Hi ASEMASTER6371 and BRIAN GREENHILL. Here's the same diagram in a different form. Use whichever one is easier to follow. My contribution has to do with an effective method of running down a short that avoids wasting a lot of fuses. I originally used this when working on tvs, then later as an electrical specialist at a car dealership.
This procedure starts with the fourth drawing. It is likely to be over-simplified for you, but this may help others researching this topic. All this is doing is connecting a bulb across the battery, with the short taking the place of the ground wire going back to the battery. I also added a notation in the third diagram that this circuit comes through the ignition switch, which is why fuse 30 doesn't blow until that switch is turned on.
When you turn the ignition switch to "run", current will still flow through the short, but it will be limited to a safe value by the light bulb. I've shown a common 3157 brake light bulb. That will allow about one amp to flow through the circuit. If you need more current, a low-beam head lamp bulb will pass around five amps. Because of the dead short, when the circuit is turned on, the bulb is going to be full brightness and hot, so be careful what it is laying on or against. Now you can disconnect the things shown on the diagram to see what makes the short go away. When it does, the bulb will get dim or go off.
One note to add to the confusion. This is a relatively high-current circuit as evidenced by needing a 30-amp fuse. Once you remove the shorted item, everything else in the circuit is going to try to function, but that bulb is still limiting current too much for that to happen. The bulb is going to drop too much of the 12 volts, leaving too little for computers to work. That normal high-current demand will result in the bulb still being very bright after the short is removed. So bright, in fact, that you may have to watch very closely to see a brightness change when you unplug items. This is where a head light bulb might be a better choice. It will be easier to see the brightness change when the short is removed.
If you find the short is gone when you unplug the "PCM Power Relay", you can reinstall fuse 30, then use this same procedure to bypass that relay instead of the fuse. The bulb will still limit current flow through the short that way, but it will restore full 12 volts to everything else protected by that fuse. If necessary, I'll find the appropriate diagram and figure out which are the two terminals for the relay's internal contacts.
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Tuesday, April 27th, 2021 AT 3:16 PM
