AC problems

[bmnelson23]

I have a 99 ford expedition, 5.4 4x4. AC works fine unless the car sits in the sun parked and turned off for a few hours. If it has been in the sun, upon starting the car, the AC and the blower motor do not turn on. After running the car for 10-20 minutes, the AC will kick on perfectly. This happens in the summer and the winter. The Heater will always turn on regardless of being in the sun or not during the summer or the winter but the AC will not turn on in the summer or winter if it has sat not running in the sun for a few hours. If the car is parked in the garage, regardless of summer or winter, the AC kicks on every time. Any help is greatly appreciated. Would a faulty resistor motor behave like this if it got too hot? Is there a sensor somewhere that is causing the AC and blower motor to not turn on? Help.

 

[99WhiteC5Coupe]

I have a 99 ford expedition, 5.4 4x4. AC works fine unless the car sits in the sun parked and turned off for a few hours. If it has been in the sun, upon starting the car, the AC and the blower motor do not turn on. After running the car for 10-20 minutes, the AC will kick on perfectly. This happens in the summer and the winter. The Heater will always turn on regardless of being in the sun or not during the summer or the winter but the AC will not turn on in the summer or winter if it has sat not running in the sun for a few hours. If the car is parked in the garage, regardless of summer or winter, the AC kicks on every time. Any help is greatly appreciated. Would a faulty resistor motor behave like this if it got too hot? Is there a sensor somewhere that is causing the AC and blower motor to not turn on? Help.

Does the a/c system compressor not run AND the HVAC blower motor not run when this happens, or only the blower motor?

Do you have rear air? Does the rear blower motor run when this happens?

A defective blower motor speed control (resistor unit) usually only affects the front HVAC blower motor.

Have you examined the wiring connector at the blower resistor and also at the front HVAC fan motor?

These links may help you:

https://static.nhtsa.gov/odi/tsbs/2019/MC-10163573-0001.pdf

And

How To Replace Blower Motor Wiring Harness 2007-2017 Ford Expedition

Shop for New Auto Parts at 1AAuto.com https://trq.video/c-157-ahThe heat and air condition fan is controlled by a separate module, called the blower motor re...

www.youtube.com

 

[bmnelson23]

Thanks for your response. The compressor does not engage if it has been out in the sun. Once the car is driven, it engages and then the controls on the dash begin to blow cold air. I do have rear air but those don’t turn on either if it has been sitting in the sun. I am truly baffled by this. The system works great as long as the vehicle is covered by a tree or garage, out of the sun. But if it sits in the sun, the clutch on the compressor does not engage, the air blower inside won’t blow any air or even turn on, until the car runs for 10 to 20 minutes.

 

[99WhiteC5Coupe]

Thanks for your response. The compressor does not engage if it has been out in the sun. Once the car is driven, it engages and then the controls on the dash begin to blow cold air. I do have rear air but those don’t turn on either if it has been sitting in the sun. I am truly baffled by this. The system works great as long as the vehicle is covered by a tree or garage, out of the sun. But if it sits in the sun, the clutch on the compressor does not engage, the air blower inside won’t blow any air or even turn on, until the car runs for 10 to 20 minutes.

A long shot. Have you checked to see if all fuses and relays are fully seated in their slot for all electrical junction boxes?

On my 2015 - there was one box on top of the radiator - just under the hood. Sun and heat could cause one or more to expand or contract, and not make full contact?

 

[bmnelson23]

Good question and yes I have checked all of those.

 

[Billme]

I gave grok your question

This sounds like a heat-soak related electrical or sensor issue in the HVAC system, common on these older Expeditions (1997-2002 era). The fact that it only happens after sitting in the sun (hot underhood/engine bay), resolves after 10-20 minutes of running (as things cool or stabilize), works fine in a garage (cooler ambient), affects AC/blower in AC mode but not heater, and occurs in both summer/winter points to something temperature-sensitive. justanswer.com ford-trucks.com


Key Observations from Your Symptoms


• Blower doesn’t run at all initially in AC mode when hot-soaked → Suggests no power to the blower or a control signal blocked when hot.


