1995 B4000 Unique Hvac problem

[tangoyankee]

This one has me totally stumped:

All modes of my hvac work great... for about 30 minutes. Blows Cold a/c, or great heat. Vent, floor, and defrost all operate as designed .... for about 30 minutes. AND THEN after about 30 minutes it is as if all intake air is shutoff. During these times, almost no air comes out of vent,floor, or defrost. Although the vacuum motors for all of these selections are operating their respective doors, and the fan is running. The outside/recirculate door also operates as designed. But no air is output.
If I turn it off for about 15 minutes, when I turn it back on, it will blow air as normal...... for another 30min.
On the included diagram (from a 98) I can see no point of failure that could cause this problem and then reset after time off.

Has anyone experienced this problem or have an inkling of what the problem could be?!??

 

[Uncle Gump]

Could be the evaporator freezing over. Do you see a frost build up on the low pressure a/C hose directly after the orifice tube?

 

[tangoyankee]

Could be the evaporator freezing over. Do you see a frost build up on the low pressure a/C hose directly after the orifice tube?

Interesting.... I'll check. Would that block all air from getting through? And what would be the fix?

 

[Uncle Gump]

It could block the majority of airflow.

Typically when they freeze the evaporator... the system is a little low. Sounds strange... but the refrigerant boils lower in the evaporator which makes the coils frost and freeze. Lots of humidity in the air speeds up the freeze over time.

Maybe try unplugging the air compressor and see if it heats properly for a longer period of time without shutting down airflow.

 

[tangoyankee]

That all sounds reasonable and explains every symptom. Thank you!!!!
So the fix may be to charge the A/C system?