defrost doesnt work

[cwillis]

I have an 84 suburban. My heater works well and the blower motor works fine. The problem i am having is that the top slider doesnt seem to do much.
I cannot redirect the sirflow to defrost.

any ideas?


Thanks
Chris

[DmaxMaverick]

There are a couple things to check. First, verify you have good vacuum. The pump should be providing at least 15" of vacuum. About 25" is optimal. The selector lever operates a vacuum selector valve. It is a plastic manifold with a rubber selector disc. These can become deformed or dry out. The valve can be purchased alone, is inexpensive, and is easy to replace. The vacuum selector is separate from the electrical switches (there are two with A/C, one w/o). If it's just dry, it can be lubed with dielectric grease and sometimes works for a long time. It is attached to the back (toward front of vehicle) of the HVAC selector cluster. The hard plastic vacuum lines can crack, or the connection at the selector valve can fail, either the line or the rubber insert at the valve. Lastly, the vacuum actuator for the HVAC doors can fail or the cranks, shafts or doors can become frozen (partially or completely) or obstructed.

If you have good vacuum, you should be able to hear the vacuum being routed or a vacuum leak. If the vacuum hisses continuously near the selector, I suggest a lube or replacement of the valve. If under the dash near the HVAC assy, it could be a line, connector, or actuator. If you hear the vacuum hiss momentarily when the selector is moved and the air direction doesn't change, or changes very little, the valve is likely good, it is likely a frozen, broken, or obstructed door, actuator, or linkage/crank.

[cwillis]

wow thank you so much for all that information, that is more than i could find on any web pages.

sorry i am a diesel newb though. . . where can i check for vacuum? i havent seen any good spots.

is the 15" at idle?

do you have any pictures of this stuff? Thanks a TON

A1C Willis

[DmaxMaverick]

Vacuum is checked at idle. The vacuum pod is located at the rear of the engine, where the distributor would be on a 350 gasser, just rear of the intake manifold. Bear in mind, a pump putting out 15" may indicate a failing pump, but is enough vacuum to operate the HVAC. A healthy pump should be drawing 20-25". Wild vacuum gage fluctuation is normal when testing at the pump, and is less the further you get from it. The vacuum can be checked at the pod, where the hose connects. Once you confirm it's good, check it at the HVAC cluster. You should be able to hear vacuum hissing when you move the HVAC selector lever, in any case. Put your ear up to it, and if you don't hear it, you have a significant leak between the valve and pump, or the pump is not pumping.

If necessary, pump replacement is simple, and should take less than an hour with common tools. Much like replacing a distributor, only you don't have to time it. IIRC, there were 2 pumps available for 1983-86. Be sure to get the correct one, and have the parts store verify by the VIN if they can. Vacuum pump failure is common if they are old. They don't last forever, and I have rarely seen one last more than 10 years, but 5 years isn't uncommon.

Note:
The only "Diesel" part of all this HVAC stuff is the source of the vacuum. All else is the same as their gasser counterparts. The gasser vacuum is sourced at the manifold, or the base of a carburetor.

[cwillis]

wow thanks again. . . thats is an great chunk of information to know!
maybe even a sticky?

do you have anything to say about converting to R-134a versus finding old or recycled r12?

I heard R-134 sucks in suburbans.

[DmaxMaverick]

You're welcome. My pleasure.

R134A. Hmmmmm......

From a cost standpoint, there's no argument. 134 is pennies, to R-12 dollars. Yeah. It sucks, though. Depends on the climate you see, and the health of the system. You can make it work, but there isn't much room for error. You can get more effective evaporators and condensers, designed to be more friendly with 134. In the end, it may be worth it to upgrade, but the initial cost is usually the stopper. A complete R-134A system upgrade will cost about the same as 2 R-12 services, or more. It depends on where you want your ends to meet, and how you weigh your priorities. If a wife is involved, it may be worth a lot more to do it right, and be done with it!

[cwillis]

say. . . south georgia summer heat, without dual AC. . . only single


im thinking R12. seems like i can get some ok deals on it on EBay. how many of those cans do it need? and what else do i need to charge my system after a compressor change?

Thanks
Chris

[DmaxMaverick]

Ebay....Ewww. Be VERY careful what/who from you buy. Many of the R-12's, and R-12 "compatibles" are not R-12, or not pure R-12. Could be "other" types of freon, or even propane. Realistically, you could replace your entire charge of R-12 with propane, and it will act just like you charged with R-12. Problem is, it has obvious drawbacks.

It is best to have a shop that does A/C servicing do it. The system needs to be evacuated (vacuum) and leak tested before you put in that gaseous gold. Even if you get lucky and have no leaks, it should still be evacuated. When you open the system, you introduce many contaminants into the system, just from the atmospheric air. The contaminants include moisture, and the air itself. If you are dead set on a DIY, there are a couple tricks to getting it done, "mostly right", at an acceptable level. It can be much less expensive than having it serviced, but there are certain steps you should not skip.

The amount you need will be listed in your owner's manual, on the A/C evaporator housing (under the hood) or on the accumulator, and in a service manual. You should get this amount as close as possible. Too much or too little not only cools less efficiently, but can cause costly damage to components. More is not better!! You will also need to get the right amount of oil into the system, and you can do this by either charging the oil itself, or get freon with the oil already in it. Either way, it's gotta be done. Be sure to also replace the orifice tube (front) and check/replace the expansion valve (rear) while you have the system open. If you need to replace them, the system has to be "empty" and open. And, ALWAYS replace the accumulator/receiver/dryer with the compressor. Most of the small junk the pump tossed out when it failed can accumulate there. The orifice tube will have a screen on it that will catch the majority of the larger stuff that passes from the pump, through the condenser, and into the system. Also, flush and blow out the condenser. It is also a good time to replace the pressure switch. They aren't expensive, and need to work for your system to operate correctly.

PM or email me for more on the charging procedure if you are dead set on doing it yourself. I can help you with the right "shade tree" method of doing it. Be careful of the advice you get that isn't pro. Components and freon are too expensive (and potentially dangerous) to overlook the necessary steps of the process.