My 1999 Ranger seems to get weaker and weaker defrost as I drive. When I start it in the AM everything seems fine. Hot air blows from defrost (or vents or wherever I select). However after about 45 mins the defroster seems to come out only as a trickle (and none of the air from elsewhere) even though the blower sounds like it's still blowing full blast. Plus somehow it seems like cold air is getting in but not clear where. If I let the truck sit for 6ish hours, it works fine again for 45 mins.

My thinking is vacuum leak based on research and I'm prepping to run a smoke test but as I understand, in a weak vacuum situation it should default to defrost and thus if the blower is going full, I'd expect to feel that full blast via defroster which is why I'm a bit skeptical.

Thoughts or ideas what might be going on?

 

Does the 1999 Ranger have Air conditioning?
If so have a look at the metal lines coming out of the heater box in engine bay when air flow is effected, if you see frozen water on the outside of the pipes the Evaporator is freezing up inside and blocking air flow, that is caused by being low in "freon"

AC is on during Defrost to help dry the air
AC should never get below 38degF or it will freeze water on the outside of Evaporator clogging air flow

Air flow in Rangers
Blower/fan>>>>>>Evaporator>>>//>>>blend door>>>>>>Vents

All air pass thru Evaporator first, then thru firewall //
The Blend Door just directs the air flow either thru the heater core(HOT) or around it(COLD), Blend Door is what TEMP control **** operates
Blend Door can't block air flow, just direct it

It's possible blower's cage fan is slipping on the motor's shaft after running a bit, long shot
It's possible leaves or other debris is blocking evaporator, but would expect that to be all the time, you can pull off the blower motor and reach in to clear any debris, longshot

Yes, no vacuum = Defrost vent
The Vent doors can't block air flow, just direct it, as you say

 

Hi Ron.

Thanks for the reply!

Yes, the truck has AC.

A couple months ago, the AC compressor kept kicking on and off and from my research it seemed low on freon. So I got one of those top it off kits and added freon to the right amount and the compressor kicking on and off stopped for a few days but then returned. Now I do indeed see the frozen water on the outside of the pipes. I had convinced myself they were unrelated to the current issues because it seemed like different systems.

I'm guessing this means that there is likely a leak in the freon system if the issue returned so quickly?

Would the AC system even affect the blowing of hot air through the defrost vents? I would have expected it only to matter if I had AC selected instead of defrost.

 

AC is on in all positions except OFF and Vent, it dries the air in the cab which helps with Defrosting windows so always on in Defrost for sure

Unplug AC compressor, drive it that way for a few days and see if the air flow issue goes away, if so you may want to have the system serviced
It may indeed have a leak, if so this issue will just go away once enough "freon" has leaked out, lol


AC works by compressing a liquid("freon") and then letting it out slowly, and as the liquid de-compresses it COOLS down, the rate at which the pressure is released decides the amount of cooling
When a system is low on liquid there is less pressure on the "low side" so more "high side" liquid is released, and over cooling happens, liquid goes down to 32degF(freezing point of water) or lower, same as your home freezer does, but in this case its bad because moisture in the air freezes on the pipes and Evaporator Vents inside the heater box and that blocks air flow from fan to Defrost Vent, or any vent in the cab
High and Low sides need to be balanced for AC system to work as designed

 

Wow, that is super helpful and all makes so much sense. Really appreciate the advice and learning.

What's the way I unplug the AC Compressor?

 

It should have a 2 wire connector on compressor, for AC clutch, unplug that connector

There is also an AC Clutch relay in the engine fuse box, you can pull that relay out

You can go here to get owners manual that has fuse and relay maps: https://www.ranger-forums.com/genera...1-models-3747/

 

Really appreciate the help, Ron!