PeachParts Mercedes-Benz Forum - View Single Post - Got a 560 SEC

tinypanzer · #12 03-10-2017, 03:54 AM

So, slow down a bit. I don't know what you're referring to as the 'thermostat' because the thermostat is a purely mechanical device. If you're talking about the temp sensor wire on top of the thermostat, that only controls the instrument cluster reading and has zero effect on how the motor runs or air/fuel ratio. Pulling it off does nothing other than peg your temp needle.

There is another temp sensor mounted closer to the throttle body, that one is used by the computer. I believe it may have four wires and two elements that both need to work. Do a little searching here about it on the forum, it's also referred to as the NTC. And also, as I stated before, if your engine is running far outside of normal parameters, like with a vacuum leak or invalid sensor readings, it might not even be trying to adjust the idle air. Before worrying about the wiring harness and or one of the computers, test the actual temp sensor by measuring its DC resistance and comparing it with the chart that is posted in countless threads here. Test the idle air valve by powering it with 12v and observing its mechanical action.

Verify they even work before proceeding, then work backwards through the harness towards the computers. Unless you have a chart of expected voltage values on those leads, taking arbitrary measurements is unlikely to lead to an answer.

CORRECTION -

Just went out and looked at my 560SEL, I was confusing the setup with my 300SE. My 560 has two temp sensors on the thermostat housing, the single wire one I'm pretty sure is for the inst. cluster only. There is a two wire one next to it (measure DC resistance and compare to chart) and there is also another 2 wire temp sensor screwed into the top of the motor located just under the air filter. Both those two wire sensors must be within spec. It's actually a really easy test once you find the NTC chart (it's here on the forum somewhere) and you just measure DC resistance and ambient temp and compare to the chart. Very quick, and will rule out the least expensive explanation.