Long and Covoluted intermittant A/C Problems with '88 300TE

[CTM VT 2K]

I have spent months trying to diagnose this problem, and spent quite a bit of time searching the AC threads on this grate site... Each time I think I have found the source of the problem, it turns out not to be it. So, here is the LONG version of the story:

I purchased this 1988 300TE (w124.090, engine 103.983) to be my primary commuter car when I lived in Kansas. It worked great, and I had no complaints. Then I moved. Two hours down the road, the AC cuts out. The cold dry air coming out the vents changed over to cool moist air, and after a little while, to hot dry air.

The first diagnosis was low freon. I could not find anyone in NW Iowa (where I was many miles from a major metro area) who was willing/able to actually do the recharge on this car in the time I had in the area. A few days later, I headed north to visit family in Northern MN, and the AC worked the whole trip (Cloudy day, relatively cool dry air). I got the system recharged up there - but I found out later they managed to botch the job, and didn't get the fittings properly sealed, about half my R134a leaked out within about two weeks - but by that point I was in Virginia, and had to pay someone else to re-do the fitting and recharge. On my way out of MN, it was a hot humid day, and the AC system cut out after about 20 minutes on the road. Further drive testing determined that at full blast, the AC would only run for about 20 minutes (from a cold engine), but on a minimum cool setting, it would run indefininately (at least at first - before the refrigerant started to get low a week or so later). By the time I was commuting in Virginia, the AC would only run for about 5-10 minutes (much hotter and more humid than on the Prarie or Plains). I took it into a shop in Virginia, and they very quickly diagnosed the leak - it was almost visible to the naked eye (even I could see the yellow dye staining everything around the low pressure fitting). They drained the system, replaced the fitting, and recharged it. That's where things start to get interesting.........

I picked up the car and drove it to another place in the county to register it (I'd only lived in the couny for about a week), and sure enough, about 20 minutes after I left the shop, the AC stopped blowing cold dry and started blowing cool moist. I turned right around and went back to the shop. They kept the car all day the next day - running the car on idle most of the day trying to force the AC to fail. It didn't. Then they got in the car to drive it, and it cut out within a block. Temps both days were high, and the humidity similar. It was the edge of a long weekend, and they woudn't be able to do much for the weekend anyway, so I was going to take the car and drop it off again the next week - except it wouldn't start. It seems the battery was dead. A quick jump-start and check of the electrical system with a voltometer, and it becomes immediately apparent that the Alternator is only putting out about 12.2V even at high RPM (e.g. not enough to run/recharge everything properly). A replaced alternator later, everything but the AC is running much better (I had been having a few other minor annoyances that were evidently electrical in nature).

So, I take the car home, and pull the wiring diagrams off the shop manuals, and start trying to trace a fault in the control lines (I'm an Engineer myself, so while this vehicle is unfamiliar, the concepts are not). I cannot find one. All the electrical lines seem to be working properly - the compressor is getting shut off after about 20 minutes of operation, and I'm stumped as to why. As I look more and more into the operation of the AC system, I suddenly realize I'm low on engine coolant - so I top it off, and immeidately take a test drive. The engine is now getting cooled better, so the engine coolant switch doesn't cut off the compressor as often, and for about an hour I can recognize the compressor cutting out at high engine temp, and turning back on once the engine cools back down. No problem. It seems to be fixed. It was a HOT and dry day.

The next day it's cool and rainy, and the AC cuts out after about 20 minutes.


So, I start checking all sorts of things, even asked a family member who is a retired GM mechanic (whom I had not spoken with in over a dozen years). I explain the whole problem to him, (as I have above), and he thinks it's either a) the dryer crystals are shot, and I need a new dryer, or b) my condensation drain may be plugged. The Humidity (both inside and out) is what he keyed off of. If I have a plugged condensation drain, then the water will back up and reduce the air-flow over the system, and not permit the proper heat-exchange, thus freezing the system and forcing the compressor to shut down. He suggested I put a temperature probe in the vent to see what temp the air coming out is. I did so the next morning, and every day since. The temp runs down to about 43.0°F at minimum. I have seen it as low as 41.7°F. The AC seems to operate between about 40°F and 50°F, but once it gets much above 50°F it has lost compression, and rapidly climbs to the ambient outside air temp, as well as becoming massively humid.

