Cranks but no start - PCM fuse keeps blowing

[Sweersa]

My 1997 Ford Ranger with 130,000 miles died on me while driving. It would crank, but not fire up. A few weeks prior, it would lose power on the road and then the engine would surge back to life and all was normal until it would happen again. The CEL would blink when this would happen. It would do it a few times a day or more, or if I went over a rough area of road (pretty easy to do in Michigan)

Today I removed the bed, replaced the fuel pump, rusted tank straps, and will also do the filler neck and shocks since I have the bed off. I originally thought it was a bad fuel pump because I didn't hear it before starting the engine, and was upset to find it was not the issue after replacing it but at least I have a new fuel pump now!

I later discovered the 30A PCM fuse under the hood was blown. When I replaced it, it blew again as soon as I tried to start her up.

I called it a day, but will jack the truck up later and check under it for shorted out wires. Is that the most likely culprit?

I'd appreciate any advice on what could cause the 30A PCM fuse to blow. I heard the wiring around the O2 sensors can melt, so I'll check there first.

Thank you!

 

[tomw]

You have a wire that is directly shorted to ground. I would suspect that you pinched a wire when working on the fuel pump or sender unit.
If I were trying to resolve this on site, I would likely be lifting the bed and inspecting the wires that were disturbed. I suspect that one of them that is supposed to lay in a depression may have been routed atop something that smushed the insulation.
tom

 

[Sweersa]

You have a wire that is directly shorted to ground. I would suspect that you pinched a wire when working on the fuel pump or sender unit.
If I were trying to resolve this on site, I would likely be lifting the bed and inspecting the wires that were disturbed. I suspect that one of them that is supposed to lay in a depression may have been routed atop something that smushed the insulation.
tom

Hi Tom,

Thank you for the information! The issues started before I even worked on it, so I don't think I crushed anything. The bed is still off, so I'll take a closer look at all wires.

 

[Mark_88]

Well it definitely is a short so whether it happened in the process or prior too, best thing is to check all the wires you can see for bare parts...wires that pass over sharp body or dash edges are prone to that sort of thing.

Think back to before this fuse started popping and recall if you did any work on anything...go to that area and check that/those areas first...as it is likely due to some wire being pulled off, pushed down, or otherwise roughed up...

 

[Sweersa]

Well it definitely is a short so whether it happened in the process or prior too, best thing is to check all the wires you can see for bare parts...wires that pass over sharp body or dash edges are prone to that sort of thing.

Think back to before this fuse started popping and recall if you did any work on anything...go to that area and check that/those areas first...as it is likely due to some wire being pulled off, pushed down, or otherwise roughed up...

I did have to cut a wire that I do not believe was factory to remove the bed. I figured since it was aftermarket, it was okay to do that, but they may have pulled the power from a critical component. The wire runs from the front of the vehicle, through the frame on the driver's side, and all the way back into the left taillight area leading to a wire harness that was used to power a topper light that has been abandoned for years when I pulled the topper off. Whoever ran it, ran the wire through the gas tank straps so the wire was messed up there a bit. I snipped it towards the end to get the bed off yesterday. Perhaps that is the problem. It begins near the fuse box under the hood mounted on something on the driver's side, fastened on a part I'm not sure what it is, and has a 5 amp fuse on it. Please see the picture:

I can re-wire this (and run it better/safer than who did it originally) and hook it back up to the bed wire to see if that was the problem.

I also found what I believe to be broken copper braided grounds. This one connects the hood to the body around the engine compartment. I found two others on each side of the vehicle, on the frame. All are broken. I'll fix all of these too by soldering new thick wires between the contacts after removing the broken copper. Can these cause problems if broken? I have noticed at least one broken before on the hood.

I'll try to fix all of these wires, and any more I find next weekend to see if it fixes the blown 30 amp PCM fuse issue.

 

[Mark_88]

The ground straps are needed for a reason I didn't even think about. Apparently everything needs to be grounded on a moving vehicle because as the air passes over the metal, it creates a static charge.

Not a big problem for the operation of the vehicle but the charge will cause excessive rust and corrosion in anything that is not grounded...and the sparks can be dangerous if you build up enough static. As you can see by the colour of the strap, it is already corroded. The ones that I have on my truck that are not broken are not corroded at all...well, very lightly.

Sounds like you may have the solution within sight...I've gone back over my work a few times over the years when other problems pop up...and, sure enough, I created the new problems myself...lol

 

[tomw]

The UAW had in their multiple contracts that the State of Michigan MUST use salt on every road with traffic in the winter. Thus the salt bath that turned your ground cabled to Cupric Oxide. In reality, that's a lot of corrosion, but I have not lived in IN or OH in 40+ years, so have lost my concern about road cancer. The ground straps carry the juice back to the battery, or reverse, and will use the path with least resistance. Given the number that were 'open circuit', the current must be passing through one ground at a higher than designed rate. It makes sense to repair the grounds to insure proper operation of the electrical things.
Most trucks had a bunch of fusible links on the batter terminal of the starter relay. Newer vehicles have a power distribution box, with fuses and relays all (most) located in that box. In either case, you can remove all the fusibles, and then touch them one at a time to see which circuit causes the short, OR in the box, remove all the fuses & relays, and insert them one at a time to see which circuit blows the fuse. Once determined, repair that circuit.
tom

 

[Sweersa]

Thank you again Mark and Tom for the advice. It will definitely come in handy the next time I'm able to work on her.

