'95 Altima air conditioning not cutting out any more

The air conditioning would cut out on hot days. I made a guess and paid to have the system evacuated and recharged. Still cut out.
Today I may have found the problem. There was a throttle rheostat, loose, in one of the two radiator cooling fan cages, on the bottom, out of sight. One hears the fan(s) running but I never checked if both were running. Now the air gets nice and cool.
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The stopped fan wasn't the problem and the obstruction to the fan may have fallen into place recently when I replaced the lower timing cover oil channel O ring. I'm still getting intermittent failures Am awaiting R134a snap fittings for an AC gauge manifold set which recently arrived; am building an R134a storage and recovery tank in case I need evacuate the system; have cleaned the evaporator which had some matted leaves accessible to removal with a partially opened evaporator; have the 1995 Altima shop manual w/troubleshooting chart. When the gauges are in place I'll have a much better shot at fixing it.
BillJ
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Manifold onto the system after it had been running for 10 minutes on full with door open. Pressures were 14/125 and receiver/drier sight glass showing clear fluid flow, compressor running, no cooling, outside air temp mid-80's, duct air >95. Underpressurized.
I charged slowly until the pressures were 20/200. Duct air was now 60.8. Here I realized I had neglected to raise from idle, now 750RPM. I raised the idle to ~1400RPM and the pressures were now 26/225, duct air 55.8. I considered it done.
I dropped to idle, 750RPM, and the pressures went to 34/218. Duct temp remained between 55 & 56. Oddly the receiver/drier sight glass now showed the fluid to be yellow milky containing oil. The receiver/ drier is not an OEM. The engine had been running for 30 minutes in mid- to-high 80's ambient temp.
I didn't road test until the next day. A trip to town, 22 miles, ambient temp probably in the low 90's. Started out fine but after 15 minutes, on cruise control, 55 MPH, overdrive (2050RPM), the air suddenly got warm and humid, then uncomfortably so to the degree I needed open windows. Same failure as in the past tho in cooler weather the air has run without failure.
I suspect the compressor is working too hard, causing the compressor temp breaker to cut out; that or increased working temp causes the pressure to rise tripping the pressure switch.
I could fire the car up, get the air running, block the condenser with cardboard, and watch the pressures but the engine might overheat.
Although I cleaned the evaporator as best I could without removing it, there's a good chance it's failing to pass air efficiently due to packed dried leaf matter within the coil fins that I can't see while it's in place. I understand current model Altimas have an air filter in the duct. I've a home window air conditioner that will stop the compressor if the filter gets matted/clogged tho the fan continues to run. This wouldn't explain how the Altima air can run without failure in cooler weather unless in cooler weather the failure just isn't apparent.
BillJ
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So revisiting the evaporator seemed like the next logical step. I prepared by testing both a drain opening agent with water and a spray oven cleaner on a mixture of pine needles and hazelnut tree leaves. Later, after disconnecting the battery, I accessed the evaporator as best I could while leaving the refrigerant system closed. It was then I decided against the caustic agents followed by club soda and instead used the power of a garden hose. The deciding factor had been the evaporator thickness. Lining the car floor with heavy plastic and crimping the garden hose until I got the nozzle in place, I blasted the evaporator with quite a bit of water. The water seemed to mostly stay within the evaporator box flowing out the drain hose. Very little water spilled onto the carpet. The under-dash is presently drying with a fan and doors open. Tomorrow I'll close and give it a test but I've a vague sense this wasn't it.
I should mention I tried measuring the temperature at both dryer/ receiver ports having read that at proper charge both would be at equal temperature. Clamping thermocouples to both aluminum tubes the measurements were a bust as I had not thermally isolated the dryer/ receiver from the forced hot air after the condenser & radiator. If this method is effective some kind of insulation would need be wrapped to isolate the dryer/receiver, aluminum tubes and thermocouples.
If it fails again I think the next good test might be to pump out some R134a to my homemade storage unit, this on a hot day while monitoring the duct temperature looking for the first indication of temp rise.
BillJ
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A closer look at my 1995 shop manual shows pressures for 90 should be 27.2/211PSI (averaging). I measured 26/225PSI. My low is lower and high is higher. Interestingly the proper ratio for an 1995 Altima seems to be 7.75:1 while mine is 8.65:1 Looking at the figures for an ambient temp of 104 the avg ratio is ~7.30:1. I looked at this number figuring that if my condenser was loaded with internal scaling it would appear as if operating at a higher temperature but the ratio drops. So, this might mean that I'm yet under charged. If I add more R134a I might expect that the high low differential would decrease, lower ratio. Maybe the lower charge means the R134a spends less time in the condenser cooling. Testing will tell. By the way, the mtc.pdf for a 2006 Altima found available on the web shows pressures @ idle not 1500RPM and have an avg ratio of ~5.13:1.
