thought the other four doors were hard controlled, but the defrost door is controlled by a vacuum actuator hidden away on the other side of the heater box and controlled by a 12 volt vacuum switch. After a lot of looking between the spare heater box out on the ground and looking under that dash, I tested the defrost door with a hand held vacuum pump that you can get from Harbor Freight and the actuator worked great and stayed at any place that wanted to put it. So I cranked up the Pup and after a few minuets the air started to go away slowly. I decide to pull the vacuum line off of the defroster door control and see what would happen if I applied the hand vacuum to it. Low and behold the air came back. The defrost door had moved real slowly and I hadn't noticed it over several minutes.

But there should have not have been any vacuum being applied in the AC selection mode. Further checking I found that the 12 volt vacuum switch had a slow leak in it from the main vacuum line. There was 24 hg of vacuum on it and when monitoring the input, and the other port, it would let it slowly go to 5 hg and stay there. The defrost door would slowly open to full open with that slow rise of 5 hg of vacuum. This has bugging me since I've had the Pup for 4 years. Even with the defrost door open it sill didn't put air up into the defrost vent because the other doors didn't switch. It was just a big heater box that had air pressure generated by the fan and the only air that could escape was out of cracks in the housing where it mated together and the defrost water outlet.

I went out for a drive yesterday and the AC worked great, with the normal air flow. The temp out of the vents was hovering between 39 and 41 at full fan, I had to slow the fan speed down after a few miles it was getting to cold.

I just want to say a big Thank You to all of you on the board that put up with people like me. .

Cool temp to all
Gerald K4NHN
Cayce, SC

Dougflas on Mon June 24, 2013 4:40 PM User is offline

Good job.

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