evaluating an AC compressor when system is currently discharged - Page 12 - PeachParts Mercedes-Benz Forum

kuene · #**166** 07-16-2018, 01:45 PM

I tested my pressures yesterday, it was fairly hot (high 80s) and after about an hour of driving with AC on and off. (I'm running r134a currently and at 80% of the weight of the r12 that is specified for my car.)

In my driveway will all windows/doors open and AC on max the pressures read at about these values - slowly going up and down within these ranges:

LOW: 37-40
HIGH: 280-295

With High side peaking once at 300 before dropping down into that range listed above.
I'll see if I can pull and test my evap temp sensor.

I feel like my compressor has a bit of a very slight grumble/grind in the moaning noise - but I know it has somewhere around 7 ounces of ester oil and that nothing has leaked since I installed.

Diseasel300 · #**167** 07-16-2018, 03:17 PM

Pressures are roughly where they should be if running the stock system. High side will be elevated due to reduced efficiency compared to the small-tube condenser or a parallel flow condenser. I'd be looking squarely at that ETS if you froze up. It's been said before, but I'll say it again. CLEAN THE EVAPORATOR COIL. You don't need to pull the dash to do it. And yes, it's worth your time to do.

kuene · #**168** 07-16-2018, 03:39 PM

I’m going to do this Diseasel - consider your advice heeded!

Replace (test first) that switch and clean. Do I need to remove the expansion valve to get to evap?

Diseasel300 · #**169** 07-16-2018, 06:18 PM

Need to remove the blower and go in through its opening. There are writeups on this forum and elsewhere on the Internet on what and how to do it.

vwnate1 · #**170** 07-16-2018, 10:12 PM

I'm confused now ~ earlier you said low side should be around 25#.......

Diseasel300 · #**171** 07-16-2018, 10:22 PM

Quote:

Originally Posted by vwnate1

I'm confused now ~ earlier you said low side should be around 25#.......

His original posting in another thread said he was running 25PSI suction with similar head pressures to what he has today. 25PSI is low. Normal suction pressure is 35-45PSI depending on ambient temperature and load on the system. Low side pressure by itself is useless, it has to be factored with head pressure, ambient temperature, and load on the system to be a valuable number.

vwnate1 · #**172** 07-16-2018, 10:29 PM

Thanx, I *think* .

Agreed, clean the evaporator, it's dead easy : remove the blower motor and use a garden hose......

Mine was nowhere as dirty as the ones I see here but I cleaned it anyway to be safe .

kuene · #**173** 07-17-2018, 08:36 AM

Cool. Looking at the forums threads. Should I pick up some condenser cleaner foam like that have at Home Depot? And then blow it out with water and air? I’m assuming this liquid wouodnthen make its way out down the trans tunnel drain tube.

Diseasel300 · #**174** 07-17-2018, 12:38 PM

The foaming stuff works well if you can get it on the coil. I used the concentrated Nu-Brite for evaporators (green bottle) in a pump up sprayer on my SDL because I couldn't get the can of foam in there, the 123 may be more accessible. Consider using a toothbrush on a stick to clean all the muck off of there. Rinse well. Don't be surprised if it's a horror show in there, 30+ years of leaves, dirt, mold, mildew and other nasties don't make for pleasant viewing. Anything you wash off should come out of the evaporator drains under the car. Don't go crazy with the water or you may be cleaning up more than you were bargaining for.

Your reward for a disgusting job? Stronger airflow and improved cooling, not to mention allergies that aren't freaking out from breathing 30+ year old mold!

vwnate1 · #**175** 07-17-2018, 09:31 PM

I cheated and bought a remote viewer at Harbor Freight, it is incredibly cheaply made but works just fine, it's designed to crap out or loose a part & stop working the *instant* you're not uber delicate with it but for looking in there it's fantastic .

I waited for a cool day and sprayed some foamy cleaner in there and let it soak a bit, flushed with the garden hose (use a nozzle or suffer the consequence !) and never a dribble on the carpets ~ every bit of crud went right out the drain tube everyone touches then complains when it crumbles .

As the evaporator is light alloy you want to try to find proper AC evaporator cleaning stuff so you don't cause corrosion .

kuene · #**176** 09-03-2018, 09:05 AM

I replaced the evap temp sensor and the compressor is now kicking on and off as is should be. I want to test my original unit, but I’m pretty sure it was the faulty component now.

Diseasel300 · #**177** 09-03-2018, 09:55 AM

If it's working properly now, I'd find the nearest trash can and insert the old one in it.

97 SL320 · #**178** 09-03-2018, 05:38 PM

Quote:

Originally Posted by Diseasel300

If it's working properly now, I'd find the nearest trash can and insert the old one in it.

Doing a failure analysis of failed parts gives one a better understanding of how things work. This is vital in building a knowledge base in order to fix something in the future that might not even be AC related. A true mechanic can fix _ anything _ down to the smallest part.

It also gives experience in taking things apart that are not normally serviced. If one is stranded on a desert island or the part is no longer made, these skills come in handy. Not to mention a better appreciation of " how do they make this at such a reasonable cost? ".

Testing would consist of a continuity meter across the terminals, dunking the sensing bulb in a water bath at low temperature and a thermometer checking water temp. I'd probably use a bit of antifreeze to keep the water liquid.

Your switch might have a temperature rating on the side so look at the numbers. It might not, for example, say 5 *C but 05 might be in the makers part number ( not MB part number ) .

The most likely failure point is welded contacts causing the compressor to run all the time allowing the evaporator to ice up. No air flow = no cold air.

If the sensing bulb had lost it's charge, the contacts would not close because the switch thinks the temp is too low and the compressor would not run at all.

Diseasel300 · #**179** 09-03-2018, 08:09 PM

Failure analysis is great on something that's worth repairing and assuming that the person doing the post-mortem has the ability to understand what it is they're looking at and WHY the piece failed. There's very little to these switches....a sensing bulb, a capillary tube, and a switch contact. You're not going to repair any of the above, so why bother? The new part fixed the problem, the old one isn't going to be repaired, so who cares?

Before jumping down my throat of being wasteful, I'm the type that actually DOES post-mortems on just about everything and have my entire life. Mechanical switches go straight to the trash when they die. When they fail, they're done.

97 SL320 · #**180** 09-03-2018, 09:30 PM

Quote:

Originally Posted by Diseasel300

Failure analysis is great on something that's worth repairing and assuming that the person doing the post-mortem has the ability to understand what it is they're looking at and WHY the piece failed. There's very little to these switches....a sensing bulb, a capillary tube, and a switch contact. You're not going to repair any of the above, so why bother? The new part fixed the problem, the old one isn't going to be repaired, so who cares?

Before jumping down my throat of being wasteful, I'm the type that actually DOES post-mortems on just about everything and have my entire life. Mechanical switches go straight to the trash when they die. When they fail, they're done.

My first two paragraphs in the prior post address this. I'm not saying to investigate every failed part, do it with a few as a learning experience.

At some point he will run across a part that can't be had at any price making repair the only option. He can develop these skills on an irreplaceable part or something that is zero loss if he fails.

Our guy can learn why the contacts welded. RE: How inductive kick is generated from a non snubbed coil and how it cause arcing / welding of the contacts. See also Mr. Miyagi for my overall reasoning.

Quote:

Doing a failure analysis of failed parts gives one a better understanding of how things work. This is vital in building a knowledge base in order to fix something in the future that might not even be AC related. A true mechanic can fix _ anything _ down to the smallest part.