I did a search on the forums here and couldnt really find a very good thread that detailed what I am dealing with right now.

My truck is a 95 F150, 5.0 V8, Auto, 2WD, standard cab.

Yesterday I started her up and she purred just as she normally would, I drove for maybe 15 houses down the street and the engine died very suddenly, no sputtering or any signs of hesitation, just dead. I still have all electrical accessories such as radio, lights, dash lights, etc... The starter is still engaging and sounds as normal as can be. It will turn over for as long as you like but no signs of starting.

First I checked for sounds of a fuel pump, btw it has dual tanks, and I do not hear any sign of a fuel pump, none of the usual whirring sound that I have heard on other cars. But then again I never really did hear the whirring sound come from my truck before so I cannot vouch for how loud it should be, but I did check with my ear almost pasted against the fuel tank. I then removed the air intake and sprayed a bit of carb/choke cleaner down the throttle body to see if it would try to start and it wouldnt. So I assumed maybe spark is my prblem if not both spark and fuel.

To try and trim down the possibilities I began to look for spark. I grounded one of the plugs against the valve cover, a large peice of metal that appears to be used for hoisting the engine, and some other pieces of metal inside the engine compartment and it shows no sign of spark.

I the tried to trace down a culprit and found that a 20 AMP MAXI fuse was blown inside the fuse panel inside the engine compartment. I replace the fuse only to have it blow again. It blows instantly and still with no signs of starting. Still the engine turns over normally. I replaced the Ignition coil (fairly cheap attempt as it was in stock locally for $10) and three (3) out of four (4)of the relays inside the fuse panel in the engine compartment (again fairly inexpensive as they were only $4 a peice, and three was all they had in stock).

After many failed attempts to get her to start again I asked a mechanic down the road what he thought it might be and he told me to look for the Heated Air Sensor and check there for shorts or a bad sensor itself. I went home, disconnected the sensor to try to rule out the sensor itslef, and alas, my last fuse blew. So I was then out of 20 AMP MAXI fuses and so are all of the local part stores.

Being out of fuses I decided to make a Jumper. I jumped the original location of the fuse, Heated Oxygen Sensor still disconnected and she FINALLY starts right away. But the wire that I used to jump the fuse begins to smoke and burn very soon after she starts so I turn her off. So if there was a fuse ot still would have blown.

Lastly, a friend stops by and he notices the Diode that is inside the same fuse panel inside the engine compartment and takes a quick look at the wiring diagram in my (well used) Chilton's manual and he thinks the diode may be bad, allowing power to return back to the fuse therefore causing it to blow.

Unfortunately the fuse panel that I am referring to does not have much detailed information about the fueses inside and it only lists them by numbers.

The fuse in question is fuse MAXI #9

I forgot to mention that many many years ago (almost 9 years) I had an aftermarket alarm installed in the truck. It is a Viper alarm and does have a starter kill. The starter kill has always worked correctly and did kill the starter only in the past, not the whole ignition system. Now im not sure if this could be a cause but I thought it worth mentioning once I thought about it.

 

I don't have info on you year truck but NEVER jump a fuse location when it fast blows a fuse unless you like buring wiring back into a harness and create a big mess to fix.' besides the original problem.

 

Understood, and I am well aware of the risks. Unfortunatley I let my frustrations get the best of me and had to try something. Wont be doing it again.

 

It's pretty obvious that something MAXI fuse #9 feeds is drawing more current than it should. It's possible that particular fuse feeds the ignition module along with whatever is sucking the juice. When you used the jumper, you fed the module, but the other piece was still sucking the juice.

I don't have schematics that go back prior to '97, so unfortunately I can't help out much.

-Joe

 

I'm looking at Mitchell's for that year. It does not have ford numbers. I see a diode, a relay and two 20 amp fuses in that box. This may not be everything, of course.

Have you checked engine ground? Have you done ANY work on the engine lately? Removed any ground straps? Had any trips into large mud puddles? Been in deep snow? Fender bender?
Chris

 

I have not checked the engine ground. I will check in a few moments.

Lately she's just been used around town. Weeks ago I had to take her through what I guess you could call a puddle. Wasn't very deep, but it was at a considerable speed so water presumably could have still made it inside the engine bay. Although she has seen much much worse in her day.

I have done zero work on the engine. I really cannot think of the last time any real work was done on her, except for the simple stuff, oil change, fluid check etc...

