A/C Pressures Equal - Causes??
 

Recently purchased a 91 SL300, and sorting through a few issues.
The only major one seems to be the A/C.
Worked "okay" when I drove it 200 miles home last weekend, but gradually getting less and less cool this week (it's 98 degrees this week, so I'm asking a LOT of it, granted) :-(

Figured I'd check the freon pressures (it's already been converted to R134a), and all I can get the gauges to read is about 105psi on both the low and high sides with the a/c on.
Any possible reasons or things to check for why I'd have no differential in comparing low and high-side pressures??
Thanks,

bad expansion valve?

The compressr is not kicking on. Test circuit.

Good luck,

Compressor not running = Bad speed sensor.
Compressor running = Bad reed valve in compressor.

(Above not absolute but probable.)



.

Doh!! Sometimes I amaze myself - in a NOT so good way!! :-(
The compressor wasn't even turning......

But at least we now need to check the speed sensor, but....
Where the heck is THAT??
I've traced the brown and yellow/green wire back into the electrical "guts" of the corner of the engine bay, but don't know what I'm looking for there, and lose track of those two wires into THAT maze....

Regardless, I figured I could short the brown to ground and the yellow/green to a 12v source, and I'll hear the clutch "clicking" like it WANTS to engage, but not much engaged movement. Also, while the car is off, I can turn the compressor by hand, for what that's worth. The compressor seems to spin relatively freely (I expected decent resistance), just that the electro-magnetic clutch doesn't seem to really try to "engage" with authority.
So, bad clutch, or bad clutch/compressor unit, or???

Thanks,

With the engine off, jump 12V to the clutch and see if it clicks. If it does not, check the clutch coil with an ohmmeter and see if the coil is open. If the coil is not open and it is still not clicking in, then check the air gap.

If the clutch kicks in when jumping 12V to it, then you need to see if the low pressure switch is making contact. Use your ohmmeter to check it.

Good luck and let us know the outcome of these tests. I believe that these tests should be done before worrying about the rotation sensor.

Good luck,

With a 12v source, it definately clicks (clutch engaging), but doesn't seem to want to "stay" engaged. (when the engine is running, the clutch engages and rotates the compressor shaft 1/4-1/2 turn and then disengages)

I'm not sure where to check the air gap (or what the gap should be), but let me know and I'll do it.

I've jumpered both switches (one at a time) located at what I believe is the drier, and each made both front cooling fans kick on, but no change in the status of the clutch. I DIDN'T try jumpering them both at the same time).
I'm sure one of them is the pressure switch, but not sure exactly what the other one is/does.
Appreciate the help Larry - it's a convertible :sun_smile , but it's also 98 degrees every freakin day here in Houston.... :eek:
This is not cool.... LOL!!
Thanks!

Okay, since the clutch IS trying to kick in, it very well could be related to the rotational switch. That does not mean that it is the rotational switch or its associated circuit.

The purpose of the rotational switch is to detect a locked up compressor and prevent it from strewing bits all through the system(I think.) A slipping belt can cause what you are seeing. Is the belt tensioned properly, in good shape and not oily?

Good luck,

Hi Larry;
The belt appears good, with no cracks or squeals or apparent slipperyness.
I presume it's tensioned properly (everything else that runs off of it seems to be working properly, and no squeals or noises), but can't say for sure.

And like I said, I can rotate the compressor (not the belt driven outer ring) by hand, so surely don't believe it's locked-up.....

I'll try another 12v source on the clutch tonight.
I'm wondering if I just wasn't using enough current on the clutch....
Mechanically, there just doesn't seem to be any reason the clutch won't engage and stay engaged.

I'll post back. Thanks.

<I'm wondering if I just wasn't using enough current on the clutch....
Mechanically, there just doesn't seem to be any reason the clutch won't engage and stay engaged.>

You may want to check the clutch draw for specs.
So . bring 12v to it with an amp gauge is series . You are looking for 3.8 to 4.2 draw for normal clutch coil draw. Don't have to run car for a coil draw test.

.. and , to see if your problem is controls , just back probe the clutch coil feed wires with a test lamp/volt meter and see if the feed drops out when the clutch does .. that means no power ..back to circuit feed checks.