BillJ
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Drove the car in 90 heat and the air eventually failed so I know it wasn't the result of leaf matter blocking air through the evaporator. While driving the duct temperature got down into the 40's.
At home I sprayed water on the compressor because the clutch was disengaged (air had stopped). I was hoping that if it reengaged I'd know it was clearly internal thermostat and not pressure. Didn't happen. then ran the hose on the condenser but again it wouldn't kick in so I just shut it off and let it sit with a fan helping the engine cool.
Later, fired it up and after 10 minutes at a steady 1500 RPM had a ratio of 7.5:1 but the pressures were rising. To test if adding R134a would improve the situation I added but the pressures got worse. I started pumping R134a out into my storage unit. Took too much, duct temp went high. So, I removed the compressor that had been pumping R134a out of the system and into a storage unit and replaced into the system from a fresh tapped can of R134.
Adding slowly then testing after 5 minutes, the duct temperature was 65.1, the pressures were 34/235 (ratio 6.91:1). I added more, waited 5 minutes and the duct temperature had gone up to 66.7, pressures 38/250 (ratio 6.58:1). This seemed crazy, it going in the wrong direction.
Then I noticed the output temp wasn't steady as it started to drop again in the next 5 minutes, not having added any refrigerant the duct temp dropped to 63.1 & 34/233 (ratio 6.85).
I stopped here. Although mid-60's duct temperature is 10 degrees higher than called for in the shop manual, I'll run like this for a while and look for the failure tomorrow after typically 15 minutes of running mid-day. I see the pressures, both low and high, just drifting up the longer the car runs. I suspect that undercharged but cooling will have a lower probability of failure. If there's debris floating around in the system requiring system evacuation, flushing and the addition of an inline filter, I wonder why the failure happens after 15 minutes of mid-day running.
BillJ
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The system failed sooner, 5 minutes rather than 15. In this case undercharged I believe. A good troubleshooting guide is found at http://www.aircondition.com/tech/questions/82 /
The present problem is undercharge (from the above mentioned guide "You can have a system evaporator pressure of 30 psi, and still be low on refrigerant. Let's assume that only half the evaporator is full of boiling, heat removing, liquid refrigerant. Only half the air traveling through the coil is being cooled. Pressure readings indicate core temperature is near thirty degrees, but half the core isn't removing any heat. The system is close to being full, but that discharge air is only slightly cool.").
I found many references to Nissan Technical Service Bulletin NTB-97007A on the web.
Although I couldn't find NTB-97007A as a full text article on the web, I did call my local dealership and a young lady in the parts department was good enough to read the 5 pages to me. Two failures in pre-1996 Altimas are noted: 1) a refrigerant short in the evaporator where the refrigerant moves from the evaporator to the low hose bypassing the evaporator causing a very cold frosting low pressure hose (I've seen this once) requiring replacement of the evaporator and valve and 2) desiccant coming free of the receiver/drier requiring replacement of the hoses, receiver/drier, evaporator and valve.
BillJ
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While the day was 90 I charged the system.
Precharge static pressures: 104/108 PSI. Ambient temp 90.5.
Early in the charging the compressor was cycling on and off as the low pressure dropped too low.
My final measurements after an hour were these:
1500 RPM Pressures: 43/308 PSI. Duct temp: 58.6. Receiver/Drier temps: 104.3 compressor side/106.3 evap. high tube side; sight glass clear no oil. Firewall evap low press tube temp: 83.7. Firewall evap high press tube temp: 107.6. Water almost pouring from evap drain; rain expected; air pressure 29.3." Added pipe insulation to evap low aluminum tube return.
Temperatures taken with an IR thermometer. The receiver/drier temps would seem to indicate somewhat overcharged. According to service manual the pressures are appropriate for an ambient temperature of 104.
Road tested it. Temperature dropped to mid-80's. Drove 15 minutes, duct temps good, in the mid-50's. Stopped and turned off engine while checking mail. Restarted engine and the AC clutch would not engage and the duct temperature rose to ambient. Turned off air and drove 5 minutes then tried air and it engaged, soon giving me mid-50's again but only for, at the most, 10 minutes then failure.
Next step will be to evacuate the system, take out the evaporator and check for desiccant or corrosion, then repair or replace.
The previous owner had a major head-on collision with the passenger side front taking the brunt of the impact. The weather was snow and rain. I suspect the repair didn't include a half hour or more evacuation of the entire system to vaporize and suck out moisture and the receiver/drier might not have been replaced either. On the web there's a photo of a Ford evaporator tube that's been subject to moisture in the refrigerant for significant period. The moisture deteriorates the tube. So, moisture in the system might not show up right away but cause failure months or years later and front end accidents in rain or snow are common.
BillJ
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