Temperature sensor
 

Hello,

I'm trying to sort out a strange hot running engine issue. It started some time ago and i tried the following: replacing the radiator, water pump, and thermostat. However it still runs at 98C constantly, and sometimes drops to 95 if i turn on the heater (doesn't matter if it's 90 or 40 degrees F outside, still the same temperature). Today i tried to eliminate the possibility of a bad temperature sensor by disconnecting it to see if the connection was corroded or poor, and noticed that there are two temperature sensors that look identical in the head. By process of elimination i found that the sensor in the middle of the head is the one connected to the gauge, and the sensor near the front of the head i don't know what it does. I then tried to connect the wire for the gauge to the other sensor to see what the gauge reads (someone suggested that on another thread) and it read 40C where the middle sensor read 100C on an engine that was just running. So now i'm wondering the following, first are both sensors the same part, what does the front one do, and does it sound like i need to replace the sensors?

One more thing, i replaced the thermostat with an 80C Behr unit, the expansion tank cap seems fine since i'm not losing any coolant, and i burped the system when i refilled with coolant by squeezing on the upper radiator hose a few times and refilling.

Thanks

And the car is a 1985 300SD......

bump

The "sensor" on the thermostat housing isn't a sensor, it's a switch. It prevents the blower from operating when "heat" is selected and the engine is cold.

This isn't the one on the thermostat. I'm talking about the two sensors on the head, drivers side. One located in the middle of the head, the other on the same side but toward the front of the head.

Have you tested the temperature with a thermometer? It may read 98 but that doesnt mean it is 98.

How can i accurately read the coolant temperature with a thermometer?

Use an IR type themometer pointed at the t'stat housing or on the head would be a good place to start.

Thank you, i will try that after work. Hopefully i can find an IR thermometer in walmart or autozone around here.

I use to have the same problem. Does your car over heat in stop and go traffic too. Because that might be the FAN CLUTCH. Do a search and find out if you need to replace it. I ended up costing me like $200 for the part. If you have replaced your raditor it's not that bad of a job.

The fan clutch will turn ON, when the tempature reachs over 80C and pull air through the raditor. This was explained to me by a MB mechanic. One I replaced my fan clutch all my over heating issues went away.

Burp it again....
 

Fill the system with antifreeze but this time instead of filling it through the upper radiator hose, fill the system through the heater hose on the rear og the engine next to the oil filter housing closest to the firewall. Some systems are just to stubborn to burp and eliminate air-lock just like mine....this trick has never failed me...

The second sensor is for the glowplug timer. How about you take a thermometer or something to it after a drive.

On an '85?

Why not? It's not on my 83, and I remember having it on an 84.

According to the 617.95 Engine Manual, the coolant temp sensor for the preglow timer was discontinued in 1981.