Thanks for the spec info; i did need that. My ets is old but still registering yet 17.5 at 72 (high end of spec); jumps around a little too. I'm going to go ahead and order me a new one...check it's resistance and adjust from there. Thanks again! if memory is correct; effective resistance is larger resistance divided by smaller resistance....take that number and divide into smaller resistance and then subtract from smaller resistance. Anyway, I get the picture.

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA

RT= (R1 X R2) devided by (R1 + R2)

use the calculator I sent ........... easier...

You are actually looking for 11.5 - 13.5k ohms at 20C [Room temp 68F]

I might have said so earlier...if it had occured to me earlier. As it was, the conversation went in a more interesting direction.

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1989 300 SEL that mostly works, but needs TLC

Here's that photo:

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Dave
Boise, ID
Check out my website photos, documents, and movies!

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"

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA

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!!

Are all thermistors universal as far as ohm reading, ie 10C @ 18.3-21.5? I'd like to find out what the ohm reading should be in the 2to4 degreeC range.

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA

I'm not sure. The factory specs are in 5°C increments down to 10C, IIRC... don't recall seeing specs for 5°C or below.

Well, if we make a few assumptions;
1. linear correlation at the range of interest y-ohms x-temp
2. Use average of known ohm specs @ known temp

we can calculate the following equation y=(-.8)x + 28

extrapolate to 2 degrees C and get 26,400 ohms. If we add the shunt, the effective resistance would end up 20,900 ohms. If we further assume the spec range at 2 degress to be 6k. the spec could be 23.4k - 29.4k @ 2 degrees C.

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA

The reason you have to experiment with values is b/c you want the circuitry that the termistors trips to be fooled by you making the resistance out of spec .. ie .-incorrect at that temp ..this trip point in the circuit is the factor that determines what value change you are looking for in the thermistor ..just as the thermistor has a +/- tol., so does the other end of the driver circuit that the thermistor is activating...
That is why I mention 100k as a base line try..you may even try a 100K variable and when you tweak it to your likeing, measure the VR tweaked value and sub a metal film in its place for final design/hardwired...

yes, we are talking about another variable that has to be included. That was my concern also, 'cause I've heard 90k has "frozen" some when 100k works on others. It definitely should be attempted car specific every time. But I wanted to confirm the "ballpark" and I like the tweaking from actual measurements. you are talking about a 100k pot aren't you?

is metal film available at Radio Shack?

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JHaburay

1972 280se 4.5 "princess"
1986 2.3-16v first 100 (099) USA

You can use a pot , but I always just use a trimmer resistor for experiments mostly cuz they come in 100k and are checp.. they just have a little screw on the end .... RS has them and that is also where I get M/Film resistor 5 paks...

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

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

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Michael McGuire
83 300d
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD
86 300E

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.