computer controls fan on at about 225 (91TPI) , block fan switch grounds at about 235. used snap-on scanner. watched the scanner temp and my temp on the dash. ( the dash is about 5 degs +-. Now the temp on the dash said about 235 fan comes on . but the scanner said 200 deg . so running off of the block temp. why is the intake so far off . ( tried two different sensors in the intake ) I know the block sensor is right cause if I disconnect the block sensor and wait for the computer to turn on the fans I start to boil over .

 

Does the primary fan in fact run at all? If you jump the ALDL connector and turn on the key to run diagnostic codes the primary fan should run, it is a self test. You could have an intermittent primary fan relay(very common). When in normal or non diagnostic mode you can pull the aux fan relay and the primary should still come on at around 215-220 or so. Later TG's also default, if the block sensor is lower temp than the ECM primary fan setting BOTH fans will come on assuming the driver in the ECM, and the fan relay are OK. I have seen many people chase thier tail because of a flaky fan relay. I am not sure if the Snap-On tool alters any fan operation, never looked into it. use the ALDL jumper then tap the pri relay and see if the fan acts up. Also don't overlook the relay plug. If you get to boilover with the aux fan not running you simply have a problem with the pri fan control circuit. Try swapping the relays, or sustitute the heater blower relay as they should be the same.