Tom, if I do the test method where I pull all of the fuses and put them in one by one to see what fries the PCM 30A fuse, should I put the key in the on position as I do this, or turn it over for each fuse separately? I remember when I first replaced that fuse, it didn't blow until I tried to start the engine (or it may have happened just with the key in the on position). I'm assuming when I pull all of the fuses, I keep the PCM 30A fuse in there the entire time so I can watch for it to blow.

 

[snoranger]

The grounding strap on the hood is there to prevent radio frequency interference. The truck will work perfectly fine without it, but you may here a little more noise coming through the speakers.
The ground straps from the frame to the body can cause many issues, including, dim/ non-functioning/ always on lights, static discharge every time you touch the door handle, instrument cluster malfunctions, power windows/locks slow or inop., etc... They should be fixed, but they generally don't cause blown fuses. Dead shorts cause instantly blown fuses. I don't have a wiring diagram for a '97, but that 30 amp fuse most likely powers more then just the PCM. Get a wiring diagram and follow the circuit. It will lead you to any and all places your short can be.

 

[tomw]

Going back to the original condition. Stopped while running down the road, no fuel pump action, pump replaced, still no workee, found 30A fuse blown. Replace fuse, and more smoke gets out, ruining fuse.
There is a short that eats fuses. It is in the ECM, the relay, or the associated wiring.
IF you have bad grounds, you can 'backfeed' stuff, powering it up after a fashion, and using whatever it is as the wire to ground 'other things'.
Given that you originally had the problem of intermittent on bumpy roads, I'd suspect something that can be moved by bumps. Wires and relays come to mind. I HOPE it is not the computer itself.
I would be checking all the bundle of wires under the hood, grabbing and yanking to see if there is a corrosion problem that has eaten through the conductor(s). You have checked the under-bed wires, I assume, but should continue to check the 'add-on' wires, and to improve reliability, remove the added wires that are no longer used. They are just a potential trouble spot, and you might find that one of the wires is routed thoughtlessly, and has a direct connection to ground, thus touching firmly now, but intermittently when bumping down the road previously.
You more than likely have a wire that is contacting something it should not. Look for that.
tom
add:
Remember, the ECM provides a GROUND to all the things that it runs. They are powered by separate fuses, breakers and fusible links. Take a look at the link
http://www.therangerstation.com/forums/showthread.php?t=162465

and follow the link to advance parts for the wiring diagram. The basic parts will be wired the same way, i.e., powering the injectors via a fusible link, but you may have a power dist box instead. Point being, the computer does grounds.
I do not have an Advance membership so can't look for a more correct diagram to figure out what additionally is run by the 30A fuse. If the computer is it, that's a bad sign...

 

[Sweersa]

Quick update - I reattached that wire I snipped that someone added years ago. I took all of the fuses out except for the the 30A MAXI PCM Power Relay fuse that blows. When I put the key in the on position (didn't try turning it over) it did not blow when it was the only fuse installed. When I added the 50A Ignition Switch fuse, and put the key in the on position, it blew the 30A MAXI PCM Power Relay fuse again.

I'm going to check the wiring more thoroughly after I jack the vehicle up to see if I can find any shorts. Couldn't find any this morning, and I checked both O2 sensor wiring. I have one fuse left, and will probably pick more up tonight. Will replace the fuel filter too while the vehicle is raised, it's 5 years old with over 51,000 on the Motorcraft fuel filter I replaced in 2010. I'm not giving up on her!

Worst case, is I put the bed back on, and have it towed (towing covered under the coverage I have on it) to my trusted local mechanic.

 

[tomw]

I hate to say it, but your logic or fuses are somewhat backward. You do not fuse a 50A circuit with a 30A fuse. The 50A fuse will never blow before the 30A does.
If the 50A is getting close to 50A of current, the 30A would have long gone and turned so smoke. The 50 will NEVER blow, as you cannot get 50A TO it.
I do not know the Power Dist Box layout nor circuits, but by design there's something backwards if a 50A is powered by a 30A.
Either way, the problem is in the circuit fused by the 50A, or OR things are not as they should be.
If the computer is hanging off the 50A circuit, I would disconnect it or the relay (WAY LESS THAN 50A ...) and see if fuse burns up.
I cannot keep all the details of this problem in my haid. To blame old. Are there any aftermarket 'things' hung onto the electrical? If so, let them say unpowered until you can figure this out would be my suggestion. If YOU did not install them, and the PO left no diagram nor schematic telling what they are there for, you are playing with juice, and possibly they could be the cause of all the problem. You cannot successfully fuse a 50A circuit with it being fed by a 30A circuit unless you consider the 50A as pure wire. It will never get hot, much less smoked.
tom

