Never heard of anyone blowing diodes on their PCM. Most people don't have the PC board needle soldering irons to be able to replace them.

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Never heard of anyone blowing diodes on their PCM. Most people don't have the PC board needle soldering irons to be able to replace them.

Sent from my Pixel 4 XL using Tapatalk

The diode that blows is in the fuse box under the hood

 

The diode that blows is in the fuse box under the hood.

 

Oh Man! That's a Good One....

I would think that the only thing that would do that is some sort of Surge where the diodes being overloaded, a short or something. But,,, why it only happens when you shut-down is hard to fathom

Can you put your finger on it and get a feeling for signs of Heat? Maybe at various times to see if it might be running overcapacity normally and something's pushing it over-the-edge when you shut down? How about an IR temp gun?

What's the fusebox position that it's in?

 

Oh Man! That's a Good One....

I would think that the only thing that would do that is some sort of Surge where the diodes being overloaded, a short or something. But,,, why it only happens when you shut-down is hard to fathom

Can you put your finger on it and get a feeling for signs of Heat? Maybe at various times to see if it might be running overcapacity normally and something's pushing it over-the-edge when you shut down? How about an IR temp gun?

What's the fusebox position that it's in?

I have felt the temp often but it is never hot. When it blows, it melts the plastic housing around it.

 

I knew exactly which diode you were having trouble with. I've attached the .pdf of the circuit, it shows the diode.

That diode isn't very large in capacity. About 1-Amp. My thought is that when you turn the key off, it is subject to a reverse current that it can't block and it smokes. The problem with this assessment is that (looking at the circuit) there isn't supposed to be any voltage source on the down-stream side of the diode. Only the two relay coils.

The power to energize the relays comes from the ignition switch. When the switch is turned off, the power ceases and the relays open, shutting off the engine. The ABS is also energized by the ignition switch. Because "the ABS light comes on when this happens", that indicates that power is back-feeding into the ignition circuit.

With the key off, remove the diode and measure the voltage of both terminals in the box. It should be zero. The fuel heater is one of the items on the circuit, but so is the wastegate solenoid. Inspect the wiring there as well.

You have a really, really odd one. I think it is going to be something like rodent damage as the root cause.

I'd begin by separating the upper fuse box from the lower housing and conducting a close visual inspection of the wiring inside the under hood fuse box. You're looking for any of the wires that go to the relay coils and diode possibly touching a live wire through damaged insulation.

 

thanks I'll check that out.

 

Are you sue you are inserting the diode the in correct orientation?
Have the PCM and IDM realys been replaced?
the 99 diagram shows the diode also powers the FP relay coil

With the diode removed there should be zero on both terminals -- key on, should be battery voltage on one terminal and zero on the other terminal

 

Are you sue you are inserting the diode the in correct orientation? If the diode is installed 'backwards', it wouldn't run. The PCM and IDM relays wouldn't close.
Have the PCM and IDM realys been replaced? It runs. The relays are fine.
the 99 diagram shows the diode also powers the FP relay coil. No. The diode also powers the IDM relay.
I am betting on a shorting coil in one of the relays pulling too much current -- the diode is acting like a fuse in this case. If the relays are pulling "too much current", the diode would smoke as soon as he turns the key on, and the engine would not start. The diode doesn't smoke until he turns the key OFF.

If "reverse current" is blowing the diode, the current will be on the relay coil side of the diode. With voltage present on the coil side of the diode, the engine should not stop running because the stray voltage will keep the PCM and IDM relays energized.

This one is a head-scratcher. I have some questions:

Is this intermittent, or every time you shut it off? How many times has it happened? Was there any work or other incident that preceded the first time it did it?

And: When you see the ABS light when you turn the key off: Does the engine immediately shut down, or does it continue to run until the diode pops?

 

Diodes fail from heat or excessive current and excessive reverse current. That PCM diode on diesel applications is in the coil side of the PCM relay and the IDM relay. Since you already have some great advice I would only add that you inspect the diode pins in the BJB for poor fitment, a failing PCM and IDM relay.

 

I am betting on a shorting coil in one of the relays pulling too much current -- the diode is acting like a fuse in this case
could also be corrosion in the relay box, so that needs to be looked at also
you can use a meter to measure the current draw across the relay terminals -- then remove one relay at a time and observe the draw
seems like about 20-30ma on each relay, if I remember right

 

I can not think of a failure scenario that would smoke the diode, but still allow the engine to run and shut down when the key is turned to OFF. If it isn't wiring damage, it would have to be a faulty (somehow) IDM or PCM relay.

Substitute both the IDM and PCM under hood relays with 2 relays removed from the under-dash fuse box. If the diode still pops, you haven't found it. If you start popping a fuse for a non-critical system (Horn, Driver window fast-down, or Power Window / Radio) then that is the "bad" relay from under the hood. Relays are cheap, but test by substitution is even cheaper.

