91 B2600i, a/c working now....strange....

Hello,

I have kind of a strange thing that happened today with my a/c working. Have a 91 b2600i, 4cyl, standard trans.

Got the truck about a month ago and noticed the a/c wasnt blowing cold and compressor was not kicking on. Previous owner told me it had been 'converted' to a r-134 setup with new compressor and other various parts, but believed it hadnt been charged after this was done.

Took the truck to a local mechanic, had the system evacuated and charged with r-134. Air blew decently cold for about a day. After an 8 hour shift at work, was blowing hot air again and compressor was cycling. Figured it had a leak. Eventually compressor wouldnt even kick on. After trying my luck with leak detector and sealant and another charge, I gave up after the A/c system was still not working.

Tonight, while tinkering with different parts on the truck, i noticed a small rubber hose coming in from the firewall and connecting to what I believe is the box housing for the evaporator core. I disconnected the hose from the box only to find it full and packed with dirt/debris. I vacuumed both the hose and inlet to the evaporator core box with a shop vac for a while until I thought everything was clear. I decided to test my a/c just for fun. Strangely enough, the compressor kicked on, remained on and air was blowing out cold.

I don't get it. It defies logic. I had assumed the r-134 i had previously put in simply had leaked out, causing the system failure. Evidently it was still in there, and a simple check at the schrader valve on the low side tube going to the compressor confirmed this right now....

Can anyone explain to me how me vacuuming this small rubber tube could have caused the system to start working again, and the compressor to remain engaged? I assumed there may have been some switch or something sensing this clog which caused the system failure, but just looking at the tube itself, it appears to be nothing more than a possible drain tube? I'm happy everything is working now, but kind of would like to know what caused this. Unless i possibly bumped something else while i was working down there that made the compressor kick back on? Any ideas???

Thanks

-Nile Sheen

There is a cycling switch that's mounted on the outside of the evap. core housing......maybe it is working intermittently and you just hit the right time it decided to work........BUT, if your evap. core is matted up with dust & hair (from pets that used to ride along?) and your not getting good airflow across the core, it could freeze up and the cycling switch will shut the compressor off........of course once the A/C hasn't been on for some time, it will turn itself back on since the core has warmed up or de-iced.

One of the things I do when I get a Mazda truck for the first time, and it has A/C, is clean the evaporator core and replace all of the seals on everything down there.......here's what a dirty core looks like!



I've had them dirtier than this also!





After pulling the lower half of the housing off and cleaning it........flows way more air through the vents as well!

I think it was coincidental timing. I recently had the AC pushbutton switch become intermittent. Sometimes it would come on after 5 minutes and other days it might not come on at all - or just decide to come on after an hour or so. Other times it would come on and the stop.

Was the light on the push button switch illuminated when you had it on but it wasn't cooling? If not, then that is probably your problem. I assume the compressor wasn't coming on. Next time check to see if you have voltage at the wire running to the compressor. Also, the next time it happens, take notice if the engine RPM changes at all. If the compressor does not come on AND there is no increase in engine RPM, then the computer isn't getting one of the two signals it needs to turn the compressor on and bump up the idle air. It needs the signal from the pushbutton switch and a ground through the fan speed switch to know that you need the compressor on. Since your fan is running on all speeds, it's not a fan or resistor problem. I'm going to think it's the push button switch or possibly the defrost switch ABE mentioned acting up.

Thanks for the replies guys, a little more info I have......

So I am thinking this is an electrical issue and that the drain tube being clogged was just coincidence. 100% positive about that now. What Im confused about now is the compressor randomly shutting off.

I would say 90% of the time now when I push the a/c button, compressor kicks on like normal and air blows out cold.

HOWEVER, at random I have been noticing the Compressor will shut off while driving and air becomes noticeably hotter. A majority of the time, this seems to occur when Ive stopped at a light or am at idle. Not all the time, but most of the time if ive been sitting for a minute or so. And It will stay disengaged until I start moving again. When the compressor does disengage the RPM's do go up slightly.

Any ideas? I did remove the casing to the evaporator core and it wasnt what i would consider 'clogged'. There was some debris I cleared out, but nothing major.

I did see that 'switch' with the metal wire running into the evap core box, and was wondering if that might have to do something with this problem?

Again, 90% of time a/c seems to be working normal now, but compressor has been randomly shutting off while stopped and wont re-engage until i start moving again. I tried looking up that pressure cycling switch on the side of the evap core box on rockauto.com, but couldnt find a replacement listed. Unless you guys think it's something else causing that problem. Ive heard under normal operation the compressor is supposed to cycle on and off, but i swear, when it does, air becomes hot quick! For what it's worth, I did check all the connections and sprayed everything with contact cleaner, but i couldnt find any cut wires or anything like that.

So, in closing, A/C seems to be working normal, except for the Compressor randomly shutting off at idle or while stopped, air becomes hot, and then it doesnt re-engage until I start moving again. Any ideas on this problem would be really appreciated. Suggestions on what to check and look for, etc. And where I can find a replacement cycle switch (the one on the side of the evap box) would be appreciated as well.

Thanks!

-Nile Sheen

I'm not so sure that the issue is the de-icer switch n the evaporator case. That can readily be temporarily jumpered to bypass that switch to check that out.

Have you actually checked, by getting out and looking, that the AC compressor is NOT engaged when you have this at idle speed and switched on? I mean that whether the center part of the AC clutch is actually turning, not just the outer pulley. Because I'm wondering if what you are experiencing might just be poor air flow due to a bad fan clutch or missing fan shroud, which will kill the AC performance at idle and low speed, but as the truck starts moving, the air gets forced through the condenser by plain old ram force.

