StangMuscle

Please! I really need some help troubleshooting a starting issue. I have a PO191 code and the fuel pump is not priming. I bought a used fuel pump with low miles and it's still not priming.Can someone please check their voltage reading at the fuel pump wiring harness!? (There is nothing on this online or in the manuals/diagrams) That would clear up whether or not I got a faulty pump or another issue.(possibly fpdm, which had some water inside the connectors)

With the key on! At my fuel pump wiring harness I have:
Pink/black- 12.07
Green/Purple- 10.76
Light blue/Yellow-? Idk what this wire is, I got 0.01 at most. (Please chime in if you have any info)

If anyone can please help me out by doing this I would be very grateful, as im sure others will be! The Po191 code is very common in our vehicles, this would be valuable information to me as well as others!

moosestang

I think the blue/yellow wire is for your gauge. I have a gt, plus gt500 pumps, so I can't really help on the voltages. I'd suspect the fpdm, I would have changed that before changing the pump. Actually I would have swapped the relay for the fuel pump with another relay in the under hood fuse box first.

StangMuscle

Yea, thats what someone suggested at autozone. I did swap out the relay a couple times, that worked once when I originally broke down.. then it stopped working again afterwards. I didn't suspect the fpdm at first bc it was providing voltage to the fuel pump through the black/pink wire. But when i discovered water in the fpdm.. that made me question whether it is the cause of my problem. Im going to see if I can test it out on another stang.

petrock

Have you hooked up a fuel pressure gauge to find out if the CEL is in fact a problem with the fuel pressure and not a problem with a sensor? You can rent a gauge from your local autoparts store.

I don't know about that specific sensor, but in general on a three wire sensor one is power, one is ground and one is the signal. From the readings you have gotten, I suspect the pink/black wire is power, Green/Purple is signal & light blue/yellow is ground. Which wire is which would also be indicated on the wiring diagram in the service manual.

You can verify the power wire by unplugging the sensor and testing each pin in the connector on the wiring harness. One wire should have power. That will be your power wire.

Either way, your voltage readings only tell part of the story. The readings you have already take are called a "voltage drop test". You need to also have the battery voltage in order to put your readings into perspective. The power wire should be within 0.5 volts of battery voltage. Any lower then that, and you have excessive resistance on the power wire between the battery and the sensor. Since you already got a reading of 12.07 volts, if your battery is between 12.07 & 12.57 volts with the key on, engine off then the power wire is fine.
The reading of 0.01 volts is normal for a ground wire. Any higher then 0.5 and it would indicate excessive resistance on the ground wire between the sensor & battery.
The signal wire is a little trickier to decipher. It depends on if a voltage closer to battery voltage means a reading of low or high pressure. Your service manual should have that info. If it means low, and the fuel pressure gauge also reads low, then the sensor is good. If it means high, and the fuel pressure is high, then the sensor is good.

Good luck...

StangMuscle

[QUOTE=petrock;8156504]Have you hooked up a fuel pressure gauge to find out if the CEL is in fact a problem with the fuel pressure and not a problem with a sensor? You can rent a gauge from your local autoparts store.

Well i figured I had no pressure in the lines because the pump isnt priming, and I had disconnected the fuel lines. That is still a good idea, and I do have one!

StangMuscle

@PETROCK

I was not able to find anything in the manual about wiring and voltages, there were several diagrams but none for the fuel pump/fpdm. I was able to test my pump today, and it does work, leading me to believe the fpdm is toast.. only other factors I can think of are a short, or the computer.

StangMuscle

I was able to test the pump by grounding the black wire, and providing 12v to the pink/black, and the green/purple wires. It immediately started pumping and it sounded normal. I believe Moosestang is right about the blue/yellow wire being for the fuel level.

moosestang

This is from the whipple install instructions. You have to pull the fuel sender wires out and plug them into the gt500 harness.

Release terminals for the LB/YE (pin 2) wire and the LG/VT (pin 4) wire from the connector housing. Do not remove terminal from wire. These are the fuel sender wires.

Pretty sure LB/YE stands for light blue/yellow.

petrock

Go to the front of the wiring diagram section of the service manual and re-read the "how to read a wiring diagram" section. The info is there, you just need to know how to read the diagram. Different manufacturers layout their wiring diagrams differently, so it is always a good idea to read the "How to read a wiring diagram" section of the manual, even if you are an experienced electrician. From my wifey's '98 Mustang service manual, power starts at the top of the page and flows down to the ground at the bottom of the page. Think of electricity flowing down the page like water. It will always take the path of least resistance & will flow down hill (and occasionally horizontally). Also, it is usually assumed that the power feed will be battery voltage (around 12 volts) unless otherwise noted for that circuit. On most vehicles, its either 12 volts or 5 voltes. There are a few circuits that can spike well over 100 volts (e.g. fuel injectors & spark plugs), and some that are less then 1 volt (e.g. O2 sensor signal wires). So you need to keep them in mind when poking around an electrical system. If your not sure what voltage is flowing through a circuit, set your meter to the highest scale it provides and take your reading, then lower the scale without going under the reading to increase the accuracy of the reading. What voltages to expect out of a signal wire (if there is one) is usually described in the description/operation section of that sensor as a min/max range.


If you got a factory service manual, and not some crappy haynes or chiltons manual, all the questions you have asked can be answered within the manual and by applying a basic understanding of how electricity & voltage drop works.