Hi,

My fiancee went to start my f150 today and said it wouldn't start due to a dead battery. I went to look at it and found a dead battery but the PCM relay (Relay #2) was also continuously clicking. I could not start the truck and ended up having to jump start it. I was able to get the clicking to stop before I jumped it by pressing on the brake for several seconds. I'm not sure if the relay clicking in was a result of the dead battery and her trying to start it or if it started the problem. It didn't look like the lights were left on so I was thinking the PCM had something to do with the dead battery.


Thanks for the help,

Jon

 

Usually when you have a rundown battery, and clicking relays, it's because the voltage is so low in the system. You can also get this with the starter relay and a dead battery or bad battery cable.

What happens is with no load, the voltage from the battery will rise up. Any load on the battery, because it's charge is very low, will cause the voltage in the system to drop.

So the PCM relay is off, the load on the system is low. When the PCM is activated, it sends power to all the computer circuits. This is a heavier load that the dead battery can't handle, so the voltage drops. The voltage dropping causes the coil in the PCM relay to drop out. Of course when the PCM relay drops out, the load on the battery goes away, and the voltage rises. When the voltage rises, the coil in the PCM relay kicks in again, and you go through the same cycle over and over. That is what the clicking is all about.

 

Thanks for the help. Was worried it would be something to fix.