You shouldn't get a check engine light by temporarily disconnecting the coolant sensor harness (presuming your car has a check engine light).

If you build the code reader, you can not only check the codes, but you can clear them afterward.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Well well, Iive and learn . First I found the CTS sensor and when I looked at the wiring I saw some wire was exposed and the black covering sorta crumbled a little more as I handled it.

So much for the condition of my upper harness ( I think ) . I proceeded to take connector off engine with engine off , temp approx 95C pre disconnecting it , restarted and drove it around a little . No difference . Engine missed at the same RPM 1400-1800 under hard acceleration. Feathered throttle produced no engine miss as usual

So then I turned it off for about 1 1/2 hours to cool down and retry. When I started it again the using K6JRF view in the climate control window on position #6 said LOW instead of a temp.
But the instrument cluster showed the temp approx 65C. When I first start in the morning the cluster gauge shows below 40C

But I learned that the car has 2 independent engine coolant temp sensors.With the CTS I disconnected the instrument cluster gauge still works , and it went from about 65 to juts over 80 and back down when I got back on the highway " EUREKA "

Then I left the sensor disconnected and drove home.
The miss/bogg was still there. Nothing seemed different at all ! Actually after a half hour of highway driving , and slowing down to 40 MPH and 1700 RPM an attempt at rapid acceleration without a downshift caused the famous miss/ bogg 2 more times and one time I was going slightly downhill.

The longer the engine is running regardless of temp at the moment , the more likely the engine will bogg down.

I am going to let the car cool down entirely overnight , so that I know I am at the same starting point and drive the car tomorrow, and see if it still NEVER MISSES COLD, and starts to miss when warm as usual , with the CTS sensor on the engine manifold disconnected .

So far we learned there are 2 engine temp sensors, and tomorrow I believe we will find out that the CTS by the fuel rail does nothing either .

Do you really believe I can get codes with the reader I will build? Even though the 2 Mb mechanics did not get any with their sophisticated scanners.

Which is the easiest and best scanner to build ? I have heard of one with radio Shack parts, that I once printed out and threw away when I saw my second mechanic get nothing with his scanner

As 400Eric pointed out, if the MB techs really knew what they were doing, I have to question why they didn't notice that the engine wiring harness was toast. This is such a common problem for that vintage car.

The home brew scanner really does work--I used mine last night. But if you have a bad wiring harness, it's probably not going to do you much good, because a bad harness will cause all kinds of weird things to happen to your car.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Please steer me to the easiest explanantion of how to build the Radio Shack scanner you might have.

I had a large printout of the parts needd etc and how to build It but I lost it. I have been searching for a full sized explanation to print out and go to radio shack and Build anyway.

I thought it was in a sticky but cant find it now