I have not. Basically, I have not done any new checking inside the cabin. I will work on checking this first thing - probably tomorrow (have the day off, so I can spend it tinkering).

Sounds like the compressor runs until the first time it cycles off, then won't restart.

I don't know if the ETR has a failure mode where it lets the compressor run, then shuts it off. I would guess the answer is no, because the compressor always runs when the car is cold started. I can't see the difference between letting the ETR sit and warm up in a parked car vs. driving down the road with the climate fan running.

To me this sounds like the compressor clutch gap is out of spec. (How many times have I written that statement?) Failure to re-engage when hot is a known fault on Nippondenso compressors when the gap is out of spec. DTDT myself.

- JimY

the failure mode on etr switch is to shut down at 36degf [supposed to come back at 39-41] and take it's own sweet time to close contact again.[just like an independent teenager].

I agree.

I lack the equipment to properly check that gap, or I would have had that addressed long ago. I am trying to isolate out everything I can, so that I don't have to have pay my mechanic to go on an open-ended hunting expedition. If I can nail down two or three specifics to check (the compressor clutch as a whole is already at the top of the list) that saves my mechanic time and me $. Either way, next month (Assuming nothing else blows up before then) I'll get the AC system properly fixed one way or another.

Would such a failure then prevent the AC from re-starting until the engine has powered off and cooled down?

i have seen that occur yes.

All you need to check the gap is a set feeler guages, as are used to set valve lash or check a spark plug gap. I haven't purchased a set since I last owned an OM617 several ages ago, so I don't recall what they cost. But 5 bucks at your local parts emporium oughta do it. We're not talking rocket science here.

- JimY

Feeler gauges are about $7 at AutoZoo. Just be sure to keep 'em oiled - I just looked at mine, and they are pitted with rust from sitting too long, so now they're useless...

Update:

My indepedant mechanic verified that the AC pressures are NOT going out of spec between running and failure.
When running in HOT weather, in bumper-to-bumper traffic, occasionally my engine temp climbs above the A/C compressor turn-off temp - turning off the compressor. When the engine temp cools back down, the compressor turns back on. He and I agree that it's most likely my ETR.

While I had the dash open to pull my instrument cluster (to repair my odometer as discussed in another thread) I tried to locate the ETR so I could bypass it. Unfortunately, the .PDF doesn't seem to reconcile with my actual system. Can anyone tell me how to properly locate the ETR to bypass it?

In this picture (looking from the driver's well towards the center column, as specified in the .PDF) the circled area is what I think MAY be the ETR.

Here is the graphic portion of the .pdf on my ETR

<When running in HOT weather, in bumper-to-bumper traffic, occasionally my engine temp climbs above the A/C compressor turn-off temp - turning off the compressor. When the engine temp cools back down, the compressor turns back on. He and I agree that it's most likely my ETR.

>>

Well, not to disagree, but if the temps in traffic are getting to a/c cut-off spec of temp sensor, then the problem is an engine coolant temp problem..the comp cut-off is working as designed..and if one has a hot engine ,the ac system is less efficient than normal temps, so how do you justify the ETR is at fault,,that just senses evap temps..which would not be low at an 'In traffic/high engine temp condition"

You want to be looking at coolant system efficiency and a/c condenser airflow for problems,,, [ my take on it].
..and be sure your comp cut-out is not the product of high side pressure cut-out at drier ; tho you said your guy has verified that one, but did he do it under condition?
The trick to this diagnosis is to be absolutely positive as to which sensor is actually cutting out the comp..I doubt it is the ETR

The reason being that I EXPECT the AC compressor to turn off at high engine temps. Seeing it turn off and then back on is the expected mode of operation.

The failure I'm trying to trace is when the engine is NOT hot, and the weather is not HOT, but the AC compressor shuts off after 20 minutes and does NOT turn back on. I am seeing a failure of the system corralated to vent temps:

At HIGH outside temps, the vent temps never get below 50, the AC never fails.
At moderate outside temps, the vent temps eventually get below 50, and the AC fails sometime after the vents reach about 47
At low outside temps the vent temps quickly get below 50 and often below 45 very quickly, and the AC fails at that point.

By failure, I mean the compressor shuts off, and does not come back on until the car has thermally reset (Turned the engine off, and let it return to close to a startup condition). If I leave the AC and blower off and drive for a while, it has no effect on the failure mode (i.e. I can drive for an hour without any vent blowing and everything will still start properly).

The thinking was that it was either the Compressor Clutch failing or the ETR in failure mode (First suggested by David S. Poole on this thread). I want to attempt to bypass the ETR to either ID it as the failure point, or rule it out as the failure point.

Very Good..
I mis-interpreted you complaint to question the hot cut-out.
I aggree..

So, is the spot I circled about right for the connector for the ETR, or am I out in left field (or do I need to take more pictures)?

The part that is circled is a hard-pastic connector between two cables. It is basically loose and accessible, and there are part numbers on it (I can go read them off it that would help - but I didn't before I came in for lunch).

