2001 jeep cherokee xj - 4wd - automatic - all items work and drives perfectly, except that the electric aux. fan does not engage when the a/c compressor engages - it will run when the temperature exceeds 212 but it is designed to engage whenever the a/c compressor engages - then disengage when the compressor cycles down. one competent tech suspects the pcm is responsible. ideas ????

 

Hi and welcome to CF from Tj...Sounds like a relay might be the problem.
Does your AC compressor kick on when you put it on? ...Tj

 

yea my first thought is relay as well. since its working with the temp sensor.

 

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Well here is the latest "update". The PCM is operative - it is receiving signal to engage the auxilliary fan and the message is going "out." The problem is that our weather is too cold to re-charge the A/C without pulling it all the way down and then putting in the proper about of refrigerant. The system may be on the low side, thus never reaching the top psi of the refrigerant, thereby negating the signal to engage the aux fan. One tech said the PCM was trashed for that circuit - but a more extensive evaluation proved that was incorrect.

Thanks for all the ideas - and all good ones too !