Follow up on a14 white fusible link keeps blowing: '94 a-body sedan

Question

QUESTION: 1994 Plymouth Sundance/ 2.2 Ltr engine/ Engine Controller/ SBEC/ 60 wire connector. (NOTE: At first, suspected fuel pump, so it was replaced with a new one and that seemed to solve the problem) HOWEVER: After running great for 130 miles, car suddenly and without warning died and would not start. The White fuseable link that connects to the Red with White Stripewire that goes to the module keeps blowing fuses whenever the key is turned on. It turns on the service engine light and the circuits that run the engine. If I unplug the 60 pin Connector from the module, there's no short until I plug it back in. My manual has no information on this except to systematically jiggle wires. ANY IDEAS WOULD BE GREATLY APPRECIATED! This is the family's only source of transportation in a very rural area. Thanx!

ANSWER: Hi Gage, That red/white wire provides current to the automatic shutdown relay (which when you turn the key to run position it closes, and the current from the relay then supplies on a dark green/orange wire the fuel injector, the field coils of the alternator, the ignition coil primary winding) and to the fuel punp relay (which closes when you turn the key to the run position and the current supplies on a dark green/white wire the fuel pump motor and the heated oxygen sensor), and as you noted on a red/white wire to pin 3 of the powertrain control module at all times. So there are many possible devices that may be faulty and shorting the current to ground and of course also many possible wires that might be shorted to ground. I don't believe it has anything to do with the module but rather the fact that the module is what closes the two relays listed above when you turn the key to run position and thus blows the fusible. If you have an ohmmeter one approach would be to go to the ASD relay and the fuel pump relay and find the dark green/orange wire and the dark green/white wires and measure the resistance to ground of those two wires with the ignition off. One or the other will read a dead short to ground and then you will know which set of devices (supplied by which is the shorted wire) to then check out individually. The fuel pump relay is the most inboard one on the firewall, and the asd relay is the most forward one on the inner fender. My thinking is that the most likely device to short to ground is the heated oxygen sensor located on the exhaust manifold because it has very fragile wires between its tip and its plug. So take a look at that device which looks like a spark plug and examine the fragile wires carefully for break or short between them. The next most likely cause would be the field coils of the alternator. That coil has a very low but non-zero resistance as compared to a short so you could measure the resistance of the pin at the alternator to ground after disconnecting the dark green/orange wire from it. If you have a digital ohmmeter it should not not frank 0 but some finite resistance, around 2 ohms. So those are some ideas as to how to find the short that is blowing the white fusible link, and without having to test by seeing if the fusible link blows or not which would be a real hassle. Please 'rate' my answer (see below). Thanks, Roland

---------- FOLLOW-UP ----------

QUESTION: Your advice was extremely helpful in narrowing down the problem. It is not the brain. By removing and reconnecting or testing wires per your advice, the problem seems to be in the wiring "before" the oxygen sensor. Once that was moved around and tested it read out on the ohm meter and i was able to start the car without blowing the fuse. However I am still unsure of exactly where the problem is or why it failed in the first place. Just that it appears to be in the wires connecting ahead of the oxygen sensor.But i wanted to thank you for responding so quickly and accurately. If you can think of anything else it might be, please let me know. Either way I am going to your "Thank and nominate" page

Answer

Hi Gage, The 4-wire plug/socket at the sensor has one black wire to a nearby ground (right side of intake manifold near throttle body) and then 3 wires (including a dark green/black which you might confuse with a black/dark green that go to a 10-pin black disconnect at rear of valve cover, where the other side of the disconnect is where you will find the dark green/black wire is now dark green/white and which connects directly to the fuel pump relay socket. So trace that pathway and also take a close look at all four wires at the sensor proper as the short might be among those 4 rather fragile wires. Thanks for the rating and nomination, I am pleased to have helped you. Roland