EDIT: Because this is intermittent, I would try the relay substitution above first. Because the ABS light staying on is associated with the diode pop, do this:

When you shut it down, watch the ABS light. If the light comes on, IMMEDIATELY turn the key back to RUN. (If you're quick enough, the diode should still be intact.) (If the diode does stay intact, it is "reverse current" overload that is smoking the diode).
Go under the the hood and remove the diode. Does the engine stop running? (It should not stop if the fault is present).
If the engine stays running with the diode removed, remove the PCM relay. Did the engine stop? If = NO, remove the IDM relay. (That better stop it).

 

I can not think of a failure scenario that would smoke the diode, but still allow the engine to run and shut down when the key is turned to OFF. If it isn't wiring damage, it would have to be a faulty (somehow) IDM or PCM relay.

Substitute both the IDM and PCM under hood relays with 2 relays removed from the under-dash fuse box. If the diode still pops, you haven't found it. If you start popping a fuse for a non-critical system (Horn, Driver window fast-down, or Power Window / Radio) then that is the "bad" relay from under the hood. Relays are cheap, but test by substitution is even cheaper.

The relays are "fine", ...you said so in post #12. Please make up your mind.

 

The circuit only powers the coils of the PCM and IDM relays. When they collapse they can send a voltage spike, which is why the diode is there. I'd replace the two relays with factory relays.

Edit, see there are other posts generated.

 

Curious what else is powered by that circuit on Splice 179 ?
possible a back feed from that part of the circuit causing a spike to hit the diode

 

I guess you "forgot" to mention the add on fuel system at the start -- that is where you should look for the problem -- provide a wiring diagram showing how it is integrated into the truck system, and we can help diagnose

That VegiStroke system has to keep those two relays energized in order for the truck to run and change out the fuel

 

-----

Hydro,
You are correct. I do appreciate you being willing to help after you know I have a non-stock truck. First, it is powered direct from the batter on a separate fused link. It has a diode on the diesel pump to protect the system from power spikes when it shuts down the diesel pump. The only other connections are a wire to get 12V key sense that is attached to the pink w/ blue stripe wire in the OBDI loom under the dash and a wire to the fuse box under the hood so that the engine can continue to run after the key is removed. It is attached to the white w/light blue stripe wire running from the pcm diode to the pcm relay. I have looked at all of these connections but not found any troubles. When the weather clears I will look into those relays to see if switching them out helps. Again, many thanks. Some folks just shut down when they hear I burn veg with an aftermarket system. I have also reached out to the forum of VegiStroke to see if there is any help there. In my search, I found no other instances of this happening with their system.

 

@DaveP. usually even in the cheap diodes, the peak inverse voltage is at least 50 - so no wonder yours didn't blow with 12 volts

I would like to know what the purpose of this diode is anyways -- it is not a fly back for the relay coils -- and since the power supply is keyed, that source is cut off when the key is turned

What is down stream of the splice S179 ?

 

edit failed.

 

The fuel heater and the waste-gate control solenoid. That is all.

I cant see what the purpose of the diode is ? -- the way it is in the circuit, it stops voltage from coming back out of the relays, but that voltage has no where to go (so it dissapates in the coil)
the fly back from the 2 relays is not gonna hurt the fuel heater (and the heater should be off anyways)
the waste gate solenoid would easily sink the voltage from the relays -- so I don't see a problem there

this voltage is only there for a few mili seconds when the key is turned off

Just thinking out loud here

 

I think look at the diode on the diesel pump , it may be popped -- that is on the side that would feed a spike thru the fuse box diode

Aand isn't the ABS module tied into the splice S179 ? -- Thanks Dave for the clarification -- the heater should not cause a back feed, and the gate solenoid should be off
EDIT: what is the heater circuit has a partial ground in it?

@Spike92 A diode is like a check valve for water or gas - but for electricity
electric is passed in one direction (the arrow in the symbol) and held in the opposite direction -- this is a simplified description

All diodes have forward current and voltage specs -- as well as inverse -- take for instance the 1N4001, it will pass 1 amp current and hold 50 volts back (again simplified)
I am sure ford used the cheapest diode they could get by with and the 1N4001 is a comparable diode I think to the one used -- it will power 2 relays just fine
but your fuel pump may be spiking voltage back enough to exceed the 50 volt limit
I would look at the fuel pump diode to see if it is still good

 

Hydro, I'll check out that diode on the diesel fuel pump. Thanks for digging into this.
All, I'm really thankful to all of you for chiming in and working on this. Hate to have her just sitting there in the driveway.

 

EDIT: what i[f] the heater circuit has a partial ground in it?[/QUOTE]
The heater element is a PTC heater. It always has a "partial ground" in it. PTC elements stop conducting at a predetermined temperature when the internal resistance rises markedly, and current flow diminishes to effectively zero. But there is still some resistance to ground.

Even if the heater is full-on (about 12 Amps) reverse current won't blow the diode as long as the voltage isn't above 50v (using your number which is correct). Which it won't be in a 12V system truck.

 

Thank you.

 

Thanks for the diagram Dave
So, the heater is always on -- no thermostat switch
unless the element burns out - common failure
The heater kinda serves a dual purpose, to heat the fuel and to sink the fly back voltage from the 2 relays and the solenoid

Anybody on the purpose of the diode ?