AC Blows Cold but electric fan never kicks on

BC_CRDtom · **Posted:** Sat Jul 21, 2018 10:19 pm

So after servicing my once defective AC system I got it all sealed up, vacuumed, and recharged. Compressor kicks on and air blows cold through the vents. However, my electric fan will not activate. Checked both hi/low relays, and they’re functional. When I pull either one low speed kicks on the electric fan.

Are there any other fuses/switches or sensors that could be causing this issue?

Mountainman · **Posted:** Sun Jul 22, 2018 12:56 am

My guess is the high or low pressure switch. Unless the fan only runs on high for the AC, and the high side of the motor is bad. See if you're getting 12v to the fan when the AC is on first. If not, maybe pressure switch, but I'm not much of an hvac guy

flash7210 · **Posted:** Sun Jul 22, 2018 9:13 am

It’s possible that everything is behaving normally.
If the AC stops blowing cold when stopped in traffic, or you start having overheating problems, then start investigating deeper.

BC_CRDtom · **Posted:** Sun Jul 22, 2018 11:38 am

From all my research the electric fan in front of the rad should always kick on when AC is active, so I’m pretty sure there is a defect in the system somewhere. I’ve never seen it running ever.

Sir Sam · **Posted:** Sun Jul 22, 2018 12:12 pm

BC_CRDtom wrote:

From all my research the electric fan in front of the rad should always kick on when AC is active, so I’m pretty sure there is a defect in the system somewhere. I’ve never seen it running ever.

No. Just because the AC is on or the compressor is on does not mean the fan should be running. The fan will run if the engine temp is high enough or if the AC high pressure switch activates the fan, or high speed if both relays activate.

If its hot outside, parked, AC is on, and its not blowing very cold and you manually jumper the AC fan relay and the AC cools down more, that would be a sign that the AC high pressure switch is not triggering the fan to come on.

WWDiesel · **Posted:** Sun Jul 22, 2018 1:05 pm

Cooling fan runs based on AC Hi side line pressure. All operations are controlled via the ECM, i.e. compressor clutch, fan relays, etc...

From FSM wrote:

A/C HIGH PRESSURE SWITCH DESCRIPTION (operation)
The A/C high pressure switch controls both A/C compressor clutch engagement/disengagement, and electric cooling fan operation when equipped with the 2.8L diesel engine.

The A/C high pressure switch controls the electric cooling fan operation by monitoring refrigerant line pressures.
When the A/C discharge line pressure rises above 1900 to 2200 kPa (280 to 320 psi) the fan will turn on*.
The cooling fan will turn off when the A/C discharge line pressure drops to 1600 kPa (235 psi).

The fitting for the A/C high pressure switch on the A/C discharge line is equipped with an O-ring seal and contains a Schrader-type valve, which allows the switch to be serviced (replaced) without discharging the refrigerant system.
If the A/C high pressure switch is faulty or damaged, it must be replaced.

See page 22, section 24 of the 2005 FSM.

*It does not state which fan speed is selected, but I think most believe it is the HIGH speed? :roll:

Without a logic flow diagram of the ECM we are left to guess on some matters... :banghead:

Sir Sam · **Posted:** Sun Jul 22, 2018 6:21 pm

WWDiesel wrote:

Cooling fan runs based on AC Hi side line pressure. All operations are controlled via the ECM, i.e. compressor clutch, fan relays, etc...

From FSM wrote:

A/C HIGH PRESSURE SWITCH DESCRIPTION (operation)
The A/C high pressure switch controls both A/C compressor clutch engagement/disengagement, and electric cooling fan operation when equipped with the 2.8L diesel engine.

The A/C high pressure switch controls the electric cooling fan operation by monitoring refrigerant line pressures.
When the A/C discharge line pressure rises above 1900 to 2200 kPa (280 to 320 psi) the fan will turn on*.
The cooling fan will turn off when the A/C discharge line pressure drops to 1600 kPa (235 psi).

The fitting for the A/C high pressure switch on the A/C discharge line is equipped with an O-ring seal and contains a Schrader-type valve, which allows the switch to be serviced (replaced) without discharging the refrigerant system.
If the A/C high pressure switch is faulty or damaged, it must be replaced.

See page 22, section 24 of the 2005 FSM.

*It does not state which fan speed is selected, but I think most believe it is the HIGH speed? :roll:

Without a logic flow diagram of the ECM we are left to guess on some matters... :banghead:

Low speed. It just turns on one coil of the motor, coolant high temp turns on the other for high speed.

BC_CRDtom · **Posted:** Sun Jul 22, 2018 7:35 pm

Interesting, I guess the first thing I should do is check my high side pressure to see if its exceeding those parameters.

Since my compressor is clutching in, it would appear the high pressure sensor is doing its job correctly on that front, would I be safe in assuming it should be correctly functioning in regards to the fan as well? Or is it possible to have the switch correctly work the compressor clutch but not the fan?

flash7210 · **Posted:** Sun Jul 22, 2018 9:03 pm

Drive it around during the hottest part of the day, with the AC on.
Make sure the engine is fully warmed up.
Then stop in a parking lot somewhere. Make sure the AC is still on.
Get out and look or listen to see if the fan is running.

The fan doesn’t come on immediately when the AC is on.
The hotter things are, the higher your AC pressures will be.

The purpose of the AC condenser in the grille is to condense the Freon into a liquid, therefore lowering the AC pressure. If the pressure is too high, the fan gets turned on improve the function of the condenser.

If you are getting cold air out the vents, then the condenser, compressor, and Freon are all doing their jobs.
Typically, on a 95F summer afternoon, with the AC on, cabin fan speed 2, I would expect the air out the vents to be between 40 and 50F.

flash7210 · **Posted:** Sun Jul 22, 2018 9:21 pm

BTW,
I got tired of guessing when the electric fan is supposed to turn on.
So I wired in a simple switch so I can manually turn the electric fan on if I feel necessary.

Also,
I see in your signature block that you are using the inline thermostat thing.
If you are doing that with a factory unmodified thermostat, you should stop.
Use one thermostat and one thermostat only.
So either go with the HDS or a factory thermostat replacement.
I did the inline thing too and wound up with a cracked head.
And I’m not the only one that’s had that happen.

WWDiesel · **Posted:** Sun Jul 22, 2018 10:03 pm

flash7210 wrote:

BTW,

Also,
I see in your signature block that you are using the inline thermostat thing.
If you are doing that with a factory unmodified thermostat, you should stop.
Use one thermostat and one thermostat only.
So either go with the HDS or a factory thermostat replacement.
I did the inline thing too and wound up with a cracked head.
And I’m not the only one that’s had that happen.

Super advice!
Heed the friendly warning; several members on LOST have reported running an inline thermostat cost them major repairs and major $$$$ to their engine.
The factory cooling system is designed in such a manner that having an inline thermostat until it opens can impede the minimum flow requirements of coolant through the head and can cause hot spots that can blow head gaskets or worse yet, crack a head. :shock:

Run only a Mopar factory thermostat or one of the HDS custom units. Anything else is asking for trouble.... :roll:

BC_CRDtom · **Posted:** Sun Jul 22, 2018 11:38 pm

Ya, that has been on my mind as of late.

I think its time to pony up the $$ for a HDS thermostat housing.

AC Blows Cold but electric fan never kicks on

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