The bright light in place of the fuse or the relay is showing it is getting the full 12 volts. That means it's getting 12 volts from the battery, AND a good ground, which is what we don't want. That's just a different way of saying there's a short to ground on that circuit. The stuff in that circuit has some electrical resistance. That is necessary to limit current flow to a safe, normal level. The short is bypassing that resistance. With no resistance to limit current flow, that current will be much too high, and the fuse will blow. Your test light or my test bulb has resistance. By sticking that in the circuit where current has to flow through it, the bulb limits current flow to a safe level. That's why the bulb is bright, and the fuse no longer blows. A brake light bulb will only allow about one amp of current to flow through it. Most bulbs in test lights pass even less current; typically 1/4 amp or less.
Regardless of which bulb you add to the circuit or where you plug it in, when you locate what is shorted and remove that short, current will have to go through the test bulb yet, and the normal circuit resistance. Think of the test bulb's resistance as one person standing on a garden hose. Water flow will be reduced because of your foot's resistance. The normal circuit resistance is like a second person standing on that garden hose. Water flow will be reduced even more. The short is like removing the second person, and their resistance, from the hose. As long as you still have your foot on the hose, (the test bulb in the fuse or relay socket), water flow will be restricted and limited to a safe level.
Even if that wondrous explanation is hard to follow, you found the important clue. The test bulb is bright, indicating there is a short in that circuit. The goal now is to not try to make the short go away, except when you are doing something that you can control and follow. I've had people bring me a car that blows a fuse once a month, and the circuit works fine the rest of the time. After I put in my test bulb, and see that it's bright, the owner pounds on the dash, or slams the hood, then says, "see, the short goes away when I do that". At that point I tell them they just removed the only clue I had, and they should bring the car back in a month when the fuse is blowing again. What do they expect I'm going to do when the short is gone? The fuse isn't blowing because there is no defect, and nothing to find. Grrr.
You have a permanent short which is a real lot easier to find because you will know exactly when you do something that makes it go away. The bulb will go out or get dim. Start by unplugging items in that circuit. I suspect the most likely cause would be an oxygen sensor heater grounded internally or to the sensor's ground wire. Unplug those sensors. The ignition coil and injectors are in this circuit. They are turned off by the Engine Computer removing their ground circuits, so even if one of those parts were shorted, the fuse wouldn't blow until you started cranking the engine. That eliminates the coil and injectors as suspects, but not the wires going to them. You can unplug each of those parts while watching the brightness of the test bulb. If the bulb dims, the better suspect is you disturbed a wire that was grounded when you moved that plug around.
The two small wires on the back of the alternator are in this circuit. There isn't much inside the alternator that can short, so if removing those wires makes the short go away, look closer at those wires.
I can't get into the online service manual web site for that last few days, so to tell you what else is on this circuit, I have to come up with suggestions from memory. Look for an EGR valve, a charcoal canister purge valve, or anything else that might get 12 volts only when the engine is running. If you do not have a separate fuel pump relay, the pump is powered through the ASD relay, then that becomes another potential suspect. Fuel pumps don't short very often. A better suspect would the wire going to it is pinched or rubbed through.
Look for a huge bundle of wires laying on the inner fender where they can slide back and forth as the engine rocks. I've seen those have multiple wires rubbed through the insulation and the paint rubbed off the inner fender. You might get that to act up by tugging on the car while it's in "park", to rock the engine.
Thursday, October 12th, 2017 AT 7:39 PM