[bismic]

Initial checks for the PCM power relay can be done by seeing if the fuel pump comes on with the key turned to "on" (not cranking), and if the reverse lights come on with the key in "on" and the gearshift in reverse. If these two do not turn on with the key in "on", it does not mean that there IS a problem with the PCM Power Relay, just that there COULD BE. More investigations would need to be done.

Of course, power does not get sent to the fuel pump electrical feed circuit until the PCM is satisfied with inputs (not in gear), and then it grounds the fuel pump relay (so the PCM needs to be working AND the TRS needs to be working).

 

[spike817]

Initial checks for the PCM power relay can be done by seeing if the fuel pump comes on with the key turned to "on" (not cranking), and if the reverse lights come on with the key in "on" and the gearshift in reverse. If these two do not turn on with the key in "on", it does not mean that there IS a problem with the PCM Power Relay, just that there COULD BE. More investigations would need to be done.

Of course, power does not get sent to the fuel pump electrical feed circuit until the PCM is satisfied with inputs (not in gear) and grounds the fuel pump relay (so the PCM needs to be working AND the TRS needs to be working).

Aha, both those thing do not, in fact, turn on.

Investigating the PCM relay itself... I can note that it is receiving battery voltage. I've swapped the relays around and checking continuity on the relays have all checked out and hasn't returned comms.

I'll send a proper response and pictures for your previous comment in a little bit but for now i'll say that the main engine harness is brand new and the chaffing point of the harness near the steering rack was addressed by the previous owner so I'm fairly confident in those two locations.

I'll look at the EBP harness and send pictures of that and the other locations here.

 

[Hydro]

Use your meter and check voltages in the PCM relay socket
#30 should have battery voltage
#86 is powered from the key switch
#85 is ground -- same ground as your data port
#87 is switched from the relay to the PCM

Check the engine circuit in this pdf for the PCM relay
Hmm, wont let me upload the file
thought is also located on this forum -- unless some rules have changed
Wiring Diagram - 2003 Powerstroke 6L - Single Alt - Full.pdf

Go here to see the file

https://www.powerstroke.org/threads/hi-res-single-page-6-0l-wiring-diagram-pdfs.1118001/#post-13497177

 

[spike817]

Use your meter and check voltages in the PCM relay socket
#30 should have battery voltage
#86 is powered from the key switch
#85 is ground -- same ground as your data port
#87 is switched from the relay to the PCM

Check the engine circuit in this pdf for the PCM relay
Hmm, wont let me upload the file
thought is also located on this forum -- unless some rules have changed
Wiring Diagram - 2003 Powerstroke 6L - Single Alt - Full.pdf

Go here to see the file

https://www.powerstroke.org/threads/hi-res-single-page-6-0l-wiring-diagram-pdfs.1118001/#post-13497177

Alright so the PCM power relay sent me down a rabbit hole.

First time I probed the relay:

• Volatge probed at pins #46 and #34 from PCM connector C1 to ground pins #11 and #23 (Key-on; C1 disconnected): Both 0.55V.
• Voltage probed from relay socket #30 to relay socket ground #85: 12.9V
• Voltage probed from relay socket #86 (w/ key on) to chassis ground: 1.4V
• Voltage probed from relay socket #86 (w/ key on) to relay socket ground #85: -11.3V??
• Voltage probed from relay ground socket #85 to chassis ground: 12.45V?

Funky voltage on the KEYPWR circuit is not switching the relay here. The -11.3V on the KEYPWR circuit and 12.45V between the grounds stumped me here. I thought the ground was maybe bad?? But I monkeyed around enough that I haven't seen these voltages since and so I never quite figured it out.

While I was monkeying around I decided to short the PCM power relay in order to see if having battery voltage on the PCM / Fuse 22 circuit would restore comms... It did not. I confirmed the short had restored battery voltage to the PCM / fuse 22 circuit - which it did - and then I unplugged the C2 and C3 PCM connectors to eliminate those connections but no luck.

Went to bed, came back the next morning and read this on the PCM power relay sockets:

• Volatge probed at pins #46 and #34 from PCM connector C1 to ground pins #11 and #23 (Key-on; C1 disconnected): Both 0.7V.
• Voltage probed from relay socket #30 to relay socket ground #85: 12.9V
• Voltage probed from relay socket #86 (w/ key on) to relay socket ground #85: 0V

The negative voltage and voltage between grounds is gone but clearly there is consistently no KEYPWR voltage. Started looking at the ignition switch, disassembled it, and disconnected it. Took a look at it and then merely plugged it back in and wham! KEYPWR voltage up to 7V and then later back to battery voltage. PCM power relay is now switching again and full battery voltage has been restored to the PCM / fuse 22 circuit. Checked to see if comms came back... nope. Low voltage on that circuit definetly wasn't the original cause of the no comms but I guess that was evident when I shorted the relay anyways.

