Chuck Haynes

The Primary Cooling Fan will NOT kick on when the car gets upto temperature. It will ONLY kick on with the ALDL jumper (Diagnostic Mode) and when the Auxiliary Fan is on because of the A/C (Default Mode). I checked the fuse in the fuse panel, replaced the CTS, and replaced the relay (actually I tried 2 relays!). BTW, the ECM is new also! The Auxiliary Fan works fine. I have no idea what else it could be... A fusable link? A fan switch? Another relay?

Note: I unplugged the CTS and the Primary Fan came on, if this helps out in any way.

It is a 1990 IROC-Z (5.7 - 700R4) with Dual Electric Fans and the Serpentine System setup. The cooling system is stock with a 195 thermostat. The relays are on the driver's side firewall (NOT on the passenger's side fender and near the radiator support).

Please, someone help me... I need my fan

-Thanks

RB83L69

It should come on at around 230-235°.... that being the engine's normal stock "start cooling me" temp. Did you let it get up to that temp?

From the rest of your description it sounds like everything is working properly. So that should too.

8Mike9

I was going to say check the CTS out...but doesn't make sense (IMO) that unplugging it turns the fan on....when the CTS is unplugged, ECM should interpet this as high resistance..aka cold coolant temp.

Test the CTS out anyways, can be done with a ohm meter. For specific values, do s search for CTS values, search with screen name "Vader", he's posted the table many times before.

Haynes and Chiltons should also have the values in the CTS diag sections.

I'm pretty sure by jumpering the ALDL and both fans coming on, you can rule out wiring, fuses,and relays...not sure if you can run out ECM though.

Chuck Haynes

Yes, I let it get past 240 and the Primary Fan never kicked on. It used to kick on way before 230-235. In fact, the FAQ on this website says the primary fan on a '90 model kicks on at 190, and that sounds about right to me...

Chuck Haynes

Could a bad thermostat or radiator cap cause the fan from kicking on? Like possibly confuse the ECM or mess with the CTS?

Damon

Stock fan turn-on temp (with A/C off) is 228*F as seen by the ECM- don't go by the dash gague. GM has used that temp for years. My 94 Formula and my brother's 87 GTA both use that temp. to turn on the primary fan.

The backup fan switch is the one screwed into the pass. side head. I think it should turn on around 240 and is there as a last-ditch backup in case the ECM doesn't turn it on for some reason. Many aftermarket low-temp fan turn on kits replace this sensor with one that kicks on at a much lower temp- about 190 or so, which makes this the main method of turning on the fans with the ECM playing backup.

If all this checks out then maybe you have some bad wiring between the ECM and the CTS, or the ECM itself could be at fault, but I wouldn't jump to conculsions too quickly.