My Problem

2011 Chevrolet Truck Silverado 1500 WT 2WD 4.3L FI OHV 6cyl is overheating while idling.

It takes about 2-3 mins, so drive thrus and multiple red lights back to back are the main culprits.

I believe that the problem is the radiator fans are not turning on when the engine temperature becomes elevated.

Troubleshooting so far

Fuses are all good.

When I idle the truck and it reaches normal operating temperature (210-212F) the fan never turns on and will eventually start creeping up (temperature measured on the dashboard)

Relays appear to be working correctly. However, I can short the circuit in the relays to get the radiator fans blowing. Short circuiting both 86 to 87 and 30 to 85 will get the fans blowing. (see image below)

After measuring the voltages in the relay socket using a multimeter, it appears that there is no input voltage in 30/87/87a. I believe; however, that there should be power in 30 when the truck is idling past 210 degrees.

My conclusion

This makes me believe that it is the ECM or the coolant temperature sensor. However, my dashbaord temperature is still providing me with (what seems to be) accurate temperature information. And I assume that the temperature information is coming from the coolant temperature sensor. (is that correct)


I don't know how to trouble shoot if it is the ECM or the coolant temperature sensor, or even if the trouble shooting I have done thus far is even in the right direction. I just am kinda stalled out right now and don't know what direction to head in towards fixing this issue.

enter image description here

  • Welcome to Motor Vehicle Maintenance & Repair! Which engine is in your truck? Mar 23, 2022 at 18:21
  • @Pᴀᴜʟsᴛᴇʀ2 thank you! and Just updated the post for exact specs, but 4.3L 6 cyl 2wd.
    – Daniel
    Mar 23, 2022 at 18:24

2 Answers 2


You shouldn't short circuit 86 to 87 and 30 to 85, that may well damage the electronics.

To bypass the relay, you should short 87 to 30.

From your experiments, you clearly know that the fans work, and that the wires from the relay to the fans are OK, so that is not the issue.

If the fans do not run with 87 shorted to 30, then you have either a blown fuse, a fault in the fuse/relay box or a faulty wire from the battery to the fuse box. It is likely not the wire from the battery to the fuse box, because other things would likely be not working.

You should have 12v on either pin 87 or 30 on the relay with the negative probe of your voltmeter on the negative side of the battery. Check that.

If shorting the relay works, then you could try a different relay. You could then check the voltage between pins 85 and 86 you should get 12v when the fans are meant to come on. You could even put a low wattage bulb between those contacts to see if the bulb lights.

Test these and report back.

Do you have a wiring diagram for this part of the vehicle. If so, can you add it to your question?


The radiator fan on many cars isn't controlled by the ECU, there's a thermostat switch which turns it on and off. Your problem sounds like a classic case of a failed switch. The switch is open when the temperature is low, then it closes when the engine heats up enough to need the fan. When it closes it supplies power to the relay coil, turning the fan on. I'd start with finding the switch, then pulling the plug from it and doing the following tests:

  1. Bridge the plug: this should turn the fan on, if it doesn't you know there's a problem with the wiring, fuse, relay, fan motor or something else downstream of the switch
  2. Test the switch: you can test the switch continuity using a multimeter. There are two pins on the sensor, when it is cool the switch will be open and there will be no continuity, when it get hot the switch closes and you will get continuity. Use alligator clip cables to clamp onto the switch pins, then clamp the other end of the alligator cables to your multimeter probe leads. Start the engine and idle it until you'd expect the fan to start and watch for continuity. If you get continuity when it's hot the switch is working, if not you know the switch needs replacement

I think it's worth running both tests even if you discover the a problem in step one, it's one of those while you're at it things.

If both tests are successful you know there's an issue in the connection between the switch and plug, try cleaning it and making sure the plug makes good contact with the sensor posts.

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