I measured the resistance of the wiring back to the PCM at the Pressure Module Plug with the ignition off.

I can measure the pressure modules tomorrow but I am not sure that matters

I physically swapped the pressure modules. All pressure was relieved at this time. There was no change in the pressure at the OBD reader 75/40 after the swap but of course that doesn't matter the results are the same if the sensor is plugged in or not.



I unplugged the good pressure sensor to see what would happen, the car recognised that and I got a error on the dash that said check fuel system, Not the same for the other side. That side is saying (without a pressure sensor plugged in) there is 75 psi here. I think because the PCM is seeing 75 lbs of fuel pressure, cant control it, and says no no lets shut this side down till this is solved.

Next step I guess is to check wiring from the bad side to the PCM

The mystery continues

 

The wiring resistance numbers did you notice how low the resistance was on the control wire on the failed circuit vs the good circuit? Does that matter?

 

If that control wire is shorted or getting 5v from somewhere perhaps that's why i am getting the 75 psi reading.

I am going to go over the ODB scan to look for differences in the numbers from side to side regarding the fuel system.

I guess I need to trace down that plug to the correct PCM to test

 

Before getting to the PCM's I Re-did the resistance check with a decent meter and the results were repeated EXCEPT the 3 wire on the bad side.... it measures over 300 Ohms. This would be the wire that communicates pressure to the PCM. If resistance over this wire is used to control fuel pressure could be a problem?...any thoughts?

 

Just reading through this thread for some education, but that was my thought. If you get 5V on the pin 3 sensor reading wire without the sensor connected and you get 75psi reading from the PCM then it seems like somehow the sensor wire is getting 5V put on it. You got a resistance value that wasn't infinite between pins 1 & 3 which would seem to indicate they are connected somehow, but that could be through the PCM. An interesting test might be to unplug the harness from the PCM and then see if you still get a resistance reading between 1 & 3. You could also do that and test for 5v, but you probably don't want to be unplugging the PCM when it has power. Could visually follow the wiring from the sensor to the PCM to see if there's any obvious pinches or rubbing. Hope that helps.

David

 

Thanks!

 

The problem is the working pressure module circuit and the faulty one shows 5v from both power supply and modulation wires. In addition this test also shows all wires have continuity in both Pressure Sensor Modules circuits to the PCM. The fact is the PCM is reporting to the OBD and I assume the Fuel Pump Control Module 75 pounds to that rail. Some where in the code/logic , I assume again, this high pressure or pressure differential between the two rails is shutting down that side of the engine

 

Yeah, if both circuits behave the same then that would seem to indicate the wiring is ok. Does the PCM report 75psi on the good side when the sensor is unplugged like the bad side? If both sides input the same voltage to the PCM, but the PCM shows different readings then the PCM is looking suspect.

 

No if I unplug the good side I get a different code and a warning on the dash that says fuel system error. However I cant get pressure info because I can only read this with the engine running and it wont run with this disconnected.

The wiring is showing a major difference in resistance, really only to that control wire that is on the defective circuit. Resistance >300 Ohms which would be like adding a 100 watt light bulb in the circuit. Im thinking that is the problem.

Perhaps I should add a 300 ohm resister in the good circuit control wire and see what happens.


Hmmmm

 

For reference

 

Not yet was going to take today off but I got trumped. Wont be able to get to the computer till saturday morning.

The question is what do I do if the wire is bad. there are two outcomes at this point the way I see it.

Bad PCM which is replace or reflash.

Bad wire replace harness?

Can 1 wire be fished through the loom?

 

As to the PCM if it is defective I would love for you to give it a shot. How would I get it reprogramed?

 

Few questions

Do you know how these pressure sensors report back to the PCM. I show 5 volts from the PCM to the pressure sensor module. How does that work?

If I unplug from the PCM and have normal resistance over the suspect wire does that point to the PCM as bad?

The manual talks about checking this wire for "noise" does this resistance on this wire point to noise, if not how do you check for noise?

I had some A/C work done involving replacing the high pressure line and the dryer. Could the harness be damaged by this operation?

This is a standard Ford pressure sensor BTW so I assume this 5v control must be used elseware I will post the part number for the module...



Thanks.

 

Got this from another group


The sensor is of a variable pressure to resistance type
As fuel pressure increases, sensor resistance decreases, allowing system voltage to increase
When fuel pressure decreases, sensor resistance increases, causing system voltage to decrease
The PCM reads these variations in system voltage as fuel pressure and reacts accordingly; adjusting fuel injector pulse width, engine timing, and drivability strategy.



Make any sense?

 

I don't get how the sense pin is showing 5V without the sensor connected. Or why it would show 5V ever unless the fuel pressure was high. You say it happens on both sides, but still seems odd to me. I would expect there to be no voltage at all on the sense pin unless the sensor was connected and reading pressure.

If the wire is suspect then you could try running a temporary wire from the sensor to the PCM to see what happens. You'd either have to cut the wire at each end or figure out how to remove the pins, though.