If that doesn't pan out, I'd suggest doing what Scotch posted, easier at night: see if the dim light in the AC push-in switch is on or off when the AC compressor is not engaging. I even wired up a dash test light (blue, of course) so i can instantly tell when the AC compressor is getting voltage (helped me diagnose an AC clutch loose, loose nut !!!).

You did good to clean that drain tube, as condensed water from the cooled humid air needs to exit the evaporator case.

The compressor just kicks off randomly, and it seems to happen mainly when i'm stopped. I can even see the rpms jump when it does disengage.

So, for testing, you guys want me to see if the light on the A/C switch stays on when I feel the compressor turn off?

Also, I have noticed a couple times what appears to be the clutch on the compressor slipping or trying to spin, but goes to slow....then i've tapped it with a wrench,etc and it seems to free up and spin normally. I assumed since compressor is new, the clutch was too....is it possible a bad clutch or slipping clutch could be causing this? Like I said, def have seen it slip several times, and appear to try to be spinning, and ive had to tap it with something to 'free' it up.

bad clutch?

Are you running R12 or 134a?

Before you mentioned the clutch slipping, I had this theory. When I converted to 134a, I put in a binary pressure switch. The factory switch will prevent the compressor from running if the pressure is too low - i.e., you are low on refrigerant. The binary switch will kick off the compressor if the pressure is to low OR to high. When you run 134a through your system, the pressures will generally run higher. If you notice, all the late model cars (134a of course) have significantly larger condensers to keep the temperature and pressure down on the refrigerant. Having a binary switch will give you added protection on the high side - so you don't blow o-rings or put excessive high loads on the compressor - the pressure can run as high as 400 or 500 psi, especially if you don't' have a larger condenser or if you fan is not adequately cooling the refrigerant as it goes through the condenser. So, if somebody put a binary switch on your system, and then you come to a long stoplight and you have reduced airflow over the condenser, the pressure will rise and then that switch will turn the compressor off until the pressure drops - a minute or so after you start driving again. When that pressure switch cycles (on or off) it will make a nice thump in your truck as it's triggering at around 400 psi. Ask me how I know this!!!! (Hint: I have a loose wire in my electric fan thermostat/switch and sometimes it doesn't turn on my electric radiator fan.) So that is one scenario.

Now that you've told us about the clutch problem, I think that the pressure is building at the light and your clutch may be slipping because of the high load on the compressor.

How is your fan and clutch working? Electric fan? If electric, any provisions to turn it on while the AC is on? How does the truck coolant temperature do at long lights and long idle times?

One more scenario for excessive pressure is overcharging. It's not uncommon for people to buy a can of refrigerant at the store with a low side pressure gauge and start adding refrigerant until it's in the "Green Arc" or until it starts cooling. But unless you know how much refrigerant is already in the system, you don't really know how much to add. I would be curious to know what the high and low side pressure readings are while idling. Have you put gauges on them or do you have access to gauges?

OK, remember in my earlier post I hinted at diagnosing an AC clutch issue? Well, I've had two slipping clutch issues.

1. The clutch nut was a little loose, all I had to do was tighten it, that was several years ago. I had already practiced on two old compressors how to R&R the AC clutch with the compressor still in the truck, so no refrigerant would be lost. And when I went to swap on my real compressor, found the nut loose, and just tightened it. So turn on AC with engine in run mode but not started, and the AC and fan on. Tap the compressor center if necessary to get the clutch engaged (to hold it), and see if the nut can be tightened.

2. Apparently I got some oil slung up into the AC clutch. I carefully directed brake cleaner at the interface to clean it, that worked.

Occasionally with wear, the AC clutch "gap" will increase, and a washer must be removed or replaced with a thinner one to narrow the gap to specifications.

Also, this little nugget of information suggests that your problem is in the pressure switch or the compressor/clutch and NOT in the AC pushbutton switch, fan speed switch, resistor, or de-ice/defrost switch on the evaporator. When the RPM jumps, that tells me the computer is still trying to send power to the compressor and consequently, has adjusted the idle air to compensate for the compressor load. So the computer recognizes that:

1 The AC pushbutton is on
2 The Fan speed switch is on
3 The resistor is good (if fan runs on all speeds)
4 The defrost switch is closed (not defrosting)

All of of these conditions must be met before the computer will send power to the compressor/clutch AND bump up the idle air to compensate. In this and your situation, the computer doesn't know that there is a problem with the pressure switch or the clutch and it's sill bumping up the idle.

NOTICE: This is true for B2600i and B2200 efi trucks. Carbureted trucks are wired slightly different and works differently. (The computer will not get the signal to bump up the idle until downstream of the defrost switch. So if there is a problem upstream of the defrost switch, then the computer wouldn't be bumping up the idle speed on a carb truck.)

Thanks for the speedy replies...gets hot in Arizona, and trying to get the system perfect as soon as possible, so I appreciate the responses. In regards to the clutch, I haven't checked the big nut to see if its loose...think I'll do that next. I have had fluids fly around under the hood too, so the idea of fluid causing it to slip may also be possible. Brake cleaner was suggested to dry it? May look into that as well. Don't know if that aftermarket pressure switch was installed or not, but it is running r134, don't think pressure is too high or low though. Again, just to clarify, check tightness of the big nut on the front of the clutch, and clean out any oil deposits on it....also, not sure how big that gap between clutch and compressor is supposed to be, or if I need a thinner washer or not.