Coolant Fan Diagnosis

[87kevroc]

The primary coolant fan on my 87 IROC is not coming on when it’s supposed to and I’m in the process of troubleshooting it. The repair manual says ground terminal B on the ALDL and the fan should run. If it does run, then go the section 6E of the manual to troubleshoot the ECM. Well, it runs when B is grounded. No codes are being set.

I don’t have a scan tool, but I am wondering what it would tell me other than my ECM is bad. If not a bad ECM, what else could the source of the problem be? I can still drive the car as long as I don’t idle for long periods with the a/c on (the secondary fan works) but I gotta get this fixed before summer. Any input appreciated.

[WS6 VERT]

Could be a bad coolant temp sender. The one that feeds the gauge is different than the ECM's. Thats the only thing the ECM goes by to turn the fans on. By grounding the "B" terminal, and the fans running, that tells you the fans are good, the relay is good, and the ECM is capable of turning them on. That means for some reason, the ECM just isn't comminding them, i.e. possible CTS.

[87kevroc]

Turns out it was the a/c pressure fan switch, once I found the right pages in the manual it was no biggie troubleshooting. Thanks.

[WS6 VERT]

I thought fan switch controls the "auxillary" fan, so it shouldn't effect operation of the primary fan, but if it works, great.

[87kevroc]

You are referring to switch between cylinders 6 & 8 controlling the auxiliary fan. The one I am talking about is in the a/c high pressure line, it sends its output to the ECM rather than directly to the fan relay.

[james85z28]

so you concern was mainly that your fan wasn't coming on with your AC and perhaps running a little to hot?

[87kevroc]

James,

Right. When both fans are working, my car rarely gets up to 220 degrees, even in mid summer. A few days ago, I noticed that while idling at a stop with the air on, I was running about 230. A minute or so after I got moving the temp started dropping and eventually got back to where it normally runs, about 200 or so. When I got home I left the car running, popped the hood and found the primary fan not running, secondary running fine.

Checking the Electrical Diagnosis service manual, I did the test described in my original post above. After reading Bernard’s response to my post and realizing I did not need a scan tool to continue, I searched section 6E of the Shop Manual for more info on the CTS. Since the primary fan is supposed to come on when the a/c is turned on, and you don’t have to wait for the engine to get up to operating temp as part of the test, I decided to troubleshoot that problem first.

With the engine idling and a/c on, if the primary fan is not on, disconnect the a/c pressure fan switch. If the primary fan comes on, the switch is faulty, according to the manual. The primary fan came on. I still may have a bad CTS though. When I got up to 230 degrees as described above, the CTS should have sent a signal to the ECM to turn the primary fan on, whether the a/c was on or not, and it did not do that