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 Old 07-26-2012, 08:21 PM   #1
 
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Default 0 Fuel Pressure/0.0 Volts HPFP Voltage...but I do have fuel pressure??

So my AP is reading that my car has 0 psi for HPFP Act Pressure...and 0.0 Volts for the HPFP Voltage...but I DO have fuel pressure. THe car drives fine in CL...and the second it goes OL it freaks out thinking there is no fuel pressure...and stops all power.

I tried replaced the fuel rail pressure sensor...but I still have the same issue.

Wierd thing is...when I disconect the pressure sensor, the HPFP Voltage jumps up to 5.0 V...shouldn't it be 0 still? Is there a short somewhere?
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 Old 07-27-2012, 10:22 AM   #2
 
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how do you know you have fuel pressure?

have you tested the harness for continuity?

checked grounds and fuses?
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 Old 07-27-2012, 10:57 AM   #3
 
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^this
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 Old 07-29-2012, 06:32 PM   #4
 
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Originally Posted by chuckms6 View Post
how do you know you have fuel pressure?

have you tested the harness for continuity?

checked grounds and fuses?
I know I have fuel pressure because the car would not be drivable with 0 psi. I have a siezed fuel pump before which gave me 50 psi and it was barely drivable. Now, when the car is in CL (when its not looking at fuel pressure) it drives perfectly normal. But when it goes into OL and starts looking at the FP...it freaks out thinking there is no pressure...when there is. Basically I know there is pressure because if it really was 0...the car would not run and/or it would run like complete ass.

How would you suggest I check the harness?

I'll have to check the fuses...should have been the first thing I did.
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 Old 07-29-2012, 06:41 PM   #5
 
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Originally Posted by AFcadet View Post
I know I have fuel pressure because the car would not be drivable with 0 psi. I have a siezed fuel pump before which gave me 50 psi and it was barely drivable. Now, when the car is in CL (when its not looking at fuel pressure) it drives perfectly normal. But when it goes into OL and starts looking at the FP...it freaks out thinking there is no pressure...when there is. Basically I know there is pressure because if it really was 0...the car would not run and/or it would run like complete ass.

How would you suggest I check the harness?

I'll have to check the fuses...should have been the first thing I did.
good point about the fuel pressure, it definitely sounds like a sensor problem.

to check the harness you will need to know the pin out and what the voltages should be at each pin, and where each pin leads to. if you donate or have access to a fsm you can access a wiring diagram that will help you out a lot.
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 Old 07-29-2012, 07:10 PM   #6
 
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Originally Posted by chuckms6 View Post
how do you know you have fuel pressure?

have you tested the harness for continuity?

checked grounds and fuses?
Originally Posted by chuckms6 View Post
good point about the fuel pressure, it definitely sounds like a sensor problem.

to check the harness you will need to know the pin out and what the voltages should be at each pin, and where each pin leads to. if you donate or have access to a fsm you can access a wiring diagram that will help you out a lot.
Yea Ill have to re-up my donation since it ran out...So when you say pin...you mean the wires? Its a 3 wire sensor...pretty sure its a resistor type...so it should be a 5 v source...a ground...and a wire for the actual readings. Sorry...I'm not good with electrical stuff...so if you could explain with more detail I'd appreciate it.
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 Old 07-29-2012, 07:33 PM   #7
 
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Originally Posted by AFcadet View Post
Yea Ill have to re-up my donation since it ran out...So when you say pin...you mean the wires? Its a 3 wire sensor...pretty sure its a resistor type...so it should be a 5 v source...a ground...and a wire for the actual readings. Sorry...I'm not good with electrical stuff...so if you could explain with more detail I'd appreciate it.
yea, basically youre just checking for continuity in the wires to eliminate the possibility of the sensor being ok and the wires being fucked. chances are your sensor is bad, but anything is possible.
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 Old 07-29-2012, 10:54 PM   #8
 
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Originally Posted by chuckms6 View Post
yea, basically youre just checking for continuity in the wires to eliminate the possibility of the sensor being ok and the wires being fucked. chances are your sensor is bad, but anything is possible.
Well I dont think it is the sensor...I bought a used CX7 fuel rail and swapped out the sensors and I still have the same problem. What would you suggest...test the voltage where the wire starts? Then test it again further down the "line"?

One thing that does confuse me, is when the sensor is plugged in, it reads 0.0 volts. When I unplug it, it jumps up to 5.0 volts. I know the sensor is a resistor type, so is this normal for the volts to jump up like that? I tried looking around online to see how this type of sensor works..but I couldn't find an answer.

