Electrical Maxi 20 Fuse Blows On Startup

[plane225]

I've got a 1996 mustang gt and after class I was driving home and my car just turned off suddenly. I coasted into a parkinglot to investigate and I found that the maxi 20 fuse (#5) for the ccrm and fuel pump had blown. I replaced it and it popped again right after the car had turned on. if I turn the key, the fuse doesn't blow. I hear the fuel pump prime and nothing sounds out of the ordinary, and it cranks over just fine and fires up, but a second later the fuse blows and it dies again. I haven't had any issues with the fuel pump, but my concern is possibly with the 02 sensor. I know the wiring had scraped on some gravel and they may be damaged. I'll be looking into it more once im home from class. also I had dropped the tank last week to replace the filler neck gasket, and I may have loosened a wire somewhere on the pump. But has anybody had experience with this? Any ideas that I could start with?
much appreciated.

 

[COramprat]

You have a short for sure. Check the wires to the O2's. Make sure they are not frayed. A loose wire on the pump won't cause the fuse to blow.

 

[plane225]

You have a short for sure. Check the wires to the O2's. Make sure they are not frayed. A loose wire on the pump won't cause the fuse to blow.

The wiring to the 02 sensor had been dangling just above or on the ground, I suppose it's possible im just now seeing the effects of the now bad wiring. But would the wiring of the 02 sensor cause that fuse to blow on startup? It makes seeing as how the car keys over and primes and all that, but once the cars running about a second later the car stalls out.

 

[COramprat]

Yes, it would. My 96 did the same thing. It would go for awhile then blow the fuse. Finally is started doing what yours did. Found a fray in the wire going to the O2 when they swapped my transmission. It was shoring on the support tray for the O2 connector. Fixed it and was all good.

 

[plane225]

Yes, it would. My 96 did the same thing. It would go for awhile then blow the fuse. Finally is started doing what yours did. Found a fray in the wire going to the O2 when they swapped my transmission. It was shoring on the support tray for the O2 connector. Fixed it and was all good.

If it turns out to not be the 02 sensor wiring, what would be my next course of action?

 

[wmburns]

If it turns out to not be the 02 sensor wiring, what would be my next course of action?

IMO you should actually hope this is your problem. Why? Because of the car's history you have a great place to start looking for a wiring harness fault (wires hanging down).

The truth of the matter is finding a wiring harness fault in an unknown location is a total bear. Basically one has to disconnect everything until the problem is found. Or perform a total visual inspection until some mechanical damage is found.

As for WHY the motor starts and then shuts off. I maybe able to help with that one. When the motor first starts the PCM is in "open loop" mode. During open loop processing the O2 sensors are not used. After a few seconds of running the PCM switches to "closed loop" mode. During closed loop mode the O2 sensors are used.

IF the wiring harness fault is at an O2 sensor it makes perfect sense to me that the fuse would blow when the PCM turns on the O2 heaters.

 

[plane225]

IMO you should actually hope this is your problem. Why? Because of the car's history you have a great place to start looking for a wiring harness fault (wires hanging down).

The truth of the matter is finding a wiring harness fault in an unknown location is a total bear. Basically one has to disconnect everything until the problem is found. Or perform a total visual inspection until some mechanical damage is found.

As for WHY the motor starts and then shuts off. I maybe able to help with that one. When the motor first starts the PCM is in "open loop" mode. During open loop processing the O2 sensors are not used. After a few seconds of running the PCM switches to "closed loop" mode. During closed loop mode the O2 sensors are used.

IF the wiring harness fault is at an O2 sensor it makes perfect sense to me that the fuse would blow when the PCM turns on the O2 heaters.

Turned out the 02 wires were at fault. Thank you guys very much, all is well.