Progress, I think.....
I hooked up a more "direct" 12v source to the clutch, and the clutch engages and stays engaged just like it should.
Surprisingly, the temp. at the blower vents didn't get any cooler, but the clutch itself definately stays engaged when there is a good current going through it.

As far as the controller unit in the dash, it's functionality is suspect also.
I can turn the blower off, and it'll respond to the different blower speed buttons. It'll also blow hot(ter) air when I adjust the temp. wheel to "Heat".
But I can only get air to come out of the side and defroster vents, no matter what directional setting I change it to (old vacuum pods, no doubt).
Also, whenever I start the car, the blower always turns itself on, and since it doesn't cool anyway, I always just turn it off..... But odd that it turns itself on every time I start the car....

Anyway, that's where I am so far.
Clutch works, blower works, off switch works, but basically no cool air....
I know it worked marginally when I bought the car a few weeks ago, but it only took a few days to get to where I am with this...

Thanks for helping me work through this.....

Finally was able to get back to this this weekend.
The compressor is running now, but I'm almost back to my original problem.
As I mentioned, the car has been converted to R-134, and with the car turned off, the pressures at both the high side and the low side @ ~95...
With the compressor running, it climbs to about 105 on both the high and low side, and still no cooling.
So, I'm thinking it's a bad expansion valve, and if not that, a bad compressor.
Any thoughts??

Is the expansion valve located right next to the driver's side top shock mount?? And last, would (should) an expansion valve be different for an R-12 system and a R-134 system.... The compressor manifold and accumulator look like they were replaced when the system was upgraded a couple of years ago, but I'm not sure on the expansion valve...
Thanks for your help.

It does indeed sound like a bad TXV, but I am not familiar with your chassis to tell you where it is located.

Good luck,

In a conversion to R-134 (from R-12), is it normal to replace the standard expansion valve with something bigger/smaller??
Is there a reason to??

I'm just trying to determine if all I need to buy is the OEM expansion valve, or if I have something different in my car and may need an A/C shop find something else for me....
Your help is greatly appreciated. Thanks.

Funny thing about freon, I have seen it not do much of anything until there was a bunch of it in a system. If you have too low of a charge, you get no refrigeration cycle. Just the chemical sitting at whatever presure you get at its current temperature. Take a peek in the sight glass in the top of the receiver/dryer with the compressor running. Any bubbles (really little ones usually-and a whole stream of them at that)? R134a is such a little molecule, it sneaks out anywhere it can. Might be worthwhile to just add some gas and see if things change. Just keep an eye on the high side presure. Since I live in N. KY, I usually don't see quite as high of temps as you poor folks. Kinda have to recalibrate my brain for the presures I would see working on your system down there. Just upper 80s withtons of humidity here.

Hi Bob-
The sight glass is just too dark to see ANYTHING.... :-(

With R-134, I believe the pressure should be about 80-85 lbs.(both sides) with the sytem off, and 20-35 lbs low side / 150-250 high side with the compressor running. Please correct me if that's wrong...

If anything, I've got a touch too much pressure, but that wouldn't be the cause of there being no differential while it's running.
At least it's not leaking out, but I still need to fix something here... :-(

Thanks.

At the 98 degrees indicated in your first post, you should have 120 psi botyht sides with the compressor off and the car just started.
Here is a pressure/temp chart even if somewhat hard to read.
http://www.prevostcar.com/DB/services/maintenance/Maintenance%20du%20syst%E8me%20CVC.pdf#search='r134a%20pressure%20chart'

Are your gauge handles closed (they should be)when you are taking the pressures? Have to ask, ;)

on my 91 190 the ac compressor had a gasket on the manifold where the high and low pressure hoses connected which seperated the two sides. the gasket had pushed out in the center portion (probably from the prev owner overcharging the system) and was letting some of the high pressure bleed over to the suction side. my guage readings were pretty close to yours.

Oh no, GOOD call on the gauges being OPEN..... LOL!!
(sorry, it's been a few years since I've had to work with A/C systems - and I just wasn't thinking....) :-(

And that Pressure Chart is VERY interesting.....
In Houston in the summer, depending on what time of day (and the weather) you check the pressure, it could be 75 degrees, or 100 degrees..... I had no idea the ambient temp affected the pressure THAT much....

Let me try this again tonight - a MUCH better informed amateur tech this time...
Thank you for your patience....