Thanks man.
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 Old 07-29-2012, 11:06 PM   #9
 
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Does the sensor from a CX-7 match the sensor for an MS3?
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 Old 07-29-2012, 11:15 PM   #10
 
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Originally Posted by theurgy View Post
Does the sensor from a CX-7 match the sensor for an MS3?
I dont know if its written in writing anywhere...but the fuel rail I got looks completely identical down to piping/hoses coming off of it, to the Speed3/6 one. The sensor fit in the fuel rail perfectly and looks identical.
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 Old 07-29-2012, 11:17 PM   #11
 
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I would suggest looking up the pin-out for the ECU harness where the sensor wires end, and depending on how many wires run to the sensor, start bridging the wires (batt disconnected and ECU unplugged) at the ECU harness and run a continuity test where the sensor meets the harness.

You're essentially creating a simple circuit for each wire that runs from the sensor to the ECU checking for a break. That's where I would start.

If something is broken you could either try to find the break or (what I would do) build a new circuit back to the ECU and hopefully the harness that plugs into the ECU is not the problem.

Hopefully that made sense.
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 Old 07-29-2012, 11:17 PM   #12
 
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Originally Posted by AFcadet View Post
Well I dont think it is the sensor...I bought a used CX7 fuel rail and swapped out the sensors and I still have the same problem. What would you suggest...test the voltage where the wire starts? Then test it again further down the "line"?

One thing that does confuse me, is when the sensor is plugged in, it reads 0.0 volts. When I unplug it, it jumps up to 5.0 volts. I know the sensor is a resistor type, so is this normal for the volts to jump up like that? I tried looking around online to see how this type of sensor works..but I couldn't find an answer.

Thanks man.
i would double check the compatibility of that sensor. second i would find out if the sensor's resistance is directly or indirectly correlated to the pressure. finally i would unplug the sensor and test the resistance with the system pressurized. if there is no resistance or infinite resistance i would say the sensor is bad.
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 Old 07-30-2012, 12:23 PM   #13
 
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Originally Posted by radioflyer View Post
I would suggest looking up the pin-out for the ECU harness where the sensor wires end, and depending on how many wires run to the sensor, start bridging the wires (batt disconnected and ECU unplugged) at the ECU harness and run a continuity test where the sensor meets the harness.

You're essentially creating a simple circuit for each wire that runs from the sensor to the ECU checking for a break. That's where I would start.

If something is broken you could either try to find the break or (what I would do) build a new circuit back to the ECU and hopefully the harness that plugs into the ECU is not the problem.

Hopefully that made sense.
There's 3 wires. It should be a 5v source, ground, and sensor wire. When you say start bridging wires...do you mean run a seperate wire from the ecu harness to the sensor? Would I want to fully disconnect the wire in question? Or leave it connected (i.e. wont that make it a parallel circuit)?

Originally Posted by chuckms6 View Post
i would double check the compatibility of that sensor. second i would find out if the sensor's resistance is directly or indirectly correlated to the pressure. finally i would unplug the sensor and test the resistance with the system pressurized. if there is no resistance or infinite resistance i would say the sensor is bad.
Thats a good idea about testing the sensor with the system pressurized. If I had a multimeter, would I just touch the 5v source wire and the sensor wire?
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 Old 07-30-2012, 12:38 PM   #14
 
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Originally Posted by AFcadet View Post


Thats a good idea about testing the sensor with the system pressurized. If I had a multimeter, would I just touch the 5v source wire and the sensor wire?
do a voltage drop from the source wire to a good ground, doesnt have to be the sensor wire. you should have anything except 0 or 5 volts from the sensor
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 Old 07-30-2012, 01:19 PM   #15
 
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Originally Posted by chuckms6 View Post
do a voltage drop from the source wire to a good ground, doesnt have to be the sensor wire. you should have anything except 0 or 5 volts from the sensor
Ok got a little confused. So touch one of the multimeter prongs to any good ground, then he other to the 5v source plug on the sensor?
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 Old 07-30-2012, 01:31 PM   #16
 
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Originally Posted by AFcadet View Post
Ok got a little confused. So touch one of the multimeter prongs to any good ground, then he other to the 5v source plug on the sensor?
yup. that will tell you the voltage in the circuit at that point.
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 Old 07-30-2012, 01:45 PM   #17
 
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Originally Posted by chuckms6 View Post
yup. that will tell you the voltage in the circuit at that point.
Either I'm doing something wrong or shit is all fucked up lol. I tried doing that...and it is still reading "1" on the multimeter. I then unplugged the sensor, and with the car on...and the multimeter in the DCV setting...touched one of the prongs to the 5v source (orange wire), and the other prong to the metal body of the car...and it didn't read anything. Am I doing something wrong? Gotta go to work...but I'll look at it more after.
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 Old 07-30-2012, 01:50 PM   #18
 
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When I said bridge the connections I meant take like a paper clip or something similar and connect say the power and ground wires that lead into the ECU (assuming the ground routes back to the ECU) and using a multimeter to pass a current through the wires at the sensor harness. Essentially creating a circuit. You can do a continuity test using the ohm measurement feature. Simply put set the meter to measure resistance, touch the wires of the multimeter together and notice what the readout changes to, this will indicate when the circuit is complete. If you get the same reading when measuring the resistance of the circuit you just created, then you know the circuit is good. Then repeat with the sensor wire, reusing one of the wires that you just verified was working in the circuit.

