Chrysler Repair: fan relay: minivan, refrigerant pressure, coolant temp

Question
I was reading your response on the 99 chrysler t&c mini van radiator fan relay.. is there a relay that kicks on when the airconditioner is switched on?? our fans do not kick on when the ac is on.. it ran hot and cracked the 2nd radiator plastic sides.. I was told there may be a relay not labeled fan relay in the under the hood fuse box also. I have changed the relay on the frame under the air cleaner housing.. THANKYOU !! does this relay fail often as i used a used relay from a salvage last time...?

Answer
Hi Gerald,
There is only one radiator fan relay. It is a solid state device and it should be fastened securely to the metal frame piece where it is mounted so that electrical heat that the relay generates will be absorbed into the frame and thus prevent overheating of the relay. There was a history of early relay failures in the latter 90's which ideally should be resolved by the present relay design and proper attachment of it. If the used relay seems to not function then it probably is defective.
The order for fans to come on comes from the engine controller which will activate the fans either as the result of coolant temperature(220F for low speed, 230F for high speed# and/or pressure of refrigerant in the A/C system (250 psi for low, 300 psi for high). Inaccuracy of the coolant temp or refrigerant pressure sensors could explain a failure of the fans to come on as needed. Check the resistance of coolant sensor to see if it is dropping to about 1000 ohms when the temp is around 200F. The refrigerant sensor will mainly control whether the compressor clutch will be activated or not, so presumably if it allows for the ac compressor to activate it will also signal the fan relay. But it is possible that if the pressure in the system were on the low side that it would fail to reach the pressure necessary to order an increase in the fan speed as needed to cool the radiator. The voltage reading of that sensor which will activate the fan and the clutch is between 0.451V and 4.519V but I don't have the break point between low and high speed fan action.
Have you tried for fault codes using the ignition key:"on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the odometer to see if the mileage is replaced by 4-digit fault code numbers. Let me know what you find.
Roland
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Thank you