Hello everyone, I just bought a '96 F150 XL, with a 4.9L 4x4 and it keeps popping the 30A maxi fuse for the PCM. I would just like to know if anyone has encountered this problem before, to save myself alot of time on troubleshooting. Thanx!!!

 

As in replace the fuse and instant "pop"
As in new fuse is ok for a while then "pop"
Which?

 

Instant pop , your wires are gounding or touching , chase it down..

 

Yes, its popping instantly. I think it's because the wiring harness is right behind the fenderwell and is getting corroded back there.

 

Yeah, prolly clean em up and you'll see were the prob is..

 

Thanks for the advice.

 

Just to let ya know, I replaced the PCM relay in the power distribution box and replaced the fuse and it fired right up, and runs like a champ.

 

That's odd that the relay was the fault. Bosch/ISO type relays are generally VERY reliable. Most of the sensors & actuators on the engine (like the injectors & the oxygen sensors) run directly off that fuse thru that relay, so keep an eye out for any other suspicious problems.

If you get a CEL, you'll have to use an OBD-II scantool to get the codes, but many parts stores do it free (assuming you can get the truck there & get the EEC powered up).

 

Don't be suprise if you have a problem again. Hope not.
There are two 30 amp fuses involved with this part of the circuit.
One would blow if the relay winding shorted. There is a diode in series with the PCM relay winding (#501). It may even be shorted now but you most likley would not see any effect from it. Only way is to meter it with a meter set on resistance scale.
The other would blow if there was a short to ground almost any place the relay contacts feed, after they close. The relay feeds all the injectors, nearly all the sensors.
If the fuse blowed after putting it in place but before the ignition was turned on , the relay contacts could have been burnt closed from the short or the other fuse was the one blowing from the fault with the relay winding.

 

I would also be skeptical that the relay replacement solved the problem. relays typically don't short to ground (or internally between the coil and contact terminals) internally.

There are two fuses that supply parts of the PCM circuit: F22 supplies the relay's coil, F9 supplies all the relay's loads via the relay's energized contacts. Please be specific as to WHICH PCM fuse is blowing. I suspect that it's F9 which opens the possiblity of a wiring or sensor fault at almost any of the underhood sensors and actuators.

Steve

 

Whenever I would replace the 30A fuse that was popping with a 50A, it would run. I thought about it all night, and it just seems like too easy of a fix, but I'll keep drivin' till it pops again.

 

It is F9. I just noticed that the coolant temp gage isn't working. Could that be part of the problem?

 

It could be related, but the temp gauge is a discrete circuit. It's possible that the EEC PWR circuit is shorted to the gauge circuit, but not likely.

 

It finally popped again today after nearly 100 miles. I am having trouble with my oil pressure gage working. When I wiggle the shifter or tap on the plastic around the steering column, the gage starts working. I think there's some grounding issues going on in there that relate to the fuse popping, gage not working and airbag light flashing 51. Now I just need more time to mess with it.

 

Look at the solid red lead all the way thru.
CMP, Can vent, vapor, all the injectors, Mass Air, IAC, Ox sensor heaters, input power to the PCM and a feed to the trans all feed thru the max fuse and relay contacts.
After you find the fault, I would change the relay to be sure the contacts won't let you down in the future from passing overload current so many times..