Well.....
Good news, and really terrible news..... :-(

The guages being open was "my" problem, and easily fixed.....
The freon had basically completely leaked out....
I was reading the pressure in my "tank", not the car's system.... DOH!!

Added some freon, brought the pressures up to (at 90 degrees ambient) 60 lbs. on the low side, and 260 on the high side....
We had cool air, and all is well..... Almost......

Anybody ever seen freon actually "drip"?? Drip so much your first thought is "I can't believe the heater hose just sprung a leak"??
It wasn't the heater hose..... :-(
I have a steady "drip" of freon, and (here's the really bad news) the drip is back by the transmission - pass side.....
I didn't get under the car, but I can see it dripping just by looking down from the engine compartment..... There's a PUDDLE of freon on my driveway....

That would be the Evaporator core, correct?? And that's the one that is like $3500 to repair?? PLEASE tell me I'm wrong...... :-(

I love the car, but I'm sick about this......

Thank you ALL for your help. Each step brought me closer to finding the REAL problem - Bad evaporator, and operator error.... :-(

A Puddle of freon in the driveway ..

I've heard them all , now..

Think maybe you have condensate coming from an operating system..

I WISH I was kidding....
It's green, and it has no smell, nor is it oily (like I'm sure coolant would), and it didn't start until I filled the A/C system with freon.....

Tonight I have reasonably cool air....
I'll let you know how long it takes to leak down....

R-12 freon at normal pressure will boil at -21 degrees F. Were it cold enough for Freon to remain liquid outside a pressurized container, you would not be worrying about your A/C.

From your pressure readings, it sounds like you may have over-charged the system a bit. Was the system completely empty? And did you then evacuate the system prior to charging?

Well, it's R-134, and I can assure you it didn't leak "coolant" BEFORE I filled it with freon tonight, and yet I'm watching green liquid drip down right beside the transmission as I'm finishing up charging the A/C system.

SOMETHING is green in my 30 lb. tank of R-134, and it's green leaking onto my driveway. Maybe it's not the freon itself, but it's coming from my A/C system, NOT the cooling system.....
To be otherwise is just TOO big of a coincidence - and the coolant overflow tank level is holding steady.....

I'm just telling you what I'm seeing - a problem that DOESN'T involve the evaporator would be a blessing as far as I'm concerned....
If I drive it for the next few days and it stays cool and my coolant level drops, then I'll be happy to admit I am wrong, but I don't think thats what's going to happen.... :-(

R-134a also boils at way low temperature. I don't happen to know exactly how low. but I'd wager it's also well below 0 degrees F. Both R-12 and R-134A are colorless, unless somebody added a leak detection dye at some point. Green, to me says antifreeze, except I understand MB uses a different color. The only thing that could possibly leak from your A/C system, and remain liquid long enough to puddle on the ground, would be the oil. Either case (antifreeze or A/C oil) leaking from that area would require a major repair, so it would be best to make certain what it is you're seeing. If it's just condensation, no worries...although I'd be curious as to how it turned green.

I'd rather believe it's just a coincidence than believe that you've somehow violated the laws of physics and created liquid refrigerant at standard pressure and summer temperature.

I'm sure you're correct about an additive, because I realize that freon is both colorless and odorless, and is a vapor at room temperature.
And while the tank only labels it as HFC-134A, there is certainly something that is green mixed in with the freon in my tank.
And that's what I'm sure is also coming out back by the transmission.

I haven't been under the car yet, but there are only two things that I can think of that would be leaking green liquid back there - the heater core, and the A/C evaporator core (filled with freon with a green additive).

I just haven't had any problems with the cooling system (no leaks whatsoever) and all of a sudden I'm adding freon (with a green dye mixed with it) and at the same time I have green fluid dripping down back by the transmission.

Like I said, I'm going to give it a few days to see what the a/c does, and also keep an eye on the coolant tank, but it took me by quite a surprise to witness what I saw....

Just an update.....
The coolant level hasn't changed, and now almost a week later I still have reasonably cool air, so must not be any major leaks coming from the A/C sytem either.... Which was my major concern....

So, the fluid coming out of the drains was certainly mostly condensate, but it was also definitely green.
And there is only two green fluids that I know of going INTO the car - engine coolant, and the freon I added also has a bright green additive in it too....

In over 1000 miles of driving, the coolant level looks to be exactly where it was when I bought the car, so I'll presume I'm okay there....

