After 15 years....yeah, might be time!

[formula_gramps]

Well, as usual I like to recharge the A/C in my bird once every 15 years or so, whether it needs it or not.....

Over the last few years I've noticed that my A/C didn't seem to put out the same degree of cold air as it used to. And if I really think back, about it, I'm pretty sure it was short cycling too--- especially last summer.

Last year it seemed pretty marginal. But it's been no big deal to me, since up here in New England I find that I can usually count on one hand how many days I desperately need A/C--- especially with being able to take the t-tops off.

Well, tried the A/C the other day and "nada"...... Compressor doesn't kick in at all. So I figured after about 15 years it might be due for a charge!

So, a couple of questions to anyone who might know:


On a 94 Formula, LT1 A/C system, any idea around what psi the pressure sensor switch drops out?

I put a gauge on the low side schrader valve and it's showing around 20 lbs. I know that's pretty low, but does anyone know what the threshold is?

My Haynes manual, ( I don't know why I even bother looking at this thing anymore..... ), suggests that if the pressure is too low, and the pcm is not allowing the compressor to come on when you recharge, to "pull the connector from the pressure switch and short the connections to force the compressor to come on".

O.K. first of all, I can think of some reasons why this might not be such a good idea to do in the first place, but assuming for the moment it's o.k., the switch has 3 conductors. It would have been "brilliant" of ol' Haynesie to mention which two to short......

Has anyone here, tried to do this before?

I'm thinking the increase in pressure by just putting the r134 in, might make the compressor kick back on during the process anyway, but I'm wondering if anyone else has actually had that happen, or if I'm just being overly optimistic.

As usual, any and all advice is greatly appreciated!!

With the definite lack of any evidence of Global Warming up here, with temps in the 50's tommmorrow, I'm not in a big rush to fix this this week anyway. But it can change pretty quickly here.

Actually....this week is reserved for the window motor replacement.........aaaaaarrrrrrggggghhh!

[Jeff_in_Atl]

Yea I don't know what PSI it drops out either. I'm guessing around 40. Yes I have put a can on the low pressure said, open the valve and had it kick the compressor on.

I never figured out which of the 3 wires to short either which is why I chose just to run 12 volts to the compressor, filled it with a can, then plugged it back in and there was enough to keep the low pressure switch from kicking off the compressor.

I am dealing with the same thing on the home A/C. I shot of can of R22 in it and it is cold again. I haven't put any in the house A/C since we bought it 8 years ago. I have a bigger problem in that the drain pan is rusted out.

Oh and the passanger window in my Vette is stuck up. The Vettes tend to get stuck up rather than getting slow.

[formula_gramps]

Well Jeff that gives me a little hope that it will kick back in by just introducing some more refrigerant. I'll try that first and keep my fingers crossed that it does the same for me....LOL.

I like your idea better for starting the compressor. I can just rig up a temporary jumper wire to it.

I am a little reluctant to try running it that way though, because there are some very good reason why the pressure switch cuts off below a certain threshold, and it would seem like you could damage the compressor running it with really low pressure.

Maybe there will be others who can advise me as to whether I'm being paranoid about that......

[Al 96 Ram Air T/A]

Of topic, but the window replacement is a piece of cake. It'll take you more time to remove/reinstall the door panel.

Look online for a tutorial. All you need to do is grind a small access hole in the door to locate the three rivets. Grind the heads of and remove the motor...simple as that. The new motor goes in with three screws and nuts. Should only take you an hour per side.

[Injuneer]

Shoebox has the wiring diagram for the refrigerant pressure sensor switch on his website. Diagram is for a 95, but I doubt the 94 is any different:

http://shbox.com/1/1995_pcm5.jpg

[Jeff_in_Atl]

Of topic, but the window replacement is a piece of cake. It'll take you more time to remove/reinstall the door panel.

Look online for a tutorial. All you need to do is grind a small access hole in the door to locate the three rivets. Grind the heads of and remove the motor...simple as that. The new motor goes in with three screws and nuts. Should only take you an hour per side.

Mine is an 2002 Vette. The motor looks the same but it uses cables instead of the gear. The motor has this think that looks like an octopus on it. I bet it comes off somehow. Actually the chances are good if I hit it with a hammer it will work for another 6 months.

[darkhorsecustoms]

Had the same a/c problem on my Z. Gauge was showing about 20, compressor wasn't kicking on. What I did was just what you were saying. I put a little refrigerent in (engine off). Started it up, turned on the a/c, and the compressor kicked on. Then I topped it off.

[formula_gramps]

Thanks everyone for the good advice as usual.

Well, you guys were luckier than me I guess.....

I topped off the 134a yesterday, and the compressor still doesn't kick on.

Next, I guess I'll start the process of troubleshooting the system electrically.

To make things a bit easier (and for anything else that goes wrong with my Formula from here on in), I finally ordered an OBD1 scanner.

But, for the time being I guess I'll get it up on the ramps and pull the compressor cable, turn on the a/c and see if I even have voltage at that point. If I do, then I guess it's likely the clutch coil that's cooked, if I don't then the process is a little more complicated.

Who knows? Maybe there's a code sitting in the PCM right now indicating a bad pressure sensor or evap sensor. I know that happened on my wife's Intrepid last year (evap. sensor). The car had a neat little self diagnostics mode you could put it into, and have any errors display on the digital odometer. Life is a heck of a lot easier when the car tells you "hey dummy, go down to the parts store and buy me a new sensor...".

Back on my Formula: I already checked all the easy stuff: fuses, swapped relays, etc.