When I had mentioned in an earlier reply to bismic that I had originally probed 12V at fuse 22, I don't think I was mistaken. I reckon that this was a new onset (possibly coincidental) issue on that KEYPWR circuit that prevented the PCM power relay from switching BUT it wasn't the original or primary reason for the no comm. Though you guys might be able to spot a common denominator here.

I doubt the KEYPWR issue was actually fixed from merely unplugging and plugging back in the ignition switch. So, I'm not sure if I need to be looking at maybe replacing the ignition switch or if its the ignition switch wiring I need to look at or if I just actually magically solved that issue. Thoughts?

I should also note that some of my instrument cluster lighting is suspiciously dimmer now then before I messed with the ignition switch; but I'm not positive that its related or even an issue.

Overall, I'm still stumped on this one guys.

 

[spike817]

Yes - the throttle position sensors should be on the list, but I have been trying to list the easiest things to troubleshoot first. With TPS issues, they are very good at throwing codes (well instrumented). Doesn't help when you can't read the PCM though - that is what makes the loss of OBDII communication so difficult.

If you have seen a P0121 in the past, then it was telling you that there is a circuit issue. A common chafe point is at the steering column - in the area around where the harness wiring goes through the firewall. Also, at the Accelerator Pedal pivot point (under dash) at steering column - adjustable pedals only.

Also, there are "mid harness connectors" in the engine well near the drivers side firewall (below the "Battery Junction Box" that sometimes get wet (coolant leak) or get damaged (bent pins if they have been removed and re-connected) that can also cause these problems.

Occasionally, a person has had a wire chafe in the main engine harness at the location under the air intake tube (in front of the turbo air inlet). This harness contains v-reference and PCM power wires that can cause your issues if they short out.

It is a 5V reference cicruit, but ALSO inspect the EBP sensor connector and as much of its wiring as you possibly can.

If you want, take some pictures and post them:

• EBP harness
• wire bundle under the air intake
• TPS wiring at steering column and at the firewall

EBP harness and wire bundle under the air intake are all brand new:

Here's the TPS wiring:

I believe these are the mid harness connectors?

 

[Hydro]

Guess I should have specified to test the relay socket with reference to the battery ground , rather than the relay ground

...but , you still should have a good solid ground on that terminal

Sounds like you still may have a loose / corroded ground

 

[Hydro]

that ground on the relay and PCM specifies a "type 4 ground" -- not sure what that means in Ford lingo

but usually would be a direct path back to the battery -- like the wires that are connected directly to the battery clamps

 

[Jeepspert]

This is like a dajavu moment, I had the same thing happen to me but my issue was intermittent no crank no start with hesitation and idling issues. What harness have you replaced? You said the EBP connector was new? The connector in the picture is not the same as mine for the EBP, drivers side underneath the coolant tank attached to a tube.

 

[spike817]

This is like a dajavu moment, I had the same thing happen to me but my issue was intermittent no crank no start with hesitation and idling issues. What harness have you replaced? You said the EBP connector was new? The connector in the picture is not the same as mine for the EBP, drivers side underneath the coolant tank attached to a tube. View attachment 799007

I was following this Video.

I replaced the main engine harness i.e., the harness that connects to the middle connector of the pcm; the one that has the EGR and fan clutch, and etc. sensors; the one that runs just under the turbo inlet.

I don't see a connector in the location you're showing:

 

[Jeepspert]

Looks the the sensor, I've heard if it being there but it threw me off.

 

[Hydro]

Maybe push the connector the rest of the way in ...
WD40 is a good lube for the connectors , so they slide easily

 

[spike817]

Maybe push the connector the rest of the way in ...
WD40 is a good lube for the connectors , so they slide easily

Had it off earlier so I just placed it there to show where it was.

Sorry Hydro but could you check my logic:
You suggested earlier to disconnect the PCM connectors C2 and C3 to see if the comms restored... If they weren't, that would imply that I can narrow down my search to within the C1 connector circuits, right?

 

[Hydro]

Yeah , was going off a process of elimination so you would not have to search the whole harness and related components

Under the weather last couple of days , and looks like not over soon -- I may repeat my self or run forums together
maybe pour a couple fingers of MM and contemplate the backside of my eyelids

Have you checked your PCM relay socket for voltages on terminal 30 and 86 ?
then the relay needs to click and send power out terminal 87 to the PCM -- when you plug it in with the key on

Half temped to go all Rambo on the PCM and advise to bench test it
you can power from a battery using jumper wires made with a butt splice on the end for the terminals -- or strip about 1/4 inch and use some shrink tube to fashion a diy connector
also fashion a connector for the DLC and see if there is comms
Just would need to be sure of the connections -- before powering anything

Dunno , maybe too much of a rabbit trail or the fever

PS here is a good guide that @bismic put together

Troubleshooting - OBDII port (DLC connector) does not...