I am obviously still going to watch this, and cross my fingers that the air blows cool for quite a while, but hopefully my worst fear of a condenser issue may have been unwarranted.... :-)

I really need to park the car in one spot for 10-15 minutes with the A/C running from time to time, and then see what kind of puddle is under the car.... And yes, I know... CLEAR water is good.... LOL!!

Onto my NEXT problems for now.... :-(

Thanks.

If you have the right coolant in the Benz its not green.

The green is prob mildue that finally got washed off the evaporater cuz you now have condensate. The condensate drains are on the sides of the tranny, so that is definately H20..
.. if you can get that freon to drip into a puddle , go see the guys at NASA , .. they want to talk to you..

The FUNNY thing is.... I "DO" work at NASA.... :)

Just not on anything mechanical.... LOL!!
Probably a "good" thing.... ;)

And I'm guessing that the clearish coolant is probably the correct fluid, but it came with green coolant when I bought the car last month, and I just haven't thought about changing it back....
Would that be a "bad" thing at this point??
(No telling how long it's had the green coolant going through the system)

Thanks.

Green fluid
 

If it is R-134A, aftermarket with a leak detector dye added, all you need is a black light to find out if this is what it is. It is a UV dye, so under a black light it will glow. I have a little battery powered flourescent UV blacklight lamp I use for this purpose.

As for the refrigerant remaining a liquid after coming out of the system...remeber that it also has disolved oil that is carried through the system as the refrigerant is pumped through. I have seen R-134A (NOT Freon! Freon is a Dupont brand name for R-12.) leaks that carried a significant amount of oil out with it (about a cup). So check to see if that fluid leak was slippery at all....

Otherwise...?? Water condensed from the air suddenly drained out the evaporator box? There happened to be a refrigerant leak before with the UV dye on the evaporator fins that got washed off with the condensed water?

Oh, final point...did the R-134A you added have a leak sealant additive as well?

Good luck.

<NOT Freon! Freon is a Dupont brand name for R-12.) >

Freon is a Trade name, but it does not designate R-12 .. It is Duponts
tag on all their CFC refrigerants .. R12 just ONE of their Freons.

.. but it is also common to see freon/refrigerant interchanged in the trade , along with ' ya need a shot of gas '..

But, Refrigerant is correct.. and adding some refrigerant w/leak sealer is not a recommendation I would agree with.

Hi Arthur;
The refridgerant is made (packaged??) by Genetron, and it's in a 30 lb. tank.
I've had it a few years, but at $140 delivered, it made sense to me to buy it like that....
But it makes no mention of any leak sealants, but then again it doesn't mention anything about leak detectors either... :confused:
I would have thought it would... and should....
But the refridgerant is a distinctly bright (almost neon-like) green, so I'm pretty sure that a leak detector agent is present.....

If they aren't terribly expensive, I will check into getting a black-light...
Might as well, it's already got the leak detector at work in there.... :o

Some place like AutoZone or O'Reilly sell blacklights??

Thanks,

Black light can be gotten in any electric supply house ..prob even a toy/hobbie store. Even ebay. I have one , but I don't use it as much as my Halogen detector..that way , I do not have to inject the dye.
Evap leaks are best spotted by sticking the detector up the drain after a long sit as the low side leaks faster when non use, as the low side has higher pressure then. Refrigerant is heavier than air , so it lays down at the drain connections ,if not disturbed. Sniffing up the drain tube after a long sit [even overnight]usually gets good results if there is a leak there.
A 30 pounder is nice to have b/c the last few weeks has seen a good price jump. Genetron is Honywells Trade name for refrigerants . If the one you have has the UV dye , it will say 134aUV on the bottle. If no dye , it will just say 134a. I guess Duponts Freon name caught on as it is common to hear Freon refered to as the refrigerant, no matter who made it .. you just don't hear someone say . "Hey, I think your aux. fan is not coming on cuz you are a little low on Genetron"... :)
You mentioned no oil feel in the liquid, so I assume it to be possible anti freeze from a pevious leak that may have just accululated in the bottom of the evap shroud and got washed out with the condensate that you now have from a working system.. specially as the drains are right on the sides of the tranny tunnel. Keep an eye on it and water drain is confirmation that you have a cold evaporator.. Good luck with the leak test