As soon as he mentioned a possible water build up, I remembered that I had heard on occasion what sounded like water sloshing around in my dashboard - but I have found no mositure inside that could be associated with it (I have found some moisture near the hatch in the back, but that is following rain and is coming in through a crack in my antenna base). I also have not seen any water dripping beneath the car, at all. Ever. He suggested I take a coat hanger or other wire and push it up through the drain hose to see if I can clear the blockage.

I did some checking, and found what looked just like a section of bycycle tire inner tube (wide and flat) coming down from about the midline of the car, directly beneath the windshield, and routing around the top of the transmission to the driver's side. It does not appear to be a straight-shot from the hose opening to the top, and I am really nervous about the idea of probing around in there with a coat hangar. I did attempt to probe it with a 1/4" plastic/nylon hose (clear, so I can see immediately what's in it, but soft enough so that I couldn't do any real damage to anything with it). I was unsuccessful.

Searching on this site seems to tell me that indeed I have a blocked condensation drain, but mine does not appear to have the insulation on it that I have seen described elsewhere, nor do I have a clear path forward from here. My read on it is this: I have a clogged condensation drain, and I need to get that water out of there, regardless of if it fixes the AC problems or not. I am an Engineer, but I do electronics and software, not mechanical, so this is kinda outside my realm of expertise. I do understand the basic concepts, and with a good set of instructions I can probably do the repair myself, but I'm missing a disk from my shop manuals on the 124 series (Gotta love moving and having everything in boxes), so I do not have any idea where to start on going in from the pasenger compartment side, nor do I really feel good about poking with a coat-hangar from the undersdie. I do understand that I may have a wrong diagnosis, but I do need to get that water out regardless - unless I am supposed to have a water-sloshing sound in my dashboard in this particular model year (1988 300TE).

It has been both hot and warm, but consistantly humid every day since the system worked for as long as I tested it; and has consistantly cut out at between 18 and 25 minutes after first getting the vent temps down into the 40s. I did have one day where after the system cut out, the humidity in the car built up to over 95% (I have a humidity meter on my probe thermometer), so I get both temp and humidity).

If you've gotten this far, thank you for your patience in reading my ramblings, and thanks in advance for any advice you can give me. I will stand ready to answer any questions, and I throw myself upon the expertise of this site.

CTM

[jcyuhn]

I've not heard of the condensate drains becoming blocked on a 124 chassis car. There are two of them, they exit out the bottom of the HVAC box, they are large diameter, and proceed straight through the transmission tunnel without and turns. See the attached photo of my '87 TD wagon receiving a new evaporator. The holes through which the condensate drains exit are circled. For reference, the gold box just rearward of the holes (the airbag trigger) is directly below the radio.

At any rate, it's easy enough to check. Idle the car in place, windows open, on a humid day. You should accumulate two puddles of water under the car, one on either side of the transmission, just aft of the front wheels.

As for the cutting out after use, I suggest you check the clutch gap on the compressor. It should be 0.50mm +-.15mm. The gap increases over time as the clutch material wears. I find that once it reaches ~.80mm or so clutch engagement becomes inconsistent. And yes, one sign of excess clutch gap is when the compressor works fine from cold, but becomes sporadic after it warms up. Any decent auto a/c shop will have a kit with a selection of new spacers to reset the gap to spec.

Another possibility is the presence of grease/oil on the compressor clutch facing. This situation can cause enough slip when engaging the clutch that the safety cutout it triggered. Try cleaning the clutch facing using Simple Green, brake cleaner, etc.

Best of luck,

- JimY

[CTM VT 2K]

I have checked. No puddles.