 

[Sweersa]

I hate to say it, but your logic or fuses are somewhat backward. You do not fuse a 50A circuit with a 30A fuse. The 50A fuse will never blow before the 30A does.
If the 50A is getting close to 50A of current, the 30A would have long gone and turned so smoke. The 50 will NEVER blow, as you cannot get 50A TO it.
I do not know the Power Dist Box layout nor circuits, but by design there's something backwards if a 50A is powered by a 30A.
Either way, the problem is in the circuit fused by the 50A, or OR things are not as they should be.
If the computer is hanging off the 50A circuit, I would disconnect it or the relay (WAY LESS THAN 50A ...) and see if fuse burns up.
I cannot keep all the details of this problem in my haid. To blame old. Are there any aftermarket 'things' hung onto the electrical? If so, let them say unpowered until you can figure this out would be my suggestion. If YOU did not install them, and the PO left no diagram nor schematic telling what they are there for, you are playing with juice, and possibly they could be the cause of all the problem. You cannot successfully fuse a 50A circuit with it being fed by a 30A circuit unless you consider the 50A as pure wire. It will never get hot, much less smoked.
tom

I did not put a 30A fuse where a 50A fuse is supposed to go, and I did not blow a 50A fuse. The 30A PCM fuse blew (again) after I re-installed the 50A fuse for the ignition switch when I was putting the fuses back one at a time to see what circuit blew the 30A fuse.

Sorry I wasn't more clear.

I didn't notice any aftermarket electrical items. Next weekend I put the bed back on the truck, and get it ready to be towed to the local trusted mechanic. I'll let him know my findings, and what I replaced (fuel pump, filter, etc.) I like to try to tackle my car issues, but I think this one is beyond me. I'll for sure keep you guys updated on what the mechanic finds.

OR in the box, remove all the fuses & relays, and insert them one at a time to see which circuit blows the fuse. Once determined, repair that circuit.
tom

This is what I was doing.

 

[tomw]

Th 50 is, I think, the fuse for all the other fuses. You have to leave IT in, and then insert all the others one at a time. That is so that there is power to the rest of the fuses in the fuse box. If you did that, and the 30A sizzled when you put it in, then disconnect the multi-pin connector from the computer, and replace the 30A one more time. If the fuse does not blow, then you know that the computer is killing the fuse.
Next, if so, question is whether it is the computer (has to be) or something the computer 'runs'. The computer provides signal 5-v to the sensors, and ground to the things that get 'powered' or 'operated' by the computer. When the computer is in place, and you turn the key to ON, a relay closes, and the computer wakes up.
If the fuse blows with just the computer in place, it's cooked, IMO. If the computer blows the fuse after key ON, something it powers (with the 5 volt reference sense voltage) must be shorted... But all this is unlikely. The computers are pretty reliable.
I would be checking to see exactly what is on the 30A fuse. Other years have multiple things on a 30A fusible, then split into multiple other fusibles or fused items. The sum of them is less than 30A. Just as the sum of all things powered by the 50A is less than 50A, or it would be 'legal' or possible to blow the 50A fuse with normal use, just turning on all the powered things at once. So the sum 'after' a fuse should be less than the fuse capacity.
tom

 

[Sweersa]

I know it has been years, but I believe it's important to update this thread. I had the no start Ranger towed to a local mechanic, and he sat on it for almost 3 years. No joke. That caused additional issues, but long story short, I recently had it towed from the shop that couldn't fix it to a far better shop, and they had it running in a few days. All they did was pop a new battery in, starter, and alternator, and it ran. It even ran fine with the old gas.

The alarming part is everything the shop replaced was working fine when the fuse kept blowing, it sitting outside for years probably killed the alt, starter, and battery. After getting my running Ranger back, it would still occasionally have the same hard shifting and slamming back into gear issue when driving, most notably over a bumpy area of the road. It really felt and sounded transmission related, and thankfully, the handful of times it happened since getting it back have not resulted in it breaking down as it had before when it blew the 30amp fuse.

I inspected all the wires I could find, and I believe I found the culprit that started it all. The wiring harness to the speed sensor in the transmission has an issue where the two wires were both partially cut/ripped near each other, and both also have some fluid (I assume tranny) running down them, so I believe this sensor was shorting out, which explains the weird transmission hard shifting, and the check engine light also would very briefly flash during these episodes. I cleaned off the wires, and wrapped them in waterproofing tape, and so far the issue has not come back. I ordered a new wiring harness to replace the old one, as the wires are cut almost to the clip so it would be hard to splice new wires in. I'm not sure why it has a small leak there either, perhaps I should just replace the entire sensor along with the harness.