Wanted to put some thoughts down on troubleshooting the issue with "no communication" at the DLC (OBDII port, connector C251). Feedback is welcome. First, try to verify that your scan tool or tuner is not the problem ...... and that it will function on another 6.0L system. Follow steps below...

www.powerstroke.org

 

[spike817]

Yeah , was going off a process of elimination so you would not have to search the whole harness and related components

Under the weather last couple of days , and looks like not over soon -- I may repeat my self or run forums together
maybe pour a couple fingers of MM and contemplate the backside of my eyelids

Have you checked your PCM relay socket for voltages on terminal 30 and 86 ?
then the relay needs to click and send power out terminal 87 to the PCM -- when you plug it in with the key on

Half temped to go all Rambo on the PCM and advise to bench test it
you can power from a battery using jumper wires made with a butt splice on the end for the terminals -- or strip about 1/4 inch and use some shrink tube to fashion a diy connector
also fashion a connector for the DLC and see if there is comms
Just would need to be sure of the connections -- before powering anything

Dunno , maybe too much of a rabbit trail or the fever

PS here is a good guide that @bismic put together

Troubleshooting - OBDII port (DLC connector) does not...

Wanted to put some thoughts down on troubleshooting the issue with "no communication" at the DLC (OBDII port, connector C251). Feedback is welcome. First, try to verify that your scan tool or tuner is not the problem ...... and that it will function on another 6.0L system. Follow steps below...

www.powerstroke.org

Haha well no worries because I'm appreciating both yours and bismics help greatly. If we run around in circles a few times that's alright with me lol, I wouldn't have gotten this far without you guys.

My original post when i opened this discussion was actually me going step by step through bismic's guide! My responses and findings are in red within where I quoted his guide.

So PCM relay voltages on terminal 30 is 12.7V.
On terminal 86 (w/ key on) the voltages are all over the place but not battery voltage. My most recent reading was 7V on terminal 86.

So my current rabbit hole is that I believe there is not enough voltage from KEYPWR to switch the relay, thus not giving me battery voltage on the PCM / fuse 22 circuit.

This has lead me to investigating the ignition switch where I found that on circuit 1044 (WH/YE), I only receive 2.55V with the key in the on position. And circuit 1044 is what powers socket 86 on the PCM relay I believe; hence, the pcm relay not switching.

So I'm trying to figure out if its my ignition switch that is troublesome or if its a short along circuit 1044.

HOWEVER, I want to note that I found earlier that even if i short the pcm relay and give the PCM / fuse 22 circuit battery voltage... Comms are still not restored. So even if I manage to fix the poor voltage on socket 86 of the pcm relay, I doubt it will fix my no comm issue.

 

[bismic]

Sorry now for me being away for a while!

Have you resolved the ignition switch power issue?

If you do provide 12V power to the "downstream" side of fuse 22, that circuit goes quite a few places. A short anywhere in that circuit STILL might drag down voltage. At 9.5V the PCM starts getting weird.

Can you be assured that you actually have good battery voltage on PCM connector C1381a (left plug) pins 46 and 34? At least those pin numbers are what are shown for my 2006 - I do NOT have the 2003 wiring manual. The circuit number in my manual states "361" - the red wires going to the PCM from Fuse 22. If you have to, maybe cut these wires (assuming the '03's have the same pin numbers as my '06 does) and spllce in battery power directly. This is doing essentially what @Hydro mentioned earlier.

I sure hope it isn't model year differences in wiring that is making this so hard! I just have not been able to find a reasonably priced 2003 wiring manual.

The items connected to Circuit 361 are:
MAF
IPR
GPCM
Fan Clutch
EGR
Shorts anywhere in these component circuits could be biting you if you are NOT measuring battery voltage at the two PCM pins mentioned earlier.

Also a short in mid harness connector C1282 could pull down voltage.

If nothing pans out in all of that, I guess I would lean towards a bad PCM.

 

[spike817]

Sorry now for me being away for a while!

Have you resolved the ignition switch power issue?

If you do provide 12V power to the "downstream" side of fuse 22, that circuit goes quite a few places. A short anywhere in that circuit STILL might drag down voltage. At 9.5V the PCM starts getting weird.

Can you be assured that you actually have good battery voltage on PCM connector C1381a (left plug) pins 46 and 34? At least those pin numbers are what are shown for my 2006 - I do NOT have the 2003 wiring manual. The circuit number in my manual states "361" - the red wires going to the PCM from Fuse 22. If you have to, maybe cut these wires (assuming the '03's have the same pin numbers as my '06 does) and spllce in battery power directly. This is doing essentially what @Hydro mentioned earlier.