[david s poole]

two things come to mind [1] have you tried shutting down the engine when the compressor shuts off and restarting the car to see if the a/c restarts.if it does,then you may have slightly loose serpentine belt or belt tensioner issues.[2]what about the evap temp sensor,when they shut down the issue is that they won't turn back on[supposed to turn off at about 34-36deg evap temp and come back on at about 40deg]based on your description #2 could explain all your problems.

[CTM VT 2K]

Originally Posted by david s poole

two things come to mind [1] have you tried shutting down the engine when the compressor shuts off and restarting the car to see if the a/c restarts.if it does,then you may have slightly loose serpentine belt or belt tensioner issues.[2]what about the evap temp sensor,when they shut down the issue is that they won't turn back on[supposed to turn off at about 34-36deg evap temp and come back on at about 40deg]based on your description #2 could explain all your problems.

I have tried shutting down the engine and restarting. It has worked once or twice, but only gets the AC rolling for about a minute or two. Is there anything I can check at a hands on level (i.e. no special equipment) to see if the belt is tight enough? I can see the belts running without apparently slipping both when the AC is working and when it isn't.

I will have to double-check on the evap temp sensor, and get back to you.

[david s poole]

if it restarts then it's probably the compressor speed sensor shutting down the relay.quick and dirty test is to see if you can lift the belt by hand and take it completely off the upper pulley in the middle.by the way you can take out the a/c relay from behind the batt and use wire to join terminals 15 and 87 and plug back in.then drive to assure yourself that nothing wrong mechanically only electric.but in this state the a/c compr will always run so remember not to shut down blower or may freeze up coil.

[CTM VT 2K]

Ok, did a quick check on the belt, and it has about 1"-1.5" of play in it across the top of the block. Wouldn't a halfway decent mechanic have discovered the belt was loose when they replaced the alternator two weeks ago? Is that too much play, or just right?

I'll try the relay bit this evening, just to test it out.

CTM

[david s poole]

yes it should have been drawn to your attention by any one of the several techs that looked at your car unless you were pleading poor to them.

[CTM VT 2K]

So is the water sloshing sound a red-herring, or is that an unrleated problem I need to look into seperately?

[brewtoo]

Originally Posted by CTM VT 2K

Ok, did a quick check on the belt, and it has about 1"-1.5" of play in it across the top of the block. Wouldn't a halfway decent mechanic have discovered the belt was loose when they replaced the alternator two weeks ago? Is that too much play, or just right?

I'll try the relay bit this evening, just to test it out.

CTM

The ordinary mechanic not familiar with MB might not have known about the speed sensor and its function.

However, I assume you had the AC problem both before and after the alternator was replaced.

The tensioner is troublesome and they might not have been able to adjust the belt properly.

For the drains, get a piece of wire (like a coat hangar wire) and GENTLY push it up in the drains. Gently because the hoses are nothing but a spring wrapped in foam.

[CTM VT 2K]

Ok... did some more checking this evening. Crawling under the car, I found the drain hoses - quite deteriorated. I know that one was at least partially obstructed, but doing some gentle probing I cleared any evident blockages. When I idled the car, I started getting condensation drip (quite a bit, actually) at 10 minutes, but the system still stopped functioning at about 20 minutes. I actually watched the compressor stop, and it never came back on. I stopped the engine, and restarted it. The AC compressor did not engage. I stopped the engine, and let it sit for a few minutes, and when I restarted, the AC compressor still did not start.

I did not get any additional time to check by jumpering the relay as suggested, due to a pending thunderstorm (don't have a garage here like I did in Kansas). Nor did I get to do more than more fully examine the belt. The belt has about 3/4" of deflection midway between the coolant pump and belt pulley. I can feel the tensioner resisting my push/pull. I did locate the tension adjustment control, but didn't have a chance to try increasing it.

