2010 Ford Taurus SHO - Low 5V Reference Circuit low voltage was fixed but back

I could really use some guidance as I'm at my witts end...

I have a 2010 Ford Taurus SHO with a low 5V Reference Signal (P0642).
I first had this issue a couple months ago.
I ruled out the three sensors on this circuit as I still had <1V with all sensors unplugged.
I changed the computer but still had this issue (Doh!).
I then isolated the issue to the engine wiring harness but could not find the fault.
I took it to a shop to see if they could find the issue in the wiring but they couldnt and suggested to change the harness.

After the above, I sourced and replaced the top engine harness last week.
I turned the key on and hallelujah, the reference circuit was reading 5 volts again, it started but ran rough..
It was throwing a code for the MAP sensor and upon inspection I realized the shop replaced the MAP sensor with the wrong one (3bar vs 2bar)

I received the new MAP sensor and was disheartened to see the 5V circuit was reading low again (P6042)....about 1 volt and the fuel rail pressure sensor is showing 5 volts

Looking for some feedback or ideas.
For some reason, the fuel pressure sensor is pegged at 5 volts while the 5V Reference signal is low (1.8V) at KOEO.
I am sharing screenshots of the voltage readings before I replaced the harness, the good readings after replacing the harness, and again now.

Looking for any fresh ideas or if anyone can see the correlation between the pegged FP sensor voltage and the 5V Ref circuit.

Thanks for reading this and any suggestions!!




INFO:

Sensors connected to reference voltage A circuit:
Fuel Rail Pressure (FRP)
Manifold Absolute Pressure (MAP/IAT2)
Turbo Boost Pressure (TCBP/CACT)

That FRP may be a substituted value. Wouldn't worry too much about it at this point.

First thing I'd do is unplug the N.E.W. MAP sensor to see if that brings the Vref back up.

If the new MAP sensor is not the problem then consider the following:

Although labeled as independent V-ref circuits, there are systems out there sharing internally ,within the control unit, the 5V voltage source. If this applies in your case any of these paralell circuits may cause a problem and you would want to check them as well.

I visited the car again yesterday and am still at a loss...

The 5-VREF circuit is still showing around 1 volt....with all sensors unplugged....AND EVEN with the engine wiring harness unplugged from the ECM.
I even swapped the computer and no change in values!

So it seems I have the low reference voltage even with a different computer AND engine wiring harness.
What is really driving me nuts is why it was showing 5 volts the day I replaced the wiring harness but not anymore...even with it unplugged

I attached the wiring diagram. Can anyone see any reason why FRP sensor would be showing 5 volts (open?) at the same time I have the low voltage reference (even with wiring disconnected).

So frustrated

How do you have the wiring harness unplugged? You have a whole connector unplugged from the pcm? You need to plug it back into the pcm cut the 5 violet ref wire at pcm see if it returns. Then check power and grounds at pcm.

Cheryl wrote:

How do you have the wiring harness unplugged? You have a whole connector unplugged from the pcm? You need to plug it back into the pcm cut the 5 violet ref wire at pcm see if it returns. Then check power and grounds at pcm.

Yes the two connectors for the top engine wiring harness with the ref circuit is unplugged and still reading the same values from ECM (streaming values via bluetooth). I'll try to see if any independent grounds or power to check for ECM. It's just the other harness that runs to the interior.

With 2 main connectors being unplugged like that I’m assuming there’s probably powers and grounds on those connectors and your taking them away with them unplugged. But they might all be on the 1 connector left in place. How many vref wires you have coming from the pcm? 1 and it splices off to other sensors? Or multiple? It’s almost looking like a bad computer again. Did you get a used one??

Yeah I unplugged the two connectors that go to the engine wiring harness: C1381E & C1381T.

The computer still has power though the remaining connector C1381B
I was still connected via OBD, and see the VREF fluctuating as before around .8-1 volts during this time...

Nothing makes sense yet but I have a hard time believing both computers have the same problem. Both seem to work fine and the new one was rebuilt/guaranteed and didnt make any difference when I originally received it. Thats why I replaced the wiring harness.

If you had one of the power probe adapters that put out 5 volts then you could put 5 volts out to the sensors see if it is working properly.

the sensors are good but I did just order a 12v to 5v converter.
I might try cutting wire 35 (5v ref) and supply the circuit with actual 5volts so i can at least get the car moved. still stumped.

If it starts and move with supplying 5 volts to the sensors then the issue is the pcm. Load test power and grounds

One last thing to check. Check terminal drag on the 5 volt ref wire make sure it’s not spread and not making good pin contact

What is the voltage at pin 35 with connector c1381E disconnected KOEO. Use a volt meter not your scan tool.

Hey guys, I tested the reference voltage at pin 35 on the computer and it was the same...about 1 volt. What's interesting is the pin next to it is unused (pin 36). When I touched them together the ref voltage would stay at 10v and when I released the prob it would sit at 5 volts and drop. Very strange. This happens with both computers.

So I cut the 5 volt wire for the circuit and used a 12v>5v converter to supply the circuit. The car ran fine with that...so as expected no problems with the sensors or wiring harness.