om617 coolant sensor thread size

[chrisgt]

I'm looking the thread size of either the coolant switch for the EGR (which is disconnected) or the sensor for turning on the aux fan on my 84 300td. My aux fan doesn't come on, it isn't the sensor but the logic on that fan is faulty in my mind, anyway. I want to put in a different sensor hooked to a microcontroller for a smarter control of the aux fan and also for a digital reading of the actual coolant temp.

[DeliveryValve]

If this is for your 1984 300TD, there is no coolant sensor to turn on the Auxiliary Fan. The auxiliary fan switch is located on the drier of the A/C system. The switch will turn on when the drier reaches a certain temperature, this happens when the A/C is on or when the engine bay gets so hot, it heats up the drier also.

If

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[chrisgt]

So what is the sensor on the thermostat housing, then?
I know one of them is the switch for EGR, but the other one doesn't seem to go to the gauge, I deduced the gauge sensor is the one on the head by the glow plugs.

If I can find out the thread size I'd like to swap one of those two for a thermistor so I can make a smart controlled fan.

[Demothen]

There is a block sensor/switch to prevent the heater from blowing in most modes until the water temperature is up to a certain temp (55C if I remember correctly).

[Mxfrank]

There are three threads commonly used on Merc sensors: 14x1.5, 16x1.5, 22x1.5.

Along the left side of the the head there will be several 22mmx1.5 ports in the water jacket. One is used for the heater source, one for the gauge sensor, and sometimes there's another sensor. The others are plugs. Remove the plug of your choice and go to town. I use one of these for a Porsche fan sensor, there is a wide choice of Porsche and VW sensors available in 22mm.

[chrisgt]

I'd rather just swap out the the EGR thermal switch for the vacuum lines because it's right on top and easy to access. But the coolant jacket ones aren't a bad idea.