During the test run, I never saw any indication of the compressor slipping, it simply ran until it was shut off. The engine temp never really got high (I kept it in idle, and had my wife occasionally throttle the engine. During this test run, the ambient air temp was about 90°F with about 45% humidity. Vent temps barely got down into the 50s, but humidity in the cabin got down to about 35%. Once the compressor shut off, vent temps spiked quickly into the 70s and 80s, and humidity in the cabin climbed above 60%.

Unless someone tells me not to, I will jumper the relay in the morning, and re-test. I don't get the feeling it's a loose belt, but I will deferr to the expertise on this forum.

[CTM VT 2K]

Another round of testing, and still no success.

Ambient Air Temp: 85°
Ambient Humidity: 45%

Engine starts, AC runs. Five minutes after startup, engine temp is running slightly above 80°C, engine oil pressue is not quite pegged, idle RPM is about 900. I can see the compressor is engaged, vent temps are down to 63°F (from an original 122°F sitting in the sun all day - original internal humidity was 29%), and internal humidity is at about 30%. The beginnings of a condensation puddle is forming on the passenger side of the transmission. All is well.

I run the car in a high idle (RPM 2500).

Ten minutes after startup, engine temp is just below the unlabled tic-mark between 80°C and 120°C (I take this to be about 100°C). When I return the engine to idle, RPMs drop to below 1000 and the oil pressue drops to below pegged. AC Compressor is still engaged, and there is a fair bit of condensation forming along the low-pressue lines - as well as in two puddles beneath the car. Vent temps are down to 58°F, and humidity is staying around 30%.

Fifteen minutes after startup, once again following a high-idle run, engine temp is not significantly changed, engine idle is down to about 800, and oil pressue is down to about the middle of the guage (between the 1 and the 2). Vent temps are down to 54°F and humidity is down to 27%.

Eighteen minutes after startup (right on time) the compressor shuts off. Engine temp is unchanged, idle oil pressue is slightly higher (just under the 2), and RPM has returned to 900. Vent temps spike quickly into the 70s, and humidity is above 48%.

An immediate engine shut-off and restart does not re-engage the compressor.

I shut the engine off, and remove the Klima Relay, and jumper terminals 15 and 87, and reinstall it.

Start the engine, and the compressor does not re-engage. Vent temps are running above 80°F and humidity is running above 60%. Engine was off for about 5 minutes during this jumpering.

Engine temp has fallen to about 90°C, and RPMs are running about 900 at idle, with oil pressue at the 2 mark.

I have removed the jumper, and will re-try the AC system in a few minutes - giving it a 30 minute break for a re-start.

I can clearly see in photos I took during this procedure (I will post in a bit) the compressor clutch engaged and dis-engaged. This indicates to me (I may be in error) that it is not a slipping clutch. I never saw the RPMs fall below 600, though they did get down to about 800 at one point. If it was just the RPM cutoff, the jumpering should have allowed the compressor to operate regardless. It was not even attempting to run.

Any other suggestions?

[Richard Wooldridge]

You could have a high pressure cutoff- this could be caused by either a clogged expansion valve or a deteriorated hose that has collapsed upon itself, shutting off the flow of refrigerant. It could also be a drier plugged with leak sealer. The thing to do is to jump out (very temporarily) the high pressure cutoff switch when the failure occurs and see if the compressor starts. If it doesn't start, ohm out the clutch coil while it is hot, it could be an intermittent coil. You should be able to locate the lack of voltage to the clutch fairly quickly, I'd think...

[CTM VT 2K]

All the hoses and lines appear to be intact. Would it not be better to get an AC Mechanic to do a pressue guage test (i.e. run the system to failure while high and low are hooked up to guages) to see what's going on?

CTM

[TimFreeh]

Originally Posted by CTM VT 2K

I shut the engine off, and remove the Klima Relay, and jumper terminals 15 and 87, and reinstall it...

Your KLIMA relay has pins that are labeled "15" and "87"?

Are you sure you removed the KLIMA relay? According to my ETM jumping pins 5 & 7 would bypass the KLIMA relay and manually activate the A/C compressor.