EDIT: Unplug the ECU, sensor, and the battery for this test

This will only tell you if the wires themselves are in tact. Measuring the voltage of the sensor power supply wire will tell you if the ECU is putting out the correct voltage to the sensor if I understand how the sensor works correctly.
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 Old 07-30-2012, 02:03 PM   #19
 
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Ok...I tested the sensor itself again and got something to work (I had the wrong resistance setting on the multimeter before). I had the black prong on the multimeter on a ground, and whenever I touched the red prong of the multimeter to either the 5v source OR the signal connector on the sensor itself, I got a reading of something other then 0 or 1. When I touched it to the middle connector (ground) I got "1"...so I'm pretty sure thats saying the sensor is working. Looks now like it might be a wiring issue.

Where would you put this bridge? All the way down the circuit? Or test it at the ECU connector first, then again down by the pressure sensor to see if you get the same reading?

EDIT:

So I was testing the DC voltage from the ECU by touching the multimeter to to the 3 individual connectors on the pressure sensor wiring harness...and I was getting sporatic readings on all 3. There were between .2 and 1.5 volts on all 3. Which is wierd...because one of them should be a ground. And if I moved the harness around, sometimes the voltages would change...or completely go away. Not sure if thats because the wiring is bad close by and I was actually moving the wires...or because I was not getting a good multimeter connection.
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 Old 07-30-2012, 06:24 PM   #20
 
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What color are the 3 wires at the sensor?

Pics throughout this process will draw more interest by the way.
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 Old 07-31-2012, 01:00 AM   #21
 
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Rog...I'll get some tomorrow.
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 Old 08-01-2012, 10:13 AM   #22
 
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Sorry it took a while...we are flying nights the next 3 weeks and its thrown off my schedule quiet a bit. Here is the picture...looks like there is a orage wire, dark yellow, and light yellow with a green stripe.
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 Old 08-02-2012, 09:34 PM   #23
 
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Alright here with an update...I tested all the wires and they all seem good. I bridged them from the ECU harness (I found a diagram) and it looks like there is not a short.

But I'm still having the same problem. Looking in the service manual...it says that the next thing to do is replace the ECU...ugh.

I am confused about one thing...when the sensor is plugged in...it reads 0 volts. When I unplug it, the ECU reads 5.0 volts.

Is there any way someone can help me out and unplug their pressure sensor and turn the key to on...and look at the fuel pressure voltage (or actual pressure...it should read super high...like 2200 psi). I just want to see if it is normal for it to read 5.0 when it is unplugged....or if it should read 0. The sensor is pretty easy to get to...if you look under the far left side of the intake manifold...it is right underneith the intake for the far left cylinder.
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 Old 08-02-2012, 10:49 PM   #24
 
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i think your sensor is effed some kind of way. your readings are saying high resistance to me. do you have access to another sensor?
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 Old 08-03-2012, 12:12 AM   #25
 
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Originally Posted by chuckms6 View Post
i think your sensor is effed some kind of way. your readings are saying high resistance to me. do you have access to another sensor?
Yea I installed that other sensor and its the same thing. We even tested the sensor I pulled off and its showing a little bit of resistance...not 1 or 0 (wouldn't expect to see a lot since there is no pressure).

We tested all the wiring...its all good. And just now I unplugged the ECU harness, and did a continuity test between the 3 different wires directly from the ECU harness plug. So this tested the entire harness...all the connectors...and the pressure sensor itself.

Fuel pressure sensor and Fuel pressure sensor reference voltage I got "1"

Fuel pressure sensor and ground I got very close to 0

Fuel pressure sensor reference voltage and ground I got a number in between 1 and 0 (showing the resistance from the sensor that is installed in the pressurized fuel rail)



I tested the wiring in multiple other ways...and like I said...it seemed find. And the final test above, tested everything all the way up to the ECU. According to the Mazda manual...the next and final thing to do is replace the ECU.

So is there any ways someone can just unplug their fuel pressure sensor turn their key to ON and tell me if their fuel rail pressure voltage is 0 or 5?
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