fans running when car is off

[warthogtech]

can anyone tell me why the fans would run with the car off? went outside the other day and heard one running so i disconected it. thanks

[mikec]

Which fan? The ON/OFF logic is a little different between the front fan and the two engine-side fans. Assuming it was an engine-side fan still running:

 

Fans get power all the time... a relay completes the ground. The magnetic coils of the relays (what "picks" the relays ON) are fed power by the ignition switch; the coils get their grounds from the temp sensors on the bottom of the radiator - directly under the fans.

 

So if either engine side fan still runs, that means either:

* relay is shorted/jammed ON.

* ignition switch is busting internally so the ON post has power when it should not.

* Wiring harness is damaged leading to a short someplace.

* Wiring has been modified and those mods suck. Sometimes folks want to adapt a time-delay relay from another car (like the Chrysler K-cars) that can run the fans for up to 10 or 15 minutes after the key is removed. Those relays have a high failure rate, sticking ON when they fail.

 

With the key OFF and the screwed-up fan running, try these tests:

1: from underneath the car, unplug the wire going to the temp sensor underneath the running fan. If the fan shuts off, the relay is working okay. The problem is the relay coil is getting power from a bad ignition switch or damaged wiring harness.

 

2: if the fan still runs after test #1, then unplug the relay itself. The two fan relays are the round relays mounted near the air filter box... they're hard to see/reach with the air filter canister in place. You'll probably have to take time to find them, and rig up some way to move them to where you can reach them, with the fan motor unplugged. Then next time the fan sticks ON, you can quickly undo these relays. If the fan stops, then the relay was holding it ON for some reason. Swap the two relays and see if the other fan runs - if so, one relay is jammed. If the original fan starts again, it's a wiring issue. If the fan still runs with the relay unplugged, it's definately a wiring issue. The black+white or blue+white wires from the fan harnesses are shorted to ground.

 

The a/c system has a different set of relays that can run the fans. These are the small relays mounted to that black metal bracket next to the ignition coil. So when the fan is stuck ON, just unplug all of these relays - if unplugging one relay kills the fan then that relay is either shorted (swap it with a neighbor as a test) or has a wiring harness issue. Normally the a/c computer turns on 3 relays at the same time: 2 fan relays and the compressor clutch relay so I doubt a wiring harness or a/c computer issue is your bug - you'd have 2 fans running.

 

mike c.