Need ac help.


Old 05-23-05, 12:28 PM
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Need ac help.

I have a 91 chevy silverado. 5.7 When I start the truck (or turn on the air) I get a flashing a/c on my display. The owners manual says this is a warning that the refridgerant is low.
As I had just bought the truck, I decided to convert it to 134a. THe sticker on the (dryer?) says it holds 2.50 lbs, I put 3 1/2 cans of 134a in it (each was 12 oz.) and the compressor still wont kick in and the display still flashes a/c for two mins.
There is no fuse in the fusebox that relates to this other than one that says heater/ac which is for the blower fan. On the firewall above the (dryer?) there was a plastic cover that had under it a terminal block with an inline 30amp fuse (blown) which I replaced with a good 25amp but it either is not for the ac or there is another problem.

Any help would be appreciated.
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Old 05-23-05, 02:08 PM
Jason R's Avatar
Join Date: Jun 2002
Posts: 261

Did your owner's manual say that the flashing a/c light always and only means low refrigerant? "Low refrigerant" doesn't mean no refrigerant.

Does your a/c work at all? Before or after your retro to r-134a?

You should have taken a pressure reading on your system before adding anything. I once bought everything needed to do a retro to r-134a because the system just didn't work (no nifty flashing lights) only to find a blown fuse upon further inspection. (luckily before I added anything… the system still works fine to this day 2 years later.)

First things first: Do you plan to keep this vehicle for more than a couple years? My advice will differ greatly depending on how long you want your a/c system to probably work.

The r-134a retro kits are commonly referred to as death kits because the instructions are so non-inclusive of several things that should be taken into account with a retro. Even how much to add varies depending on how tough your vehicle’s cooling system is.

If you actually do have to add refrigerant that means you have a leak (not unheard of for a vehicle this old). Just adding r-134a is almost never the right thing to do, even if you're a hack like me .

It's been since last summer since I've delved into the subject area in any great detail, but I believe there is an issue with excessive amounts of r-12 mixing the the POE oil that comes with the retro kit. I believe it may cause acids to form internally which will shorten the life of your compressor valves and the this heat transfer surfaces (condenser and evaporator).

If your system was actually completely out of r-12 that means you almost certainly have introduced air and moisture to the system. A little moisture will be absorbed by the dryer / receiver (provided the desiccant isn't saturated already). Air in the system will drive up the pressure quite a bit, as well as chemically react with the r-134a to produce more acids in the system.

R-134a operates at quite a higher pressure than r-12, and it has smaller molecules. Both of which means it will leak faster than the r-12 did. All of this adding refrigerant willy nilly leads me to another issue: The proper amount of oil in the system.

Your system will likely tolerate a little excessive oil in the system, but too much and you're going to kill your cooling capability (it will coat the inside of your evaporator), too little... and you know what happens. Since you've left the remaining mineral oil in the system from the r-12 you will have 2 types of oil in your system (normal and relatively harmless for a DIY retro). The mineral oil will just be sitting around in the low spots and the POE will be circulating around.

Buy a pressure gauge that hooks up to both the low and high pressure sides. Unless you have some serious money to throw at your a/c system you'll need them to be more precise on the refrigerant charge in your system.

To be proper you should take this system in to a professional and be prepared to shell out some dough. They'll probably want to flush your system (a good idea) and dispose of your refrigerant soup (r-12 and r-134a). They'll change the dryer to one that is r-134a compatible. (most are, some are absolutely not) They'll fix the leak, pull a deep vacuum on your system (which will remove moisture and air) and will charge with the exact amount of refrigerant necessary. An automotive system is most properly charged by weight and not by pressure, but by pressure is by far more precise than just dumping refrigerant in until it works again.

So..... if your compressor will kick in, take pressure readings on both the high and low sides ($17 at meijer for a gauge that will hook to both sides separately). We'll start from there.
Old 05-23-05, 04:36 PM
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I didn't make myself as clear as I should have. I did drain the system of r12. I don't have gauges although I may be able to come up with some. My problem is that he compressor will not turn on. At all. I've only had the truck a month or so and it didnt work when I got it. I guess what I'm saying is, what else could stop the compressor from kicking on? Switches, fuses etc? I know that once I get it running, I"ll have some work to do to determine the amount of refridgerant that I need to have in there. My point was that it has enough to have allowed the compressor to start, so I'm assuming that I have some other problem other than refridgerant at this point. I was hoping someone would perhaps be familiar with this model truck to tell me where any fuses might be other than the fusebox and also perhaps any pressure switches etc to check?
Old 05-24-05, 12:39 AM
nola mike
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your truck probably has low and high pressure protection switches; too much or too little will cause compressor to not work; not sure specifics of your system, but the first step is to make sure your compressor is getting 12v when system is turned on. sounds like you overfilled it. iirc, you should fill a retro with 3/4 or so 134a as you would r-12 since 134a is higher pressure to begin with.
Old 05-24-05, 05:57 AM
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Join Date: Nov 2003
Location: Shenandoah Valley
Posts: 1,147
start with...

...a wiring diagram. there is a low pressure switch located on the accumulator (which should have been replaced with the retro). this switch cycles the compressor based on low side pressure to prevent evap freeze alos provides compressor protection in the event of low charge. i don't think they used a high pressure cut out switch til would be located in the back of the compressor.

when you "request" ac operation...the ECM grounds the ac clutch relay coil and turns on the relay. this, in turn, connects the clutch circuit to battery voltage...which must flow thru the cycling switch to get to the compressor. for your flashing display...that's a c60 electronic system and different parts of the display can flash. so, it depends on which part, the part that indicates temp (across the bottom of the display), the mode part (the verticle part at the right of the display). usually when they flash, it means that one of the feedback actuators has malfunctioned. another thing altogether.

the bottom line is...the wiring diagram will help to understand circuit operation. there is also a diagnostic chart for "compressor doesn't engage" which leads the tech thru the logical process of elimination...does the control head produce a signal when ac is requested?...does that signal reach the ECM?...does the ECM ground the relay?...does the relay have voltage available on the control AND the load side?? lots of questions to answer, i know...but that's the nature of automotive electronics. try a library or perhaps ALLDATA for service information that can guide you. if you have a good understanding of DC electricity...just the wiring diagram may suffice.
Old 05-24-05, 06:29 AM
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Thank you carguyinva, thats the kind of answer I needed.
Old 05-24-05, 08:37 AM
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just disconnect the battery for a few seconds and reconnect this should reset it and will allow the compressor to kick in and work normally assuming there is sufficient pressure in the system now. this commonly has to be done on these models after repairing the a/c system when it has had low to no freon in the system it will not recognize the system now has sufficient pressure without resetting by disconnecting the battery.
Old 05-24-05, 11:47 AM
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If the compressor isnt running, how did you get 134 into the system?
Old 05-27-05, 08:30 AM
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Originally Posted by bejay
just disconnect the battery for a few seconds and reconnect this should reset it and will allow the compressor to kick in and work normally assuming there is sufficient pressure in the system now. this commonly has to be done on these models after repairing the a/c system when it has had low to no freon in the system it will not recognize the system now has sufficient pressure without resetting by disconnecting the battery.

Awesome advice Bejay, its cooling great now!!!! Thank you so much and thank everyone for their advice.

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