a/c unit runs great, but engine fan running constantly

Ok, normal if you are parked or stop and go in the city, should not be on while at highway speeds. So how are you driving it?

 

Humidity and outside temps drive up the head pressure on the AC, fan kicks on to lower that pressure in your AC system
Just be glad it kicks in and out. Not coming on will cause a AC line to break.
Then no AC and a grand or more to fix it

 

Bingo

Usually if they run that much it's got a pretty healthy charge in it.

As mentioned it's the high pressure switch that engages the fan..... It runs until the timer cut out in the ECM or the pressure drops and stays down.

You want to see it running that means the AC it's working properly...

 

Check the ac line between the compressor and condenser to see if there are two pressure switches. One will be a high pressure cut off to the compressor and the other will be your high pressure input to the ECM to turn the fan on. If the fan is on too much, you basically need to get pressure gauges hooked up to see what pressure the fan is kicking on at. Generally, even on a hot day, the pressure will increase rather rapidly, but the fan won't kick in until the pressure is about 300-350 psi. With the fan on, the pressure will rapidly drop down the 150-200 psi range depending on how hot it is outside. Once the pressure drops, the pressure switch resets. Most engines will run for a minimum of 3 minutes after the high pressure switch resets to prevent rapid fan clutch cycling. This can cause the fan to seem like it is running too much as once it turns off, it is only a brief time before the pressure is back up to 300 again, and the fan is once again commanded on. If your fan is kicking in at way less than 300, the pressure switch is faulty. If the pressure rises way too quickly, the condenser may not be cooling properly. It is common for older condensers to have the cooling fins starts to separate from the cooling tubes which causes a lowered efficiency of the condenser itself.

 

if you just had the compressor replaced it may be overcharged.

 

That's what my o6 IH 9400 did, the fan ran constantly because it was overcharged. I had half a pound drained out then it worked great.