http://catalog.worldpac.com/mercedesshop/sophio/image.jsp?title=Water%20Temp.%20Sensor&url=http%3A//img.eautopartscatalog.com/live/C602012373FEB.JPG

In the water jacket ..I think in the head on that chassis..you will find it

Continuous Injection System

The sensor is 2 thermistors in one case , so they have to read the same , regardless of temp... the ohm reading is taken diag across the pins for each sensor..

Thanks again Arthur, much appreciated. I'll check it tomorrow.

Very Good...
.......post what you find or the outcome...

When you say "button" what exactly do you mean and where would I be looking for these "blinks"?

Model/Year needed for that info.

1991 300E 3.0 liter

Cal. Version??

It is a W124. If that is what you mean.

All300e are 124 chassis.
I mean is yours California Emmissions version. They have the led/Button.

I am not sure. How would I be able to tell/ find out?

Look at the Emmissions sticker under the hood .
Do you have a CE lamp?

I am not with my car right now but can call somebody who is. Where is the sticker exactly found?

Use the Search Feature..
It is all there .....pages and pages.

And here are the codes for your model.

http://www.geocities.com/MotorCity/Downs/1039/mercedes/cs1000_mb.pdf

I cant find a page that tells me if I have one. Would you happen to know off hand where the sticker might be?

Just answer the second question.. POST #26..........................
Does your car have a CE lamp?????????????????????????????

You get in the car and you turn ON the key...you look at the dash and you see a bunch of lights.... and if you see a light that says "CHECK ENGINE" , then you have a CE lamp..if you do, then you have the LED/SW for code access, if you don't , then you don't.