Attempted R-152a "Duster" A/C Conversion....some advice....

I have to ask why you changed your mind? You seemed dead set r152 was the way to go.

I changed back simply because of the broader spectrum of information available about R134 being used in an r12 system, and more specifically, knowing pretty much exactly how much of the refrigerant I needed to use. With the Duster, I was never quite sure how much to add. Although I must say, the R152 did put far less of a load on the compressor. I hardly even felt it kick on, and while operating, the drag on the engine was far less.

That all being said, now that I have the R134 back in, with correct charge amount, knowing there are no leaks in the system, and having installed an electric fan on the condenser, I am still concerned with what seems like an excessive amount cycling of the compressor due to the evaporator de-frost switch engaging.

It seems as if A) This defrost/deice switch is cutting out the compressor far too often, sometimes within just 2-3 mins after the last cut out, and b)When this compressor shut down occurs, the air coming out of my vents seems to warm up far too quickly, usually 5-7 degrees warmer in a matter of seconds. One wonders why the evaporator isnt holding onto all of the 'coldness' for lack of a better term. Ive owned other cars which the ac compressor cycles on and off at times, but there usually isn't the sudden rise in vent temps I'm seeing with my truck.

(Without fail, this 'defrost' switch seems to always cut out the compressor when my vent temperatures read approx 50 degrees. Simply won't allow them to get any colder, regardless of blower speed)

Since i've charged with correct amount of R134, I know pressures are right where they should be, and I have no leaks in the system, AND have sealed any possible duct work cracks near the blower and evaporator casings, where should I be looking now? Or is what i'm describing normal operation? Is this simply put, as good as I'll get out of the system? Only other item I think I could possibly replace would be the expansion valve...everything else appears fairly new including compressor, evaporator, condenser, receiver/dryer.

Thanks again for replies.

Some vehicles have an adjustable temp sensor with a probe (capillary tube) that have a knob that adjusts the cycling. (temperature cycling switch) I do not think the B series does though. But the expansion valve has a spring in it and opens sort of like a thermostat and if the shaft inside has any wear or damage it could just not be opening all the way or sticking.

If you have any kind of blockage inside the evaporator the low side pressure would not stay at 40 or higher for long. And the high side would go way up and if it has a high side pressure switch it would shut off the compressor. Do you know for sure the evaporator is freezing up?

You could also have a stopped up drain and it's retaining water. That would cause it to blow warmer air when he compressor is not running. But there are several factors that that can cause one to not cool correctly like: stopped up condenser from a bad drier at some earlier time, damaged fins in the condenser, paint or oil on the condenser, fan clutch bad, no fan shroud, radiator stopped up, thermostat stuck closed or sticking partially closed, blocked intake vents in the evap box, dirt lint or plastic bag sucked up inside the box, degraded fins in the evaporator, stopped up evaporator from a bad drier at some earlier time, stuck expansion valve, bad capillary tube on the expansion valve, bad drier, weak reed valves in the compressor, bad piston rings/seals in the compressor, improperlay spaced clutch, weak magnet on the clutch, slipping belt, bad pressure switch(es) etc etc etc.

You could also have had someone at a dealer install the evaporator incorrectly and not put the capillary tube in the evaporator where it's supposed to be. Or if it's the style that clips onto a tube in the evaporator core box it may have come loose. We used to wrap those with a tar like substance.

Yes Mazda B series came from Japan with no air. The dealers installed it. There was an aftermarket version as well with a copper and aluminum coil type evaporator available then as well. But the lines coming through the firewall would also be copper if that were the one you had.

If you sprayed brake cleaner in the evaporator it could have removed the clear coat from the aluminum fins and tubes in the evaporator (no it's not a coil) and that can make them have a rough surface which is bad for cooling. Plus they would also then be more likely to catch dirt and dust which degrades heat transfer.

The evaporator will not "hold the cold" it merely transfers heat to the refrigerant. Think of it in those terms as heat transferring to the refrigerant and it becomes more clear. as I stated before refrigerant sucks in heat in the evaporator and releases heat in the condenser. Proper pressures

If you have the low side at 40 is that on the R-134s scale on the gauges? If you have it at 40 to 50 on the R-134a scale it will be cold out the vents. Now you have some other issue

You see there are many many things that can cause a system not to cool properly. When you have an unknown problem you have to eliminate them one by one. If I were the tech working on it and I had the system open I would blow through the evap and condenser and make sure they were not stopped up, replaced the drier and expansion valve and flushed the system and changed the oil. Assuming the compressor was good it should cool. If it did not then you start feeling lines here and there to see where it's hot and cold and listen to the switches click on and off while you watch the gauges to see what pressures are doing while these switches click on and off. All this and experience knowing what it all means leads to a better diagnosis.

No matter how many years someone spent at school learning the technical side of this there is still on the job learning. The same reasons doctors do a residency instead of going straight from med school to operating on people. It takes on the job learning not just classroom or book learning.

He is correct: the original AC kits for these had a pre-set de-icer switch, not adjustable. Yes, it's possible that this is bad; I doubt that it's good but in a wrong position because it would then be reaching mid 30s temperatures to shut off.

OK, poster says the system cycles too often. What poster could do would be to bypass this de-icer switch and install a toggle switch, and use that to control de-icing. In Tucson, humidity is pretty low, and the temperatures are high enough that should he feel too cold, he could flip the toggle or just press in the AC on/AC off button on the dashboard instead of adding heat with the heater. That would be under $10 "fix".

In fact, he could just jump that de-icer switch and just shut off with the AC on/AC off button on the dashboard. If he's not feeling too cold, then the air passing through is not cold enough to do any freezing up. Note: I do not recommend this in humid parts of the world, but don't see any negatives for Arizona. This would essentially be a "free" fix.

Very good post. You could also install a low side pressure switch from a GM car, mid 80s and see if that controls the compressor better. Unfortunately the low side only has one schrader valve port on the low side unless the compressor has been replaced with a different/similar model with a port on the compressor. I like the idea of jumping it and then using the A/C switch to control freeze up.

If you do that try to get a visual on the evaporator and see if it is actually freezing up.

OK, can i bypass the deicer switch by simply connecting the two wires that are attached to that switch? Also, since I did have to tape my blend door shut, could I be losing cool temp out that door? I thought I sealed good, but maybe not? I didn't tape the top of the door up...

Finally would a clogged expansion valve benefit from running a flush on the system? The kind sold in cans?

No the expansion valve is not going to benefit from a flush. If it has a screen in it the screen may be clogged. If you have to drain the system again for any reason just replace it and the drier. The system needs to be on recirculate for best results.

Yes, can bypass that way and then control using the push on/push off AC switch.

Have you also established that NO HEAT is "contaminating" the system from the heater core? If so you can also bypass the heater core for the summer with a short piece of 5/8 inch heater hose, hose clamps, and hose connectors.

Most auto parts stores also sell shut off valves for the heater core. Thats a good place to look as well. You may be loosing cooling to the heater core. Didnt think of that.

Well bypassed the deicer and was able to achieve 40 degree vent temps on 3rd blower speed.

That's an improvement. If I can confirm the blend door is shut all the way, and no air is escaping thru it, wouldn't that rule our heater core contamination? Or would you recommend still doing the hose bypass to ensure there is no hot coolant flowing thru it?
I've done heater core bypasses before, just want to better understand the thinking into bypassing it as related to ac.