A\C Stuck in Bi-Level

[AugustDiesel]

Looking for some help with my AC,

My 1988 6.5 Goodwrench Suburban blows air out both the dash vents and the lower heater vents regardless of what setting I put the selector on. Any ideas on where the problem is or any schematics for this? It’s quite warm here in Florida with AC just trickling out the vents!

[DmaxMaverick]

Is your HVAC system vacuum or electric controlled? 1988 was a transition year, so it could be either, and Sub's and 1/2 tons are a foggy area (vacuum before, electric after).

If vacuum, listen carefully near the control head while adjusting the mode settings. If vacuum, it should hiss during mode changes, then stop hissing once the mode is complete. If it continues to hiss, the rotary valve (disc) is leaking, a line is broken/loose, or a diaphragm actuator is disconnected or damaged (they are normally extremely reliable). If it seems to act normally, but the modes don't change, it could be a broken bellcrank or plastic linkage connector. If no sound, the vacuum is leaking at some point prior to the control head. Lastly, if the vacuum pump has failed or the system has excessive leaks, almost none of the HVAC mode features will work.

If electric, you should hear the servos and doors moving while changing modes. Check electrical connections at the control head and harness. A burned/deformed connector would not be uncommon.

[Robyn]

Maverick has covered the gamut.

Myself I would start at the vacuum pump on the engine (back where the distributor would normally go)

Get a vacuum gauge on the thing and with the engine idling you should see a steady reading of about 26"hg or so.

If the needle is wobbling all over you need a fresh pump before moving on..

Make sure the Vacuum line from the pump to the firewall is in good order.

As you have a 1/2 ton the tranny is the 700R4 and does not use vacuum to shift.

Check the vacuum pump first before chasing your tail.

The heater controls are pretty good in the early Burbs and when nothing works its usually a vacuum pump or a hose issue.

Good luck