Evaporator Thermistor - PeachParts Mercedes-Benz Forum

KATSCAN · #1 08-21-2005, 09:19 AM

Any advice on modifying thermistor; sucess stories, failures, etc?

t walgamuth · #2 08-21-2005, 04:17 PM

is either a typo or a part i am not familiar with.

tom w

DanielW · #3 08-21-2005, 05:09 PM

Are you aware of :

http://pages.prodigy.net/jforgione/MB_S500.html check item 18, It's something kind of similar to what I believe you are speaking of.

I did read your post and link about Ford Scorpio AC modifications. Sounded just like what I wish my W140 had. But , where on my 95 S420 is that thermister located. Likely way down in the bowels of the evaporator housing. Anyone know which wires (colors), where they come out, and where they go, and what relays (switching on-off the compressor) they control?

Good luck,
DanielW

skiier3_9 · #4 08-21-2005, 10:03 PM

Quote:

Originally Posted by t walgamuth

is either a typo or a part i am not familiar with.

tom w

a thermistor is a resistor designed to maximize the resistance change with temperature

KATSCAN · #5 08-21-2005, 11:23 PM

I know there is at least one who has adapted his ets (evaporator temperature sensor) as I read his thread of how he was unfortunate/amazed to make a block of ice. Didn't hurt his system, but it has the potential to starve the compressor (not good) so it should be "tuned" carefully. I believe he had it set (ets) at 0 degrees and was going to raise it a bit. ASAMOF what is the compressor cut out temperature on a 201 ets?

I know it's not hard to adjust; I'm just looking for what size resistor for w201 ets. He's close.

Moneypit SEL · #6 08-21-2005, 11:27 PM

Quote:

Originally Posted by KATSCAN

Didn't hurt his system, but it has the potential to starve the compressor (not good)

Y'know...you ought to try and understand air conditioning before you attempt to modify it. How, exactly, will the compressor 'starve'?

KATSCAN · #7 08-22-2005, 08:28 AM

Well, perhaps starve is not the correct term..but if the evaporator is frozen, lets say it's because you've modified the Evap Temp Sensor too low of a value. the TXV is closed yet the compressor keeps running putting a lot of stress on it.

gsxr · #8 08-23-2005, 01:45 AM

Here's that photo:

KATSCAN · #9 08-23-2005, 07:55 AM

Thanks, my thermistor a little different looking ("Meat thermometer looking) but same mod. However, that being said, this is what I've learned to help optimize/analyze your particular car's ac system.;

1. Check ETS; make sure you know if it's working and what side of the spec
it's operating on.
2. Remove ETS from system, just pull it out of evaporator, do not unplug
and see the capability of your ac. Run Low air flow and take temp
reading of evaporator/center vent/etc. What you would really like to do is
get a "map" of the temperatures at different locations of the evap. That
would tell you if your temp sensor is in a cool spot or a hot spot relatively
speaking. I've have had the evaporator out of my car and have
seen how the core fins of the evaporator are bent from the act of
inserting/shoving the probe (ets meat thermometer). Certain flow
characteristics inherent to your particular evaporator and/or operating
system may have developed to create hot/cold spots. If your probe is in
a cold spot relative to the average evap temp....you won't get as cold as
you like. Likewise, if your ETS is itself at the top of it's specification.
Others who have no AC problems have been fortunate to have a correctly
spec'd thermistor and a "good location"; (or for that matter; may be in
a "hot spot"). I think that's why the MB specs are "wide". Some people
are just "lucky".
3. Take your new found data, postulate your best soln (conservative or
liberal), and try it.
4. You still need to be "lucky" to hit your mark first try. But you may be
close enough and should be cold enough now to live with it comfortably.
5. You may in fact "overshoot"
6. Some have set the cut off at -2 degrees*, others 0 degrees, still others
are fine with 3 degrees C. I can speculate that this variation is due to
the variability caused by limitations of sensors and location. Therefore,
the best soln is an informed one.
7. -2 degrees does not neccessarily mean you're operating at -2 degrees; it
will not cut the compressor out until the sensor reads -2degrees;
remember, you probe might be in a "cold spot"

gsxr · #10 08-23-2005, 11:56 AM

Unless you are part polar bear or penguin, a 3°C cutoff point is probably fine... that will give 36-38°F vent temps. That's a huge improvement over 49-53°F!!

mario 3 · #11 02-13-2006, 08:45 AM

Evaporator seems to be freezing up causing low air flow out of vents. Need help in how to replace ETS.

samiam4 · #12 02-13-2006, 12:58 PM

If you are experiencing freezing- you need to make sure the system is not OVER filled with freon. R134a is more sensitive than an R12.

Michael

gsxr · #13 02-13-2006, 09:14 PM

Also make sure the evaporator is clean... when I removed the fan to lubricate a squeaky bearing, I discovered half the fins were plugged with a layer of dust & dirt, severely blocking airflow. I'd pull the fan out to clean things properly... definitely worth checking before you tear too far into troubleshooting other items. Low airflow will cause icing.

mario 3 · #14 02-18-2006, 06:04 PM

Can the quanity of freon, 134 be checked by pressure readings, hi and lo side? 124 300 TD. What should these pressures be if correct amount of freon is in the system?

Kebowers · #15 02-18-2006, 11:48 PM

There are several reasons to have the 'compressor cut off' temperature (as measured by the evaporator temp sensor) a ways above 32F. First is the fact that the ETS does not measure the actual surface temp of the evaporator tubes. It will always be a little warmer. Then there is a small 'production tolerance' in the individual ETS units. The goal is to keep the evaporator from icing up under conditions of low air flow -like when its 40F outside and you need to dehumidify (defog) the inside of the car. The ON/OFF EST switch system is not a very good way to achieve steady or precise evap temp control. A cycling evap temp is one reason so many MBZ evaporators fail--continual expansion/contraction. Temp cycling causes fatigue cracks. A far better system uses a throttling expansion valve which senses both evaporator operating pressure AND temperature. This allows one (The designer)to 'set' the evaporator inlet or outlet temperature at what works best for the particular installation. I once had a Imperial LeBaron, which had front and rear evaporators, and Chrysler had a VERY smooth temperature control system on that baby--throttling expansion valve on the evaporator INLETS AND an electrically operated compressor suction throttling valve-- with that valve working off a sesitive temperature sensor on the evaporator outlet lines --. That way they could drive the evaporators right to freezing (actually 33F) on inlet and outlet to maximise performance. The AC worked 'at maximum capacity for the air flow and temp conditions' with re-heat to modulate cabin temp, and variable outside air intake to modulate relative humidity!

However, with fuel economy penalties,etc, such a thermally inefficient system is not gonna happen in a measely $65K car. You are gonna be stuck with a minimal capacity system designed to use the least amount of engine power to meet the 'requirements.' Passenger comfort considerations seems to be forgotten, or certainly lower in importance, even in these very expensive 'luxury' cars.

OH, what some would give for a good old GM/Fridgedaire A-6 TXV/reheat system. MBZ actually licensed that system from GM in about 1975, just like they licensed automatic transmission shifting technology from GM Hydramatic Div--because GM had a much better systems and their technical know-how was the best in the world.