# AC and radiator cooling fans



## commuter kw (May 22, 2020)

91 Sentra E, 1.6 liter, 4 speed manual, 203k, completely stock. 
Have owned for about 20 years. Air conditioning stopped working about 10 years ago as compressor would not come on. Recently I started fixing up the car (the list was long) and now enjoy driving it again so I decided to tackle the AC problem. It is the original R12.

I checked the low side pressure with the engine off and it was about 110 on a 70 degree day. This seemed very high so I thought that the system overcharge prevented the compressor engaging. 

I removed some Freon to reduce static pressure to about 65-70 on low side. The AC compressor now comes on, has a low side pressure of about 35 with engine running and AC on, blows cold (35-38 degrees) for about one minute, then shuts off. This cycle (on and off) repeats about every minute but it never stays on more than 1-2 minutes. Is this normal? 

I noticed when compressor comes on, the driver side fan also comes on. Shouldn’t the passenger side fan come on as well? Thanks in advance for any advice.


----------



## 302831 (Nov 17, 2020)

One fan is for air cooling the AC condenser while the other fan is for maintaining engine coolant temperature. Both fan will must likely maintain engine coolant temperature since the engine will have the likeliness to idle 50rpm above normal engine rpm. The compressor clutch has a great effect on engine temperature base on if it's NEW or USED. A new compressor will need the additional 50rpm while a used compressor may not. The force to dive a new compressor is much great than a used compressor. That 50rpm force use to drive the new compressor is not need on a used compressor. The additional 50rpm use to drive a used compressor is only generating engine heat that is no needed at engine idle. So it will be best to idle the engine has normal non AC operation with the AC operating.


----------



## VStar650CL (Nov 12, 2020)

91 Sentra AC Pg1.pdf







drive.google.com












91 Sentra AC Pg2.pdf







drive.google.com





Links are for wiring diagrams for your AC. They're a little hard to wade through because they show all the possible permutations for A/T and M/T, but the short story is, it's two single-speed types using only the Sub-Relay if you have M/T, a pair of two-speed types using three relays if you have A/T. I'm assuming you're A/T, so Relay1 will run both fans together on low speed, Relay2 will run Fan1 on high speed and Relay3 will run Fan2 on high speed. I think Fan2 is your A/C fan and should run on high anytime the compressor is on. Fan1 and the Fan2 low speed winding will respond to engine conditions and not the A/C. You can see there's a thermal cutout on the compressor line that the ECM has no control over, and unless that's active the ECM controls the compressor based on the Dual-Pressure Switch and Thermo Switch inputs (the Thermo is usually on the evap. By monitoring the cutout, Dual-Pressure and Thermo, you can probably figure out what's causing the long cycle times (they do seem a bit longer than normal for a cycling-clutch system). Hope this helps you.


----------



## rogoman (Dec 16, 2004)

commuter kw said:


> 91 Sentra E, 1.6 liter, 4 speed manual, 203k, completely stock.
> Have owned for about 20 years. Air conditioning stopped working about 10 years ago as compressor would not come on. Recently I started fixing up the car (the list was long) and now enjoy driving it again so I decided to tackle the AC problem. It is the original R12.
> 
> I checked the low side pressure with the engine off and it was about 110 on a 70 degree day. This seemed very high so I thought that the system overcharge prevented the compressor engaging.
> ...


Check the pressures with the engine fully warmed up and idling. Here's a performance chart:










Here's a cooling fans operation chart taken from a newer Sentra but may help you in understanding it's operation:


----------



## IanH (Feb 11, 2003)

If the freon is low it will shut off, the low pressure switch activates. 
If you charge to get the pressures in chart above it should run fine. If not I would suspect the pressure switches not working correctly but I personally have not seen this. 
Good luck !


----------



## commuter kw (May 22, 2020)

Thank to all for the excellent information. BTW the car is a manual transmission. I removed a little more Freon today and the pressures are now within parameters of the chart posted by rogoman. I did check them with engine at normal operating temperature, which I did not previously. 
I now realize from the schematics and further research that the driver side fan receives a signal from the ecu when AC is turned on, so that seems to function correctly. 
The AC now stays on a little longer with the correct pressures and blows very cold, but still cycles on and off every 1-2 minutes.
However I still have one question. Is the AC supposed to cycle on and off to keep the inside temperature somewhat stable? I haven’t had AC on a car for a long time so I’m not sure if this is normal (so I’m not chasing a problem that is just normal operation). THanks again for the helpful responses!


