I have a Pontiac Grand Am. After getting a hose replaced, I noticed that my fans don't come on now. I checked the fuses and cooling fan #2's fuse is burnt out, which (I think) should be for AC only. It's winter here, so that's not a big deal. So now I'm left wondering why cooling fan #1 isn't coming on when my car is plenty hot. The only thing I see for fan #1 is relays. How can I test these relays? Is it practical to do, or do some auto part stores test relays or anything?

4 Answers 4


You can easily test a relay in a few simple steps. The required materials are as follows

  • Digital Multimeter (with continuity setting)
  • 12V power supply
  • (Optional) Aligator clips for hands-free testing

DO NOT USE YOUR BATTERY DIRECTLY FOR THE 12V SUPPLY, if you have nothing on hand and want to be safe please use a fused line from a harness or through the fuse box and not the battery terminals directly, a 12 V wallwart (dc adapter for powering your typical consumer devices) with the connector cut and leads stripped is a good option as well. Crowbarring a 12V battery is a catastrophe which is scarilly easy to do on a relay with redundant pins and poor pin markings.

The test procedure is as follows

  1. Determine relay number and pin out

  2. Connect 12 V supply to Control pins on the relay and listen for a physical click (Gross Test). A relay that passes this test will probably be ok, but if you still suspect the relay you can easily verify finer electrical properties by continuing.

  3. Disconnect Relay

  4. Verify that all the Normally Closed connections read continuity and low resistance ( a few ohms), verify that all the Normally Open connections read no-continuity and infinite resistance. The control pins should read a few ohms typically, but refer to the datasheet for the relay. A Relay may click from raw 12V, but a dying relay may not actuate under say ECU drive signal

  5. Reconnect relay

  6. Verify continuity across relevant signal pins (should read at most a few ohms and the DMM buzzer should go off). NO opens should now be closed and NC pins should now be open, for more complicated relays refer to the datasheet

This will verify the actuation of the relay, that a connection is established and isn't degraded, and that the relay succesfully disconnects (no sticking) when unpowered.

A relay diagram looks generally as follows: enter image description here

The bouncy line indicates the inductive control element, and the straight lines with the break indicate the signal connections. A relay may have several redundant or parallel switching lines and possibly redundant control lines. This particular relay switches between 87A and 87 when the proper voltage is applied between pin 85 and 86. In this case,for step 4, Pin 30-87A is normally closed (NC) and pin 30->87 is normally open (NO)


Why are you assuming that the fuse is only for the AC fan? It seems really strange that only one of the fans would have a fuse on its circuit. Replace the fuse, since you'll have to do it anyway - if the fan still doesn't come on, then start looking at other possible causes.

  • Oddly enough, I replaced the fuse and now both fans work
    – Earlz
    Dec 12, 2011 at 15:43
  • Not odd at all. You have one fuse for both fans. Same as on my Honda. Dec 12, 2011 at 17:09
  • Yet the fuse was explicitly labeled "COOL FAN #2 FUSE"... Even more odd was a fuse labeled "COOL FAN #2 GND FUSE" which wasn't blown
    – Earlz
    Dec 12, 2011 at 17:27
  • Maybe it's just badly labeled? The people who design cars have their off days too, just like everyone else. If you really want to know what's going on, try to find a wiring diagram for your model. Dec 12, 2011 at 17:35
  • @Earlz please accept this answer
    – Nilabja
    Nov 8, 2017 at 10:32

It can be done with a power source and a standard VOM. Apply appropriate power source to appropriate pins while listening for click and looking for the desired effect (could be presence vs. absence of voltage or continuity/lack of) on the VOM which would be attached to the appropriate pins.

You'll want a repair manual to find out what to replace "appropriate" with in the above, especially in regards to power supply and pins...

Also, you need to be very careful and make good connections that can't short out against each other while testing. A typical test involves 12v battery voltage, and it won't go well for you if you accidently short that out... Use good electrical safety practices and caution.

  • A simpler way: find another relay of the same type, then swap it in. If that fixes whatever problem you were having, then you know the other relay was bad. Obviously you can't do it if you don't have an identical relay. Dec 11, 2011 at 22:34
  • 1
    @MikeBaranczak You also risk damaging/destroying a perfectly good relay if the original relay failed through too much current draw so...
    – Treffynnon
    Dec 12, 2011 at 16:15
  • Ah. Good point, I haven't thought of that. Dec 12, 2011 at 17:35

I had to do this recently, like Brian said you'll need a power source and a multimeter. Most relays have a wiring diagram printed on them and most automotive relays are 4 pronged. Two prongs will be your positive and negative power and the other two will be the bridge that is made. What I did, and this was a very very cheap method and suggest for possible risk of electrocution that you try a safer method...I took speaker wire and stripped the ends. I wrapped a wire each around the positive and negative prongs on the relay. Then while touching the multimeter leads (set to test for continuity) to the bridge prongs, I touched the speaker wire to the car battery posts (thus the risk of electrocution, I DONT RECOMMEND THIS AND TAKE NO RESPONSIBILITY IF YOU DECIDE TO DISREGARD MY WARNING). Maybe someone knows a set of clips that will fit a car battery on one end the small alligator clips on the other end. If the you hear a click and have continuity, then the relay is most likely fine. If you only have one of the above or have no click and no continuity, it is probably kaput.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .