OK cooling experts!

I've been through many threads, lots of searching and lots of on-car testing. My basic problem is that no fans come on until 108 degrees C, (based on my aftermarket temperature gauge). They should come on, (low fan speed), via the ECU, at around 103 degrees C. I've now finally verified that the ECU "ON" condition for relays 2 and 4 (low speed) does not happen until 108 degrees C, (per my aftermarket gauge). This is for sure because I physically verified relays 2 and 4 do indeed switch at this point and also these are the same relays that switch when I short Diagnosis Connector "TFA" to ground. So the ECU appears to be doing its job, but at the "wrong" temperature.

As I see it there are 2 possible causes - either (1) the Thermosensor is faulty and giving the wrong reading to the ECU, or (2) the aftermarket temperature gauge is "showing" a higher temperature than the Thermosensor shows to the ECU. I need to decide which.

As an aside, I will shortly replace the Thermoswitch to the FC version. This will give me some fan cut-in below 100 degrees C, (via relay # 3). However, if relays 2 and 4 do not come ON until 108 degrees C, the FC Thermoswitch will not give me medium speed fans at the desirable less than 100 degrees C because, per the manual and multiple threads, in addition to requiring relay 3 to be ON, medium speed also requires relays 2 and 4 to be ON. In my situation this will not happen until 108 degrees C. So, although I will get some early fan-on benefit from the FC Thermoswitch, I will not get the full benefit of medium speed fan operation at this desirable lower temperature.

My guess is that condition (2) above applies, such that my aftermarket gauge is showing a higher temperature than the Thermosensor. The sensor for the aftermarket gauge is mounted in the top pipe of my Koyo radiator, whereas the Thermosensor is mounted at the back of the water pump housing, (I think). My question is, "could this account for about 5 degrees C difference?" Despite the threads, (though gratefully appreciated), I have been unable to find a clear indication of which way the coolant flows through the radiator - top to bottom or the opposite. Also, I need to know what is the hottest point in the cooling system. Somebody please help me on these 2 items.

I've seen Rob Robinette's article on mounting the gauge sensor in the thermostat housing or the throttle body coolant lines, and I will do this if, indeed, there is likely to be a 5 degree temperature difference, thus causing my problem. The real question is whether this 5-degree difference is real or likely.

Help would be much appreciated


Thanks

Dave

 

The coolant enters the radiator at the top...

The coolant leaving the t-stat usually "leads" the block temp by a few degrees (F). The stock ecu/thermosensor will turn the fans on to LOW at 221F (105C). The fans should switch to MED at 108C.

 

Rynberg -

Thanks for the input.

What would you estimate to be the temp difference between the top of the radiator and the Thermosensor? Is there just a pipe between them? If so should they be virtually the same temp?

Thanks

Dave

 

They should be close enough to not make a difference. Time to go through the troubleshooting procedures in the FSM...

 

FYI, the thermoswitch does not turn the fans on, it only increases their speed. More info here: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/2-trigger-cooling-fans-462760/


It's very possible that one or both of your sensors are not calibrated perfectly. Also, remember how the coolant sensors work. The metal absorbs heat from the coolant, so if the sensors are made of different metals, they will absorb the heat at different rates. For instance, an aluminum or copper sensor might pick up a temperature change pretty quickly, while a brass sensor will take a bit longer to heat up.

-s-