• Heater (blower) always works → Blower motor and basic power/ground are probably okay; issue is likely mode-specific (AC request or compressor-related interlock).


• AC compressor doesn’t engage initially (common with blower issue or safety sensors).


• Resolves after running → Cooling or voltage stabilization fixes it temporarily.


• Garage parking fixes it → Rules out low refrigerant (would be consistent) or basic mechanical failure.


Would a Faulty Blower Motor Resistor Cause This?


Unlikely as the primary cause. The resistor (or control module) mainly affects fan speeds (especially lower ones failing, high still working). It gets hot in normal use and can fail intermittently or cause no blower on certain settings, but it’s not typically heat-soak sensitive in this exact way (sun exposure only). A bad resistor + failing blower motor can overheat/melt connectors, but your symptoms are more all-or-nothing in AC mode when hot. go-parts.com go-parts.com


However, check it anyway—it’s a cheap/common failure on these trucks (located behind glovebox near blower). Inspect the connector for melting/burning, a very common issue.


Most Likely Culprits (Heat-Related)


1. AC Pressure Switches or Cycling Switch (Low/High Side): These can be temperature-sensitive. When hot-soaked, pressures might read oddly (e.g., low pressure switch opens to protect compressor), preventing AC engagement and sometimes blower in AC mode. Common on these Fords. Jumping the low-pressure switch (on accumulator) is a quick test—if AC/blower then works, replace the switch. justanswer.com


2. Evaporator Temperature Sensor (or related thermistor): This monitors evaporator coil temp. If faulty or heat-affected, it can prevent compressor clutch engagement (and tie into blower logic) to avoid freezing. Heat-soak could throw off its readings. expeditionforum.com


3. AC Compressor Clutch Relay or Wiring: Relays can stick or have heat-sensitive contacts/coils. Power/ground issues that appear when hot. The WOT (Wide Open Throttle) relay or AC clutch relay in the underhood box is worth swapping/testing. ford-trucks.com


4. Blower Motor Itself or Power Feed: Rare, but a failing motor can draw excessive current when hot (bearings seize more), tripping protection or blowing a fuse intermittently. Less likely since heater works.


5. HVAC Control Head / Module or Wiring: Less common, but connections or internal components can act up when hot. Vacuum lines (for mode doors on these models) can leak more when hot/expanded. go-parts.com


6. Ambient Temperature Sensor or PCM Input: The PCM controls AC enable based on various sensors. Heat-soak affecting an underhood sensor could disable AC request. dot.report


Low refrigerant is possible but less likely (should affect consistently or show other signs; garage test helps rule it out somewhat).


Recommended Diagnostic Steps (Start Simple/Cheap)


• Check fuses/relays first (AC clutch, blower, HVAC). Swap AC relay with a similar one.


• Test blower directly: Access blower (passenger footwell, remove panel). Apply 12V power to pins when issue occurs—if it spins, problem is upstream (resistor, switch, control).


• Scan for codes: Use an OBD2 scanner (even basic) for HVAC/PCM codes like blower control or AC-related.


• Pressure switches test: Locate low-pressure switch on accumulator (passenger side firewall area). Unplug and jumper terminals—see if AC engages immediately when hot-soaked. justanswer.com


• Inspect connections: Look for corroded/melted plugs at blower resistor, motor, relays, and underhood.


• Monitor voltages: With key on, check power at blower connector and AC clutch when problem is present.


If you’re not comfortable with electrical testing, a shop with HVAC gauges and a scan tool can diagnose quickly (pressures, sensor readings).


Parts to consider (inexpensive starters): Blower resistor + pigtail, AC pressure switches, relays. Blower motor if it tests bad.


This is a classic intermittent heat-related failure on these vehicles—start with the switches/relays and resistor inspection. Let me know what you find or more details (e.g., does blower work on any setting in AC mode when hot? Any noises/smells?), and I can narrow it further. Good luck