98 ford ranger 4.0 v6 4×4 ac turns to heat when driving only

Start with the basics first, make sure the freon charge is OK.

Does the temperature control on this truck use a vacuum to actuate the blend door by any chance? I seem to remember something similar where a vehicle’s A/C (or heat) would change depending on the engine RPM, it turned out to be a vacuum leak where the actuator only had enough vacuum at idle but not at higher RPMs.

I think the first step is to check the freon charge before getting too deep into this. If that is OK, find out if the heater temp control works using a cable or vacuum. You can either look underneath the dash where the controls are or, if that’s closed off, you may have to just pull out the panel the controls are on. If it uses cables then a vacuum leak or a faulty actuator aren’t the problem. If they do use vacuum, then it’s something to look at for leaks.

BTW – What happens if you have the AC on in park and rev the engine over 2,500 rpm? Maybe someone can look at the compressor clutch and see if it’s disengaging.

Quoted From Trcustoms719:

All great post’s.
+1 on the freon and the bend door.
Also the A/C compressor clutch may be slipping.

…maybe even the drive belt slipping as load increases with rpms????? I would think it would make some noise, but it’s a possibility.

A low refrigerant charge seems like the most reasonable first check. The PCM will turn off the A/C compressor if it’s seeing too low pressure. The pressure in the HVAC system would increase with higher RPM’s, but if the PCM isn’t seeing enough pressure, it will kill the power to the A/C clutch coil. I hate to recommend any work on air conditioning systems other than to take it to a certified professional, which is what I would recommend to you. If you aren’t experiencing any rough running issues, I wouldn’t think a vacuum controlled blend door was the issue, because a vacuum leak of any kind should give you issues. Did I read it right that you said you have a check engine light? The blend door changes how much heat is allowed into the cabin, and is located inside the HVAC air box under the dash. On full cold, It would be shut completely, allowing air to travel passed the evaporator, but not the heater core, but it can vary depending on the system. You may have a valve that controls coolant flow through the heater core, but that’s another story.

oh yes when i put it in park and rev up to 2500 it does the same thing just turns to heat

Is it hot air like heater hot or just warm outside air? I’d check the freon charge. I’d pop the hood in park running ac on and give it gas and see if the ac compressor disengages. If it doesn’t the problem is in the blend door. If it does disengage its low on 134 or low pressure switch could be faulty.