If you have a late model Jeep Grand Cherokee, chances are your cooling fan relay has failed on you at least once by now. If It has not, then you have been lucky so far. Jeep engineers for some reason decided to use a solid-state relay, rather than a tried and trusted conventional mechanical relay.
If the relay has failed more than once, there is a very good chance that you have something else going on in your cooling fan circuit that is causing these repetitive failures.
These solid-state relays are extremely sensitive to various conditions. One of the most common reasons for failure is heat; if the relay becomes too hot, it can overheat and instantly damage the internal circuitry. This can usually be solved by ensuring there is a very clean surface between the bottom of the relay and the chassis where it mounts. An optional step but a good idea is to use a thermal-transfer paste on the underside of the relay. This thermal paste allows heat to conduct more easily from the relay to the chassis. You can pick this paste up at a local Radio Shack or computer store. The relay gets rid of its heat by transferring it to the vehicle’s chassis, and the more efficient it can transfer heat, the cooler it will stay.
Typical thermal grease.
Another important factor is that the connector (often called pigtail) is in good condition; if the pins inside the connector are corroded or damaged at all, it will cause a higher resistance at the point where the connector contacts the relay pin. This again will cause excessive heat and can quickly burn up a brand new relay. A poor connection can also cause voltage spikes which are extremely hard on any solid-state device. The kits sold from APDTY include a brand new connector that MUST be replaced when installing the relay. This will help ensure the relay will continue to operate trouble-free.
The third thing that can cause repeated failure of the fan relay is if the cooling fan itself is going bad. If the cooling fan is starting to fail, often it will draw more electrical current than a fan in good condition. When this happens, it places a much heavier load upon the cooling fan relay, which again will cause the relay to overheat and fail. The simplest test to see if the fan may be going bad is to manually spin the blades. (Only do this if you have the battery disconnected to prevent injury!) If the blade feels tight at all, or if it feels as if it is binding in any spot, then you need to replace the fan. It can also fail internally without any physical symptoms as described prior. The only way to tell this is to do an amperage test on the fan while it is running to check how much current the fan is drawing. If the fan draws more than 20 amps at max, then you have a problem with it and need to replace it or you will continue burning up the relays.