Rear ac works sporadically

Tonochio

My 2004 Suburban z71 rear control was only working on 1 and 3. Then only 3. Now it shuts off and turns on whenever it feels like it. Still only working on 3. Any ideas?

imidazol97

Need more information. Is it the operation of the heat or cooling that's changing, or is it the operation of the blower motor fan speed that is the problem. I'll assume it's the blower motor and fan giving trouble.

There are two types of blower fan speed controls: one is a resistor set of a series of resistors with taps along the connections to give various amounts of power to the blower fan motor; the other is an electronic unit called a blower control module.

The resistors can fail losing lower speeds but usually all speeds below a certain speed. Sometimes only the high speed is left because of where in the resistor path the break occurs.

The modules do strange things. The blower speeds may not be the same as they used to be. I.e. speed #2 may be different one day from the next, or may change while driving. That may be what yours is doing. The problem is to find where the resistor is located. It usually is screwed to the air box close to the blower motor because the end of the power transistor involved has to stick into the path of the moving air for cooling--there's a flat plate that's a heat sink involved.

So if you look on Rockauto.com you'll see pictures of these.

You could also have a failed connection that the blower module plugs into. The high current passing can cause the contacts to heat up if there's some corrosion. The heat damages the plastic around that contact by softening or burning. The contacts are often corroded more by the heat.

Often if you've located the connector, with the blower fan running by squeezing and twisting the connector you can cause a change in the contacts inside and hear the blower fan change speeds. For many applications the connectors are available to be patched in to replace a bad one.

Also the blower motors themselves can act up with poor contacts between the brushes and the commutator. Usually that's a case where all speeds fail and a road bump or a hit from a hand causes the contact between brushes and commutator to complete the circuit again and the blower starts working for a while.

Tonochio

Thanks. This really helps. It's not a hot/cold issue. It's the blower fan speed.