Hello,
My 97 f-150 completely died going down the road, and will crank over but will not start. We replaced the fuel pump, thats not the problem (it was working fine). We tested the fuel pump relay, not faulty. There is no power going to the fuel pump relay or to the fuel injectors. There is no spark at the spark plugs either. When we plug in a diagnostic port tester it fails to link to the tester too.

Anyone have any ideas on what the problem could be?

Thanks! Kevin dylanabby@charter.net

 

Man talk about a coincidence. The same exact thing is happening to my 97. No fuel pressure and no spark, Code reader won't work either.

Check the Maxi-Fuse under the hood for the PCM and see if it is blown. Mine looked ok but when I pulled it it had a hairline crack. I replaced it, tried to start it and pop it blew again.

Check yours and let me know, I am getting ready to order a new computer as I think there is an internal short in it somewhere.

 

I checked the maxi-fuse, it looks okay.
How sure are you that it is the PCM?

 

What engine are we talking about here?

My guess is fuse #30 (30A) in the passenger fuse panel.

Fuse #30 is in the lower right of my 1999 5.4L F150.

 

I am about 99% positive that it is my PCM. The only common denominator between the fuel and spark system is the PCM which is what got me looking at it.

Once I found the blown fuse and I put another one in it blew as well so unless I have a short somewhere in the wiring it has to be the PCM for me atleast.

 

TEMP1, I did check that #30 fuse and it was blown, replaced it and keeps blowing that fuse!! What is that fuse for, and would you know why it keeps blowing? Engine is a 4.6 V8.

Thanks again,

Kevin

 

It keeps blowing because you have a short somewhere between that fuse and the PCM. Just as I keep blowing the Maxi under the hood. I am sure it is inside my PCM, and as soon as I get a chance I will let you know for sure.

Kevin I tried to e-mail you back but it got rejected. A new PCM from Ford is 280.00, I figure to tow it to ford and have them tell me it is the PCM it will cost me almost that same $280.

 

Unplug the PCM and replace the fuse. If it blows the problem is not the PCM. Note: the battery must be connected to perform this test.

JMC

 

Check the radio noise capacitors for a short to ground.

JMC

 

It is extremely unlikely that a PCM would have a short in it that would blow the Maxi-fuse..... You can have it replaced, but you most likely will be wasting your time and money....

F30 does not supply the PCM anyway. It supplies a relay (via PCM power diode) that when energized, supplies PCM power via F24.

F30 supplies the ignition coils and the associated RFI caps.

Follow JMC's advice first. Just unplug the caps and see if the short circuit goes away. If it does, the engine will run just fine without the caps. They're just for RFI noise suppression.

Steve

 

I plan on disconnecting the PCM and trying to start it in the next few days and see if the Maxi Fuse blows. If it does then I am not going to get a new PCM, I will start looking for the problem elsewhere.

How would the Radio Noise Capacitors blow the Maxi Fuse under the hood for the PCM?

Also where are the Radio Noise Capacitor located?

 

We need to clarify a couple of points.

Fuses:

There are two mega-fuses located near the battery. One is between the battery and the alternator, the other supplies every other electrical circuit in the truck.

The main fuse panel under the hood contains a number of maxi-fuses, mini-fuses, relays, and a pair of diodes. Maxi-fuse #24 is the one I mentioned earlier as suppling the PCM via the PCM power relay. In addition to the PCM, it also supplies almost every sensor and actuator (solenoids, valves, injectors, etc.) under the hood in one way or another.

The under-dash fuse panel is pictured in the post by temp1. It includes the fuse F30 I previously mentioned.

In order to keep from discussing the wrong circuit, you must be explicit about WHICH fuse is blown. Continually referring it to as "the maxifuse" does not make clear what you are discussing.

Let's get clear info before proceeding further.

Klogan: check the RFI caps.

01trublu--: provide exact info.

Also, both of you need to specify which engine is under discussion. There are differencces in many cases...

Steve

 

I don't have the truck or the manual with me at the moment so I don't know the exact fuse number. In my truck it is the Maxi Fuse in the fuse box under the hood that supplys the PCM, I would assume it is F24.

All the fuses in the interior fuse box are ok.

The factory F24 is a 30amp. I only had a 40 so I put it in and it opoed as well. It was late and I wanted to try to pull a code one more time so I put a 100amp in and turned the key to key on engine off. The coad reader still showed Error, so I figured I would try to start it once more. It cranked for about 10-15 seconds before it blew the 100amp. I am figuring, and I could be wrong, but if the computer was working and getting power for thoes 10-15 seconds before the fuse blew it should have started but it didn't.

 

Okay, now we have specifics on your problem.

Hopefully, that 100A fuse didn't cause wiring damage....don't do that again or you will release all the magic smoke that makes electrical stuff work.

Use an ohmmeter (use one with an audible continuity beeper, if you can) connected to the load side of F24's terminal. It will show a short to ground. That short needs to be identified and removed.

Start by looking for O2 sensor wiring that may have gotten up against a manifold and melted.

Look for wires near moving steering, suspension, or driveline components that may be rubbing.

Wiggle the harnesses and see if you can get the short to clear.

If none of those help, start unplugging every sensor and actuator under the hood, one at a time to see if one of them is shorted to ground.

This circuit is very large. Don't be surprised if you eventually end up doing more drastic stuff.

Steve

 

OK A few questions. Somehow through my life I have learned how to install stereo systems, and turn a wrench to anything that needs a repair, upgrade, or just to see how things work. Along that path I have never really gotten into electrical systems too much. I have a few questions that might be elementary to some so please bare with me.

I have a DMM, could this be used?

What side of the terminal is the load side?

If I put the DMM to the load terminal what should I be looking for that will show a short?

While checking this should the car be off or KOEO?

I know the damage that I could have caused with the 100amp fuse. It was late I was mad and didn't care, I had my brother watching under the hood and was almost hoping to see smoke as it might have lead me to my problem, but alas no smoke.