My gut feeling is it's not a module but don't bet the farm on it. When you turn the ignition switch to "Run", all the modules get 12 volts from the ignition switch so a shorted module would blow the fuse before you went all the way to "Crank". In addition, as soon as you hit Run, the engine computer (powertrain control module / PCM) turns on the Automatic Shutdown (ASD) relay for two seconds. This relay sends voltage to the ignition coils, fuel injectors, alternator field winding, fuel pump or pump relay, oxygen sensor heaters, and back to the PCM on a another terminal. None of these things typically short but there could be a grounded wire going to one of them. Grounded wires usually are caused by rubbing against sharp metal brackets, harnesses falling down onto hot exhaust parts, or wires getting pinched and crushed during other service.
During that two-second blip of the ASD relay, you might hear the fuel pump run. That is further proof nothing is shorted to ground in that circuit. The ASD relay is turned on again when the PCM sees engine rotation. If you don't hear the pump run, there could be a low resistance short, (as opposed to a dead short), that causes a small delay before the fuse blows. Some things to try are:
Remove the cover from a relay and install it in the ASD socket. Squeeze the contact to power up all the stuff on that circuit. You should hear a hiss from the fuel pressure regulator, and a second person should be able to hear the fuel pump run by listening under the tank or by removing the fill cap. Hold the contact applied for half a minute. If the fuse blows after a long delay, you'll need to look at the diagram to see what's on that circuit and unplug one of them at a time.
Instead of removing the cover from a relay, you can jump the terminals with a paper clip or short piece of wire. The two terminals on the sides, and parallel to each other are for the relay's coil. You want to jump the two terminals that form the letter "T". If there's a fifth terminal in the center, it's not used in these applications.
If the fuse does not blow, move that relay, (or jumper wire), to the starter relay socket. Turn the ignition switch OFF, then squeeze the contacts to verify the starter cranks the engine. The 10 amp fuse has nothing to do with the starter circuit so you're just verifying this circuit is ok. Now turn the ignition switch to Run, then squeeze the starter relay contacts. If the engine starts and runs without blowing the fuse, the problem has to be in the ignition switch or the wire going from the ignition switch to the starter relay. Since this is such a small part of the whole system, I would expect something before this step caused the fuse to blow. Another test would be to remove the starter and the ASD relays, then turn the ignition switch to Crank and see if the fuse still blows.
If the fuse only blows in the Crank position, a trick I learned at a tv servicing class many years ago is to replace the fuse with a light bulb. Tail light bulbs work best because they will limit current to about an amp so no circuitry will be damaged. Solder or crimp two wires to thin spade terminals and plug the terminals into the fuse socket. Solder the other ends of the wires to a light bulb or use a bulb socket with wires already attached. The brightness of the bulb will indicate how much current is flowing in the circuit. Now, do whatever it takes to make the bulb go to full brightness. If it only occurs in Crank, have someone hold the switch there while you flex different parts of the wiring harness and unplug various things. When the bulb dims, you've narrowed down the location of the short.
Ignition switches have given a lot of problems in the past, but typically the contacts become overheated and don't allow enough current to pass to run things on that circuit. This usually affects power windows, the radio, and the heater fan, especially if you like to use the highest fan speed often. In the worst case, the plastic housing around the electrical contacts or the electrical connector will melt. Although I've never seen it, I could imagine a switch melting enough that a ground terminal touches the starter terminal. On older cars, there was a switch built into the ignition switch that grounded the parking brake warning light to turn it on as a bulb test in the Crank position. I looked at a '99 Dakota service manual which uses the same ignition switch, and I don't see any ground wire so I really don't think the switch is the problem. You might try tilting the steering wheel to see if a wire has rubbed through and is grounded. If that stops the fuse from blowing, it would be just a coincidence that it happened at the same time you changed the battery.
I'm waiting to hear about the solution.
Sunday, March 22nd, 2009 AT 5:04 PM