Cummins NTC-400 Operating temp and thermostat location

**sk187** · 05.12.2010

This is my first post, great forum wish I knew about it earlier.

One of our older yard trucks has a Cummins ntc-400 in it from 1984.

The temperature gauge always started at 0 and when under a load would make its way to 175 and stay there.

I recently replaced the temperature sending unit with a Stewart Warner 280ea and now under load the temperature is going all the way up to 240 and then the fan coming on to lower it a touch and right back to 240.

I called a local diesel shop and they said typical operating temp is between 170-190.

I guess my question is ............. is there a thermostat or something I should change to lower the temperature or is it possible the second temperature sending unit that gets to 240 is wrong?

There is also a sensor in the side of the radiator that opens and closes the air flow to engage and disengage the fan, could this be the problem?

Thanks in advance, this is driving me nuts.

**Brickhauler** · 05.12.2010

Does it boil out any coolant? It sounds like you might just have gotten the wrong sending unit. If it were me, I would try to double check the temp with a mechanical gauge, thermometer or infra-red temp gun. You might need to buy the sending unit from the dealer that matches the gauge in the truck.

Also, its probably not the case but those engines have a small hose going from the top of the water manifold up to the radiator top tank, this lets the air bleed out of the engines cooling system when you fill it back up with coolant. If it happens to be plugged, you might have an air pocket in the cooling system that could possibly make it act funny