Rob's site has another schematic showing the whole control circuit for the A/C.....not that complicated really. Funny thing though....you would think there would be some kind of feedback to the PCM regarding the fans being operable, but I don't see that in this schematic, unless I'm missing something.

(http://shbox.com/1/hvac_compressor_control.jpg)

Stay tuned....

[formula_gramps]

Maybe some progress.......

Some developments in my no A/c saga........

Got my Autoxray OBD1 scanner today. Wow.....I love it......where have you been all my life?....LOL.

I must have spent the first 20 minutes looking at the live data from the various sensors. Damn, what a geek! I'm glad no one could tell what I was doing. I'll get over this after a while!

Anyways, my compressor is now kicking on. Funny, but it didn't the other day after I topped up the refridgerant.

The scanner indicates no error codes, so probably the sensor in the evaporator, and the low pressure switch are o.k.
It also shows my the a/c pressure, which logically increases when the compressor is engaged, then decreases when I switch A/C off.

So far so good.

Now here's what I think is wrong:

When I switch the mode control to any of the A/c settings (blue markings on dial) I hear the familiar hiss of the vacuum actuators switching things around under the dash, but if I listen carefully, the hissing never stops as long as the knob is on one of the A/c settings. We don't own the quietest cars in the world, and that is why I didn't notice this the other day.

I pulled the control panel and the radio and can see the bundle of vac tubes, but no visible leaks there. It sounds like it's coming from somewhere else. Now there has to be a door (or valve if you prefer) that switches the air flow from passing over the heater core to over the evap. core when the control is set for A/C. I'm thinking that is where the hissing is coming from, and why I am getting no cold air out of the vents. In other words it's stuck in the position to supply heat.

Does this sound like a reasonable diagnosis?

I really don't relish pulling out more stuff from under the dash, but I guess that's my next move.

I also don't like working around those scary yellow cables for the airbags. If I disconnect the battery, I'm still not out of the forest with the airbags.....isn't that correct? How do I disable that for real, and keep myself from being launched into the back seat if I mess something up?

I'm thinking that logically, the door that adjusts where the blower air is directed should be somewhere close to the blower itself. I'm actually amazed they can jam all this stuff under the dash.....

Has anyone had any experience with this that could make any recommendations?

Thanks!

[BrdWAtti2d]

As for disabling the airbags the proper procedure is to disconnect the battery, pull the airbag fuse and then disconnect the yellow connectors, one under the dash on each side. As far as the a/c goes, my wife works for an appliance repair place, I get a/c work for beer, and I like that arrangement.

[Injuneer]

Shoebox also has the diagram of the HVAC vacuum system and flapper doors:

http://shbox.com/1/hvac_vacuum.gif

[formula_gramps]

Thanks Fred....

But now I have come to realize that there probably isn't a vacuum issue after all. It seems that hissing sound that I hear when the mode switch is set to any A/c setting stops after a short while. It may have done that all along and I just didn't notice, because I wasn't going looking for trouble....LOL.

Additionally, I realize now that the "mixer door" as I think it's called, is operated by a cable--- not vacuum. And in reality it seems to be working o.k. because to one side I get a lot of heat, to the "cold" side I just get ambient air.

I think it's a case of the "cold stuff" is not even getting to the evaporator.
Regarding the two metal pressure lines that go to the expansion valve: I think it's the bottom one which is the one that should be sending cold refridgerant to the evaporator. As such, I should think that pipe should feel pretty cold after the a/c has been on for a few minutes -- but alas, it doesn't.

One question for anyone who knows:

Regarding the compressor pressure reading that you are able to get from OBD1 scanning: Is that supposed to reflect pressure on the high side or the low side? And if anyone knows, approximately, what kind of pressure should I be looking for?

[Jeff_in_Atl]

One question for anyone who knows:

Regarding the compressor pressure reading that you are able to get from OBD1 scanning: Is that supposed to reflect pressure on the high side or the low side? And if anyone knows, approximately, what kind of pressure should I be looking for?

It's the high side. The high pressure shut off is around 420 PSI. It will drop to about 150 and turn back on. If it goes up to 420 really fast as in a couple of seconds and shut off you either have too much Freon in it or the orifice valve is plugged up usually by compressor seal parts that have come off. R134 is funny in you can overcharge a system and it will quick cycle and not cool. R12 wouldn't do that. If it is constantly running but less than 350 PSI it's low on Freon. The way to tell if the orifice valve is plugged is to look at the low side pressure which you can't get from the scanner. If you put several cans of Freon in it and the pressure never gets to 420 and shuts off then the compressor is shot.

[formula_gramps]

Thanks Jeff, that's all good stuff to know!

Well, the pressure indicated by the obd1 was really low....like below 100psi.

So, I decided to jumper the compressor, and put a pressure gauge on the low end schrader. Way down in the blue zone (low pressure).

Interestingly now, when I look at "history" error codes I get a "61" low refridgerant--- no current error code though....(?)

Well, I re-attached the jumper (jumpering the relay contacts) and attached a can of 134a. Everything headed up (both low and high). I'll probably need another can, but the first one stopped feeding before it was empty. There was ice on the outside of the can.

I'll retry the process a little later when things warm up a bit more.

I guess I could use a hot towel around the can to prevent freeze up too...

[Jeff_in_Atl]

You probably know this but you fill the freon from the low pressure side with the can up so you get gas. If you put the can down it will feed liquid and the compressor can't compress a liquid. Here is a trick, get a pot of scalding hot water and put the can in it when you are filling it. The hot water will cause the freon to expand and push it into the system.