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.