I have a DMM that I will use to read the status of the ETR - once I find it - and determine if it has indeed failed; and I plan to bypass it during testing to verify that the AC works without failure when it is bypassed. I just need to know that THIS connector (among the dozen or so cables, all of which are black) is the right one.

There are 3 sensors there [heat/cabin/etr]
B10/6 is ETR ...it is a 2 wire thermistor and the wires feeding it are gy/rd on one side and br/yl on the other side. [ 2br/yl]
It is to the right of the steering column.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU24S0UPKUQ&year=1988&make=MB&model=300-TE-001&category=R&part=A%2FC+E.T.R.+Switch

That's the one. Which side goes to the ETR?

Did you monitor the voltage at S31 to see if the ETR is at fault yet??

..if you think the ETR is cutting out the comp b/c of low temp, then just pull it off the evap and leave it hanging in the ambient cabin air..leave it plugged in, b/c it can not cycle the comp if it is not there to sense evap low temp.

The one with the 2 single wires [ that is sensor side]..the other side of the connector has one wire on one side and two on the other. [ that would be feed side]

That's it. Take off the plug and slide it out.

Ok. Spent this week monitoring the voltage drop over and resistance of the ETR before, during, and after operation and failure.

One day's typical results:
Initial ETR resistance: 7.1kOhm
initial circuit resistance (once ETR was connected): 4.36kOhm
initial voltage drop over ETR: 2.085v

Max Voltage Drop: 3.5v (this is also the point at which the system fails, roughly corresponding with a vent temp in the mid 40s)
Circuit Resistance shortly after failure: 5.6kOhm
ETR Resistance shortly after failure: 11.7kOhm

The highest voltage drop I've seen is 3.554v, and the system fails at that point.
The highest circuit resistance I've seen is 7.8kOhm
the highest ETR resistance I've seen is 12.7kOhm

Today I disconnected the ETR from the core, and let it hang while plugged in.
Vent temps very quickly dropped into the low 40s, and even into the mid 30s before I shut the system off to prevent actual damage (switched to Economy setting). Once the vent temps returned to the 50s I switched back to AC and the vent temps again returned into the 30s.

At no time did the disconnected ETR voltage change significantly.

I brought the ETR inside, and placed it in my freezer. It went to Open Circuit shortly after 40.25kOhm. The chart indicates nothing higher than 20.5kOhm (corresponding to +10C, or roughly +50F). If I extrapolate out, 40kOhm comes to about -400C (not a realistic reading, obviously).

Sitting in my Air Conditioned living room at 75F, it is reading 10.5kOhms, or roughly 72F.

How I read these results: the ETR itself seems to work, but I think the Control Unit is forcing the system to failure based on a bad reading of the temperature. Certainly the system is able to blow far colder with the ETR removed than it was with the ETR in place - but at the risk of freezing the evaporator core, and forcing the system to failure by that method.

My question: Is it possible my ETR has gone out of spec, and is leading the Control Unit to think it is colder than it actually is? Or has my Control Unit failed, and I need to replace that instead?

Do a Temp/Ohms camparison test on sensor.

How do you mean? Do you mean for me to measure the temp, and the resistance from room temp down to failure (or at least down to freezing)? Or is there some other specific test you mean?

There is a test chart that you use to verify accuracy of sensor [ Thermistor Chart}
It is in the CD , or it is here in the archieves .I posted it several times , but years ago.
If you can't find it , I will try to post later , but I am back-logged up right now..

I did that. The ETR seems to read a degree or two lower than actual. Is that a significant enough difference to cause a problem?

What does your chart say for 20C ?

<<Sitting in my Air Conditioned living room at 75F, it is reading 10.5kOhms, or roughly 72F>>

That is within spec.

Two other possibles before condemning CC panels interpretation of ETS resistance.

Sensor may have an intermittant open. If CC see open or <2.5K ETR value at any time,
it defaults compressor and will not come back on until Key is cycled.
Other possible is low air-flow over evap at low ambients or blower flow blockage of any type at sensor.

An extended measurement of temp and resistance (about 150 data points), the 24 datapoint temps around 20°C (+/- 1°C) has an average resistance of 12.71kΩ and 20.25°C. This is well within the spec.

How might I detect an intermittant open with a basic Fluke DMM? Any open is so short I have not noticed one during any of my direct testing.

Most intermittent shorts/opens would show by twisting the leads or tapping sensor. This would prob show up if you tried this with the sensor hanging rather than evap temp sensing.
You certainly have done a thorough job on spec measures , so i would be inclined to look into the CC as it not responding to correct resistance values..the only other possibles are there are other thermistor inputs to the CC that tweak the control..that would be both the aspirator thermistor and the ambient outside temp senor.
Do you have the schematic for your model???

I do have the complete Climate Control Wiring Diagram for my model, but I don't have a good schematic on where each of the components actually resides (I only have one of the two CDs - so while I may see on the wiring diagram part B10/4 actually LOCATING that part is challenging). Does the outside temp sensor feed both the CCU and the dashboard display, or are those seperate sensors? If they are the same, then the outside sensor is feeding reasonably accurate data (within a few degrees). It is my hunch that the inside sensor is also reading correctly, as the blower fan speed does varry appropriately in other operating modes.

