: How do you check a relay?

03-24-06, 05:32 PM
In response to my earlier post about a blower fan that always runs on high, even with the car turned off, it was sugeested that I check the realys, which makes sense. My question is one that someone asked in response to that post. How exactly do you check a relay? I have no idea how to even start, and I dont think most people just have extra relays sitting around that they can just plug in and try. Anybody know?

03-24-06, 09:20 PM
Well, you are right. The straightforward way to check a relay is by substitution.

Otherwise you have to remove and attempt energization of the relay manually to see if the contacts close, or open.

Relays have two common failure modes:

The points can stick. They sort of become welded together sometimes. Often this can be put right by rapping the relay, or its enclosure, sharply. The vibration sometimes will free stuck contacts. Of course they can stick again next time the relay is energized.

Another failure mode is a burned out coil. In this case the low level signal which usually energizes the relay cannot do so. The points in this situation cannot close or cannot open; they cannot do whatever they are supposed to do when the relay is energized. Burned out coils usually are detected by checking continutiy with a meter. I doubt this is your problem.

Do not neglect this possibility:

The problem might not be the fan relay. It might be that the signal that energizes the relay is present when it should not be present. The relay may just be followng (incorrect) "orders".

Also full disclosure:

This post is an effort to answer your question, and no more. I am not stipulating to the premise you have a bad relay. Truth is, I do not know.

I do not even know if it is a mechanical relay controlling your fan. It might, instead, be a solid state device. And they can fail closed, too.

Good luck!

El Dobro
03-25-06, 08:02 AM
The relays usually have a part number on them and most cars have the same relay used in several positions. Just find another one on your car with the same part number and swap them. You can then see if it made any difference.

04-11-06, 10:38 AM
How do you actually REMOVE a relay? In particular, relay #5 (A/C) under the hood. I can't figure out how to physically remove it. There's a tab on the quarterpanel side of the relay that you can push with a screwdriver, but that allows the relay, fuses and the board they mount to to come free, not just the relay. Help??