----------



## IanH (Feb 11, 2003)

The AC will cycle if the evaporator is fully charged with liquid and pressure drops so that the low side switch cuts compressor off but it should continue to blow cold air. My Sentra, SER and Sunny behaved that way. Just out of interest my 2003 Pathfinder had a variable capacity compressor that reduced pumping when pressure was low, and so did not turn off.

So long as it cools well all is well.

So biggest Sentra Problems I had was low freon on the Senta and overcharged Freon on the Sunny from new. Long sad story there with the service dept in Singapore.

When the variable capacity compressor malfunctioned it was difficult to diagnose.


----------



## rogoman (Dec 16, 2004)

commuter kw said:


> However I still have one question. Is the AC supposed to cycle on and off to keep the inside temperature somewhat stable? I haven’t had AC on a car for a long time so I’m not sure if this is normal (so I’m not chasing a problem that is just normal operation). THanks again for the helpful responses!


Yes, this is the way our late model vehicles are supposed to operate in the interest of improved fuel economy. Back in the old days the A/C systems were rather primitive in the sense that there wasn't much concern to fuel economy so the compressor ran continuously to maintain a constant interior temperature.


----------



## VStar650CL (Nov 12, 2020)

commuter kw said:


> However I still have one question. Is the AC supposed to cycle on and off to keep the inside temperature somewhat stable? I haven’t had AC on a car for a long time so I’m not sure if this is normal (so I’m not chasing a problem that is just normal operation). THanks again for the helpful responses!


Yes, as long as the vent temperature inside the car isn't fluctuating wildly. The thermo sensor on the evap largely controls that. When the evap gets warm the compressor turns back on. The system is what's called "cycling clutch", meaning the expansion orifice for the freon is a fixed size and the compressor cycles off and on to control the evap temperature. It cycles off when the evap temperature is low enough or the high-side pressure is too high, back on when the evap temperature rises too high. That's as opposed to the "swash plate" systems used on later models, where the compressor runs constantly and a movable plate changes the volume of freon it pumps according to the cooling load. If your system is cycling properly, the outlet temp should only fluctuate within limits and the compressor should stay on longer when the cooling demand is higher.


----------



## IanH (Feb 11, 2003)

Well now you are really opening pandora's box.
The OLD fixed orifice systems were primarily Ford type systems while GM used an expansion valve and not an orifice. This is from my experience in 1970's models.

As I mentioned the cycling system used by Nissan was replaced by the time my 2003 Pathfinder was designed.


----------



## VStar650CL (Nov 12, 2020)

IanH said:


> Well now you are really opening pandora's box.
> The OLD fixed orifice systems were primarily Ford type systems while GM used an expansion valve and not an orifice. This is from my experience in 1970's models.
> 
> As I mentioned the cycling system used by Nissan was replaced by the time my 2003 Pathfinder was designed.


True, and a lot of Nissan systems still use an expansion valve along with the swash plates. For those who want to know, TXV's basically accomplish thermostatically what a thermo switch or amplifier does electrically. A stopped or frozen one will constipate your AC to the point where the low side pressure will go negative. Even within the Nissan lines there were and are _many_ variants for thermal regulation.


----------



## commuter kw (May 22, 2020)

WOW, I am truly thankful for the very good information! I am learning a lot from all. I’m calling this one “fixed”, thanks to this forum!


----------



## VStar650CL (Nov 12, 2020)

You're most welcome, happy to help!


----------



## 302831 (Nov 17, 2020)

😂😂😂😂😂😂


----------



## 302831 (Nov 17, 2020)

Did you burn it? Buy new never fix.


----------



## commuter kw (May 22, 2020)

stevexnyc said:


> Did you burn it? Buy new never fix.


It’s now my daily driver, 40 mpg on hwy 😎


----------



## 302831 (Nov 17, 2020)

40mph?.go eletric.


----------

