Hi all, Today I pulled the ETS (evaporator temp sensor) from the problem car as well as my parts car. I stuck both into an AC vent in my house along with a thermometer, let them settle in, then measured with a meter. To my surprise, both had about the same readings, and both were in spec - barely. At 15°C (60°F), spec is 15.3-17.2kohm, and mine measured about 17.2k... wrong end of spec for cold air, but within spec nonetheless.
With that depressing fact in mind (I was expecting the sensor in the problem car to be out of spec), I decided to try the resistor shunt trick Marshall posted. I added an 82kohm resistor pack in parallel with the sensor. Now at the same temp, it measured approx 14.2kohm, a full 3k less. So far so good - now to re-install. Back in the car, remember my old temps were in the range of 50°-54°F and would not get below 50 regardless of ambient temps. With the shunt-modified ETS, those temps dropped a full TEN degrees! I now get a range of 40°-44°F with an occasional dip to 38-39°F. I can't wait to try it on a hot day... of course, now we have a cooling trend starting tomorrow, no more 100°F+ days to really strain the system. This should make a HUGE difference in cooling! I'm still confused though, since my diesel runs at 38-42F without the shunt modification. I believe that since the ETS sends a signal to the pushbutton unit, which then decides when to turn the compressor on/off, there must be something that wears out or fails with age in the pushbutton units that causes the temp range to creep up, even though the ETS is within spec (albeit the wrong end of allowable spec). My diesel has a recently rebuilt p/b unit, which could explain the good performance. The 300E p/b unit is of unknown age (but it works fine otherwise.) Side note - my E500 with stock R-134a system cycles in the 42-44°F range with no shunt. I'll probably add one to that car when I replace the vac pods and bump it down to the high 30's. Moral of the story... if you are confident that your refrigerant charge is normal, and you can see the temps cycle in a 3-5° (F) range and observe the compressor cutting out even though min temps are nowhere near the low 40's (F), the ETS shunt trick may be just what the doctor ordered. Just remember to re-test if you ever replace the p/b unit afterwards, in case my theory is correct... in that case the shunt would need to be modified or removed to prevent evaporator icing. :-) Best regards, Dave M. Boise, ID 1994 E500 - 94kmi (Q-ship) 1987 300D - 258kmi (Sportline)
