What does your chart say for 20C ?

<>

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.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

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.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

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!

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

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?

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

<>

Yes, it does..

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

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

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?

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

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

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!