I checked the Heated oxygen sensor as one guy suggested and I followed the Chilton's on how to do so. In my Chilton's however it describes the Heated Oxygen Sensor and an Oxygen sensor in the same place. The manual said to NOT TEST THE OXYGEN sensor (it does not indicate what test not to do but I assume ohms) as it would damage it. But it does give a simple test for checking the HEATED OXYGEN sensor. I removed the Oxygen Sensor, or Heated oxygen sensor (not clear on which I have), and tested it with an ohmeter anyways. The chilton's said it should be around 6ohms, mine showed 4.5 or so. Could this ssensor, if it were bad cause the problems I'm having? Also there is some very small damage on the sensor. It has a very small dent on it as if it had been struck by something in the past, although the small dent seems pretty old. I will change it anyways to make sure I guess.

Thanks for the help and the good ideas Chris.

 

I think you are barking up the wrong tree. An O2 sensor with 1.5 ohms off is not something that can burn more than 20 amps. You have a dead short. Not an open, not a long, not a 'out of spec' component, but a dead short. Or in other terms you have something that is melted, touching, burned, not just a little hinky.

Also, whatever it is, it is a thick something. Look at the little wire off your Oxygen sensor. If that was shorted, it would burn through long before a twenty amp fuse, much less the 30 plus amps it took to smoke your wire jumper. Something is grossly wrong, not just out of kilter. Look for a bigger, more obvious problem, like a wire in the harness shorted, or a wet patch in a connector. If a component is down a tiny wire, look somewhere else. A relay stuck could do it, although that is not likely. Look around the exhaust manifold, the starter relay, even the horn relay area. Look for melted wires. Get a strong light down in the areas beside the engine.

If you have a specific wire or component to look at, I can look up the part or the wires and at least tell you something about it. The Mitchells is not as specific as the Ford CD's but for 95 that is all I have.

Good luck
Chris

 

Just re read my post and realized I came across as harsh or autocratic.
Didn't mean it that way, I just forget my manners when my mind is on electrical problems that are out of reach of my ohm meter...

Remote troubleshooting a dead short scares me a little. Too likely to cause a fire or burn up an expensive part if I say the wrong thing.

Hoping you whip this before it louses up your Christmas,
Chris

 

That's the fuse for the PCM. You said you replaced 3 of the 4 relays under the hood, did that include the one farthest from the MAXI fuse that kept blowing? That is the first thing you should inspect/replace (PCM Power Relay).

 

They must have recently updated the database.... I now have them for a '96, but not a '95....

According to the OEM manual, the #9 fuse under the hood feeds the PCM and the fuel pump relay. Now, it ALSo calls for a 30A fuse for '96, so I would double-check the manual for yours and make sure it's supposed to be a 20A.

Now, if it still manages to blow a 30A, I'd yank the fuel pump relay and try another fuse and see if she still blows. If so, the PCM is shot and shorted. If you pull the fuel pump relay and replace the fuse and it stays in there, replace the fuel pump relay. (The circuit triggers the relay, but does not feed the pump itself)

 

I'm looking at the factory 1995 Electrical/Vacuum manual right now, and it's not quite like GIJoe described. There seems to be differences between '95 and '96. Also, it is a 20 amp fuse. DO NOT try a 30 amp fuse.

Like I said, it's the PCM fuse, and the most likely source of your problem is the PCM Power Relay. If that's good, then it's either a bad PCM, or a short in any of the dozen or so places that receive power from the PCM relay (the fuel pump relay is one of many).

It's not that tough to diagnose, but start with the PCM Power Relay.

Do you have a test lamp? If so, attach the aligator clip to the positive battery post, and touch the tip of the probe to the red wire that goes to your IAC (or the red wire that goes to your MAF, or the one that goes to your EGR solenoid, or your EVAP purge solenoid, or any of your injectors, or either of the AIR solenoids. They all share that same circuit), and see if it lights up. If it does, there is a short either in the red wire, or one of the items I just mentioned.

 

Since you have the manual in front of you, could you let him know which one is the PCM relay too? I suspect that'll be the next question.....

And if it calls for a 20, ignore my comment about the 30. Like I said, it was for a '96....

I'd probably unplug the PCM and try another fuse. If it shorts out with the PCM unplugged, then it's not the PCM causing it.... something else in the circuit is short.

And I agree wholeheartedly with the dead-short comments too. You're talking a major current draw, not some puny little O2 sensor being a little out of whack (which won't prevent it from running anyways)

-Joe

 

I already did.

I just checked the times, and I suspect you missed my first post (it was just a few minutes before your next to last post).

 

You did? I'm not seeing it.... I meant which RELAY is the PCM relay.... not which fuse.... I think....

 

If that wasn't good enough, here's a picture...