97 4.2 V6 5spd.

I keep blowing fuse #24 under the hood it is the PCM Fuse.

I can put a new fuse in and it is fine, however as soon as I turn the key to the on position it blows.

I have a DMM and am not sure if I am using it corrctly to detect a short in the wiring, I also have the wiring diagram but not being familiar enough with car electronics I really can't tell what items are feeding off of that fuse. I pulled the PCM harness and turned the key and the fuse still popped so I am assuming it is nothing in the computer itself. I tried testing the ohms on the MAF as the red wire from it gets its power through the 24 fuse. I put one lead in the load side of fuse 24 and touched the other end to the red MAF wire and the DMM read a number then another number then it went to 1 which I am assuming is out of range. Turned it up to the next highest and same thing, turned it up again and all it showed was a 1 no numbers at all. I was thinking if there was a short in the wires it would have showed a 0 reading?? Anyone with experience want to help me out please.

BTW No recent additions of any electrical equipment or anything like that. I was driving the truck and it killed while sitting at a red light.

 

To use the DMM, short the test leads together. You should get a reading of between 0.0 and 1.0 ohms. That's a good representation of what a short circuit will look like. Then, observe the reading with the probes not touching. That's what an open circuit should read.

F24 supplies, via the PCM Power relay, many of the sensors and actuators on the engine in addition to the PCM itself. You cannot use the load side of F24, you have to attach one lead to to the output of the PCM power relay since the relay is not energized. Pull the PCM power relay. Identify terminal "87" and which connector it plugs into. On my 97 5.4, it's the one closest to F29 (unused) slot. Plug your positive lead into this slot taking care you don't stretch the connectors open too much. Connect the negative lead to a chassis ground. Your meter will likely read less than 1.0 ohm. That's your short circuit. The goal will be to get rid of it. When you find the fault this reading will go much higher, in the order of several hundred ohms - mine reads around 335 ohms.

Follow these suggested steps:

First, inspect all four O2 sensor harnesses to see if any are lying on an exhaust manifold and have melted to the hot exhaust. If not, disconnect each O2 sensor and see if the short goes away. This circuit powers the heater element in each sensor and thay are known to short out.

If it wasn't the O2 sensors, you will need to disconnect each of the following devices as they all feed from this circuit.

MAF
PCM
Vapor Management Valve
Canister vent solenoid at canister under rear of truck
Fuel pump relay (coil)
Intake Manifold tuning valve
EGR Vacuum Regulator
All six fuel injectors
IAC valve
IMRC monitors 1 & 2
Cam sensor

If, after disconnecting all of them and the short is still there, you have a wiring harness fault what will require a LOT of work to diagnose.

Good luck, it will come in handy.

Steve

EDIT: Didn't we cover all this back in April????

 

Thanks that is just what I needed. Hopefully I can find it without too much headache. I do have a few questions for clarification though.



Ok so I put the red lead into terminal 87 and then put the black lead to a chassis ground. Now as for the ground can it be anywhere like if I were to put it to one of the hood bolts or something like that? Or do I need to put it to a specific type of chassis ground like the one from the motor to the frame?

Now while I am testing do I just leave the DMM hooked up to 87 and a chassis ground and disconnect things one at a time until it goes away? Example leave it hooked to 87 and the ground it is showing less than 1 ohm, now disconnect 1 O2 sensor and it is still showing less than 1 ohm so that is not the short. Move on and disconnect another O2 and the Ohms jump so the short is in the O2 that I just unplugged right.


So does that mean that I just need to replace the O2 sensor?




I am not sure lol. I keep getting so many different answers, some say use the load side of 24 and probe each harness and I have gotten different stories on the DMM readings it is a PITA. I have built and put in probably 6 or 7 motors, done rear ends, etc..etc... but electrical wiring other than stereos is like looking at a Russian Play written in German for me.

 

Negative battery terminal, any grounding strap or lug, engine block, etc....

Yep, that's the idea. Hopefully you'll not end up with a harness short.

Yes, or whatever component that may have been the source of the short circuit.

Once you get some practice with a DMM, you'll be able to use it as readily as you use your remote for your video/audo system. It's just a skill to be learned.

I think I had originally suggested the load side of F24 without having looked at the schematic. If I had looked first, I would have given more accurate information. Mea culpa.

If you get bored, use your meter to see what the resistance of your speakers are. You chould get a reading that is close to the rated impedence (ie, 4 or 8 ohms). Yeah, I know impedence is not the same as resistance (and I do know the difference) but for DMM practice and training, the difference is not important. A little practice will help build your confidence.

Remember, do not use a meter on the resistance function on any circuit that is power. If you do, you will either damage the meter, or, at the very least, get erroneous readings.

Steve

 

Thanks again Steve, hopefully I will get some time in this week to start fooling with it again. I can't imagine it will take forever to find like I said it just stopped all of a sudden, with no pre existing problems. Maybe I just hit a bump the wrong way or something and a wire got screwed up???

I am wondering though if it is a wire issue wouldn't getting a whole new harness be easier than trying to pinpoin the short in the maze of wires??


Oh yeah one last thing what Ohms scale should I set it on??? 200? 2K? 200K??

 

Steve

I just wanted to thank you for taking the time to help me out. I got the truck fixed using the DMM to find the short.

I hooked it up and it showed a short, so I started unhooking things starting with the easiest. MAF, PCM, IAC, O2 sensors, and it was still there. GOt through mostly everything except for the injectors. I was getting the tools out to remove anything I needed to in order to access the injectors. I figured I would get under the truck and check it one last time. Wouldn't you know it the truck has 4 O2 sensors... Well I got the top drivers side off and the short was still there. I reached up to disconnect the front passenger side and as my hand hit the wire harness I felt the flex loom kind of pop away from the exhaust pipe. Wouldn't you know it the wire touched the exhaust, melted the loom, and out of all of the wires in that harness only the red wire had its insulation melted and touching the pipe.

Got from under the truck and what do you know...the short was gone. So I wrapped the wire in electrical tape, re-loomed about 6" section of it and zip tied it up to keep it from happening again.

So it really was kind of simple in the end, glad I didn't start taking things apart to get to the injectors lol.

So thanks again Steve, you truely are top notch.

Jeremy

 

Glad to be of help.

If you'll notice, I specifically said to check those harnesses FIRST all the way back in my first post... (where's that trout-slap smilie). I even said there were four of them...

Now that you fixed it, go play with your DMM so you get some experience with it.

Steve

 

HAHA Trout slap. I thought you said there were 4 of them but I thought you were talking about a 5.4.... The front two on the 4.2 are so damn high up on the pipes I acctually thought it was the EGR tie into the exhaust at first glance.

Oh well it is fixed now...somewhat. While under the truck I noticed that the bottom leaf on the drivers side is sitting crooked compared to the rest of the leaves. I guess I need to take them off, straightne it back out and re-install.....Or maybe now is a time to get a new set of leaf springs that can handle a better load without raising the truck up.