B10/5 is outside AT..B10/6 is ETR, and B10/4 is cabin. They are all Temp inputs to N22 [CC]..and I think B10/5 changes/tweaks the values at CC for comp cut-out at low ambients, but not sure..it would make sense b/c there would be a better chance of evap freeze up at lower ambients than at high ones.
I think the B10/5 on that model is at the blower.
I will see if I can find the exact place. I don't think it is display.

OK
B10/5 is located in the air intake above the blower .
But the connector is behind the glove box. The reading should be easy to get from the connector.
25C-2.4K/3.0K
20C -3.1K/3.9K
Prob worth checking before looking into N22 control...

Now, to figure out how to remove the glovebox... :-D Off to the DIY section I go!

Ok, I looked at what I think is the B10/5 - it is directly behind the glovebox, and appears to be the same type of connector as the ETR. The initial resistance was 1.9kΩ, and voltage during a test run was erratic, varying between 0.1 and 2.2v, but typically was in the neighborhood of 2.0-2.2v. Final voltage was 1.3kΩ. Ambient air temps were between 25°C and 30°C.

Does this appear to be off spec?

I had reconnected the ETR in the correct place, and the system shutdown again shortly after vent temps got into the mid 40s F.

The interior air temp sensor/asperator appears to at least be mechanically functional - the 1cm^2 paper stayed in place for the duration of that test.

The cable from the B10/5 connector appears to run through the firewall, and does not appear to be removeable at my skill level - or am I missing something obvious?

<<Does this appear to be off spec?>>

Yes, it does..

So, how do I go about replacing it? Or is there a way I can bypass it?

Might want to try subbing a resistor with a value of normal ambient and see how that effects the ETR.
25C maybe??? 2.3-2.5 ish..

Or, try a variable 4/5K wire wound and experiment

Ok, B10/5 does not appear to be the problem. A detailed check of the output with properly connected cables (One of my DMM cables had developed a break and was intermittantly dropping conneciton - thus my inaccurate results) shows that B10/5 is working within spec.

My next two sensors to check are B10/4 and B10/1.

It is my understanding that B10/4 feeds the ACCU (N22) and that dictates vent fan blower speed, and has some input variable to the compressor cutoff control unit (N6). As my blower fan seems to run at an appropriate speed for the cabin temp, am I safe to assume that B10/4 is working, or should I figure out how to pull it and do a detailed check on it as well?

B10/1 measures the Heat Exchanger tempreature. Where can I find it (or the wires for it)? Does this play a role in the Cooling mode, or is it just used during heating (I have had no problems with running the heat in the winter)?

I suppose I could also actually check B11/7 (Engine Coolant Temp Sensor) to verify what I am seeing on my instrument cluster as well.

Or should I just go ahead and look at replacing N6 and/or N22?

a/c repair
 

anyone close enough to louisville ky with a a/c problem on the 124 cars that no one can fix i will be glad to repair it.
larry perkins lou ky

You probably have a bad engine temp sensor

You can access the sensors more easily for checking them by pulling the right side plug off the pusbutton unit after removing it (two screws underneath the trim above the radio, lift off the wood trim, remove the six screws holding the pushbutton unit it, slide out).

Specs for all except the outside air and engine temp are:

20C 11.5-13.5 kohm
25C 9.5-10.5 kohm
30C 7.5-8.5 kohm
35C 6.0-7.0 kohm

engine temp:
40C 6-8 kohm
60C 900-1800 ohm
85C 460-650 ohm


Don't have the rest of the temp range, but watch for very low resistance on the engine temp sensor. Note that which one is used depends on chassis number and date, the earlier one is MUCH lower resistance, and if you have the wrong one installed, the compressor will cut out and stay out until the engine cools off.

Exterior air sensor:

20C 3.1-3.9 kohm
25C 2.4-3.0 kohm
30C 1.9-2.3 kohm
35C 1.6-2.0 kohm.

I suspect you need to replace the belt tensioner AND the fluid clutch for the fan -- if your coolant temp gets up to 100C (the mark between 80 and 120) in traffic, you don't have adequate airflow because the fluid clutch isn't engaging. This will cause high head pressure (no air flow over rad) and high engine temp -- sounds like between the bad fluid clutch and possibly a bad or wrong engine temp sensor, you have compressor disengagement. May not fix the real problem though, see below.

However, you should check the drains again (get both of them clear, you may need to remove the console cover, once that's out they are easy to replace) as water can short the evaporator temp sensor.

Also, double-check the interior temp sensor -- setting the temp wheel to MIN over-rides temp control, as you've noticed, and everything seems to work fine at that setting, so you DON'T have a KLIMA relay, belt, clutch gap, or major cooling problem (compressor will be switched off by failures there)

Better than what I'm battleing on the same car -- no AC compressor signal, ever.

Peter

I know I'm bumping an old thread, but I hate watching a movie and missing the last 5 minutes! Was there a fix here?

I'm doing my own movie, need to figure out how to end it and looking for a happy ending!