MAP Sensor, very very strange!

[SebOber]

I have a 2001 Jeep Grand Cherokee 4.7L V8. It has been starting VERY hard, not dependent on the ambient or engine temperature. Finally the PCM threw out a P0108 which is MAP sensor.

I measured the feedback to the PCM and found no voltage across the sensor. So I took off the connector and measured positive on the connector to negative on the battery and found 5 volts. so i deduced that it was the ground, i proceeded to jump the negative on the connector to the negative on the battery.

Plugged the connector back in and the car started right up! I thought i had fixed it, not so, at this point the jeep idles normally for a few seconds then drops and cuts out.

So I measured the ground on the connector(where I had cut the ground to jump the negative) and the positive on the battery and got ~6.6V. Now, I would think that I would get a reading of 12V since I have positive straight to the battery.

So then i measure the voltage on the feedback wire again, and i have a reading of .01V constant!

So to get it to normal operation conditions(not stalling, but hard starting) I rewired back to normal.

I took off the connector and measured the voltage across from the PCM, I got .01V again! then i measured positive from the connector to negative on the battery and got 5.14V.

Then i measured from the negative on the connector to the positive on the battery. I got ~6.6V.

And now it runs how it did before I did any of this troubleshooting

I am so totally confused by this. I know that if the MAP is malfunctioning and providing feedback readings that are completely out of range the PCM will supply it's own "estimated" values. So I figure that is why I get a good start but bad idle when i rewired because the ground straight from the battery doesn't have enough impedence and therefore causes the sensor to completely fault. but, i get a reading of .01V on the feedback no matter how I am wired but when i go back to original wiring it runs "normally" again.

Any thoughts?!

[xj31]

Ok a couple of things.First of all the ground for the map sensor is inside th pcm.It's alos shred by a few other sensors like the oil pressure sensor and coolant sensor.So when you are measuring from the battery to the sensor ground,you are still going thru the other sensors.Sensor ground is not battery negative teminal.I don't know what you were expecting on the signal wire,but there is always going to be some voltage on it.
As for the hard start,the map sensor also measures barometric pressure when you turn the key on in the short time before you start the engine.It seems like that might be what the problem is.

[xj31]

Ok a couple of things.First of all the ground for the map sensor is inside th pcm.It's alos shred by a few other sensors like the oil pressure sensor and coolant sensor.So when you are measuring from the battery to the sensor ground,you are still going thru the other sensors.Sensor ground is not battery negative teminal.I don't know what you were expecting on the signal wire,but there is always going to be some voltage on it.
As for the hard start,the map sensor also measures barometric pressure when you turn the key on in the short time before you start the engine.It seems like that might be what the problem is.

[SebOber]

I figured out what the problem was. In short the PCM is bad. It is putting out a constant 5V on the signal wire. What i was doing wrong in the first place was that the signal wire and ground were on opposite pins, according to the Hanes. I tested the MAP's on a rig I made with a vacuum pump and a variable power supply. So now I have a new PCM on order but I'm worried about the keys. I don't know if I'm going to need to get them recoded.