hey. it never turns on automatically, via temps. ran some tests, here we go. wiring diagrams don't tell me what i want to know. Test / results AB diagnostic terminals jumped - both fans operate jump primary fan relay - primary fan operates grounded coolant temp sensor - no fan operation i have 20ms voltage coming from the black wire, and it was about 4 volts coming off of the yellow wire with the key on. from the wiring diagrams it looks like grounding the yellow wire would turn on the fan, but i can't tell what's going on in the ECM. i just see yellow as the power, black wire going all over the place to multiple sensors then back to the ECM. anyways, i grounded both terminals and neither turned the fan on. any guesses at what to look for? i'm lost, if that part of the ECM is toast i'll just throw a switch in there for that primary fan. can't think of anything besides the ECM being at fault here. 1989 IROC

10 looks no replies, guys must be just as stumped as me.

tested both coolant wires for continuity from CTS to ECU, both checked out less then an ohm.

pulled ECU, no signs of anything blown up.

fans turn on while i pull codes, so can i rule out a faulty computer circuit or not yet?

If I remember right, you need to ground the green wire with a white stripe on one relay and the grey wire on the other one to get thefans to turn on on demand. Give that a try so you can rule out the relays

I have the wiring diagrams for a 1992 if you need them. they should be the same.

b4c

when you say ground the green wire with the white stripe, what does the grounding exactly mean?

and if the fan does come on when i ground that green/white wire, then does that mean the relay is good or bad?

Wezzles, disconnect the relay so you can see the connector terminals. use a jumper wire across the two wires.

this shows you that the fans will work, if you take the relay out of the equation by grounding the fans manually.

typically if the fans never come on, but they come on when you do what b4c has stated then it means a bad relay... but in my case this was not the problem.

by the way, i took out my ECU, looked at it, put it back in and both fans seem to work now.

If you do it my way and the fan comes on, then the relay works. If it doesn't come on, then do what Choobinator said and that will bypass the relay. If the fan comes on then, then you know you have a bad relay.

i tried jumping the ab connector under the dash and the fans didnt come on.
i tried grounding the green/white stripe wire to a metal bolt in the engine bay and the fans didnt come on.
when i grounded the wire that goes into the fan switch under the passenger manifolds, then the fans do come on.
also i did all this with the ignition on and i did check the fan fuse under the dash.
when i ground the other two connectors in the relay to the black ground wire, then the fans do come on.
also my car has the single fan.