Ford mondeo 2.0 diesel. 5 Volt reference issue

Hi.

This was first posted on facebook. So this is a copy paste one.

Hi Paul. Have a "quick" question for you. We have a car with 7 volts on the 5 volt reference for the egr position sensor. All the other sensors on this ref. Curcuit show the same voltage. I thought it was a bad pcm ground but it was not. So for the question. How do you measure if the elevated voltage comes from the pcm OR if it is bad wiring? The fault is periodic. It comes more often with vibration/bad roads.

Paul Danner: What are the sensor ground voltage levels when the reference circuit goes to 7v?

Me: Stays close to 0. The pcm has 4 external grounds the goes into 1 bolt to the frame. My first thought was bad ground since it was elevated above 5 volts. If it is shorted to voltage 7 volts is strange. How would you rule out the pcm? I thought about cutting the ref wire by the pcm and measure both ends, but is there a better way?

Paul: every single reference circuit that I've seen with higher than normal voltage was a bad pcm ground. That's not to say it couldn't be a pcm board issue or some type of external short to power. That's going to be tough to isolate since it's intermittent. Can you duplicate it my tapping on the PCM or by wiggling harnesses?
Anything more brother, this needs to go to my forum. I can't keep up here anymore.

Me: Thanks Paul. This helps a lot. Was leaning towards the pcm, but it is always a great aid to listen to your experience! Will test some more and post my findings on the forum.

Any input would be great. Will post an update as soon as there is one.

Hey Inge! Happy to see you again, sir

So it's an intermittent problem... I'm guessing you were able to catch it at one point, and saw the 7V on the 5V reference circuit?

Just thinking out loud here... You could try running redundant grounds on all four of those PCM grounds, and watch for the problem to reoccur. Problem is that it doesn't tell you where the problem was, or if there's more wiring damage in the same area

I could see a resistive short causing voltage to bleed into the 5V circuit, but pinning it down will be tough. Components with 5V AND 12V fed to them would be prime suspects. I remember getting my butt handed to me by an Isuzu that was shorting 12V in to the 5V reference circuit through green crusties at the EGR solenoid connector. Thing is, it only shorted when the PCM activated the EGR valve driving down the road.