I sure hope it isn't model year differences in wiring that is making this so hard! I just have not been able to find a reasonably priced 2003 wiring manual.

The items connected to Circuit 361 are:
MAF
IPR
GPCM
Fan Clutch
EGR
Shorts anywhere in these component circuits could be biting you if you are NOT measuring battery voltage at the two PCM pins mentioned earlier.

Also a short in mid harness connector C1282 could pull down voltage.

If nothing pans out in all of that, I guess I would lean towards a bad PCM.

Haven't fixed the ignition switch power issue yet. Ordered a new ignition switch so we'll see if that resolves that issue when it arrives.

Can you be assured that you actually have good battery voltage on PCM connector C1381a (left plug) pins 46 and 34?

I think so yes.
So here's me shorting the PCM relay for context:

Here's the voltage I receive at pins 46 and 34 on C1381a:

(12.61V)

And here's me back probing it while connected for good measure:

Still no comms though. But I'm pretty confident in the voltage in circuit 361.

Going back to your troubleshooting guide, Bismic, the last loose end I want to look at before throwing in the towel on this PCM is the ISO Bus circuit. Which, apart from disconnecting the ABS module, we all haven't talked much about. This is the last thing I'd like to confirm with you and Hydro before I start a search for a new PCM.

And just to make sure... Is it worth it for me to resolve the low voltage on the ISO bus (i.e., Pin #7 of the DLC connector)? Or are we confident in saying that I should just be looking for a new PCM at this point?

For Hydro - this is the voltage I've read on pin #7 of the DLC connector for reference:

Verify 3.5 volts at pin #7 (ISO bus) of the DLC. The ABS, TBC, 4WD module, Parking Aid Module, the Restraints Control Module, and the Message Center are on the ISO Bus. The HEC is also connected to the SCP and UBP networks.

Pin #7 voltage: 1V key off; 1.26V key on.

More recently I've measured 1.11V on that circuit.

 

[bismic]

I am going to defer to @Hydro on that question. He truly is a network expert.

 

[bismic]

I will say that when I look at the manual section on network communications:
Under the ISO bus are listed:
Restraints Control Module (RCM)
Parking Aid Module (PAM)
4WD Module

and to add the rest ........

The CAN Bus list:
PCM
Antilock Brake Module, ABS
Instrument Cluster or Hybrid Electronic Cluster (IC, or HEC)
Trailer Brake Controller (TEBC)

Universal Power Bus:
Instrument Cluster
Vehicle Security Module (VSM)
Electronic Automatic Temperature Control (EATC)

 

[Hydro]

Really have not had a PCM that I can short out and test
I mean -- look lets short this customers PCM -- "oops, that let the magic smoke out "
"Hey Dude, ...we found a problem and you will need to buy a new PCM"

The PCM has internal regulated power for the communication
but does that share with some of the other power outputs ? -- who knows for sure

We know 5 volt regulated power and 12 volt "clean" power is used -- but there are likely more than one source for each
Raw power is input which varies quite a bit -- from starting voltage and charging voltage

The internals need stable power , so may run from yet another power tap internal

Disconnecting all of the external loads from the networks would be a sure test
or bench testing like I hinted at earlier

Then the alternative is a new PCM
thing is ...without proving that the rest of the system is good
you are gambling -- maybe you get lucky

 

[bismic]

Ignition switch power is fed to the PCM Power relay, and to the FICM.

PCM Power Relay 12V power is sent to two separate PCM connector pins (C1381a pins 46 and 34) - per my 2006 (from the Wiring Manual).

The same PCM Power relay output also goes directly to many critical sensors and actuators. We know what "havoc" can happen if one of these circuits shorts out and drops this 12V power voltage!!! (Fan Clutch, GPCM "VPower" {note - this is NOT the "VBatt" Power to the GPCM, the "VPower" is the control power}, EGR actuator, IPR, MAF).

The Transmission Range Sensor, TRS, is also reported to be on this same 12V PCM Power circuit, but that happens inside the PCM SOMEWHERE! This TRS power also feeds the Output Shaft Speed Sensor and the Transmission Speed Sensor Assembly.

The PC/ED manual talks about a "Buffered" 12V power, and there are a few 12V circuits labeled "Reference Voltage" and we KNOW that these circuits are in fact 12V and separate from the 5V Reference circuits. The TRS 12V power is labeled "Reference Voltage".

That is about all I know about the PCM power system architecture. I have asked a few experienced Techs if they had ever seen any details on the design of the PCM Power System, but none had.

At one time we had access to a Ford Engineer on one of the 6.0L forums. I sure wish we still did!!!