MAP sensor below 0 volts
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Fleet Mechanic
Joined: Jul 2004
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From: va
MAP sensor below 0 volts
87 f150 4.9 EFI . Got 5.5 volts at plug when unpluged. MAP sensor back probed, when I checked the middle wire blue to the black wire, I get a negative volts each time. With my cheap 
volt meter set on 10 dcv I put the poss. to the blue wire on the MAP and neg. to the black wire on MAP. Should get 5 volts or close right? My MAP sensor has three wires orange, blue and black wire, is black wire is ground and or SIG. Return?
volt meter set on 10 dcv I put the poss. to the blue wire on the MAP and neg. to the black wire on MAP. Should get 5 volts or close right? My MAP sensor has three wires orange, blue and black wire, is black wire is ground and or SIG. Return?
Elder User
Joined: Sep 2004
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The Ford MAP sensor does not work the same way as most of the other ones (TPS, IAT, CTS, etc.). It is also not the same as a GM MAP sensor.
The Ford MAP sensor sends out a variable frequency, not a variable voltage. You should see a square wave of about 100 hz on the signal line. You cannot measure this easily with a DC voltmeter. Since the output swings from VREF to ground, some voltmeters will average the high and the low, and give you about 2.5 volts. It will be +2.5 volts relative to ground, and -2.5 volts relative to VREF. This will be the same regardless of manifold vacuum.
I don't have the wire color codes handy. But you can figure it out as follows: disconnect the MAP sensor
probe the connector in the harness. Put the negative lead of your voltmeter on the battery ground post and probe with the positive lead.
Two wires will read Vref (about +5 volts)
The other wire will read about 0 volts. That's the sig-rtn (ground) wire.
Connect the sensor and backprobe.
One wire will be +5 volts, one will be some other voltage.
The +5 volt wire is VREF. the other is the sense line.
The Ford MAP sensor sends out a variable frequency, not a variable voltage. You should see a square wave of about 100 hz on the signal line. You cannot measure this easily with a DC voltmeter. Since the output swings from VREF to ground, some voltmeters will average the high and the low, and give you about 2.5 volts. It will be +2.5 volts relative to ground, and -2.5 volts relative to VREF. This will be the same regardless of manifold vacuum.
I don't have the wire color codes handy. But you can figure it out as follows: disconnect the MAP sensor
probe the connector in the harness. Put the negative lead of your voltmeter on the battery ground post and probe with the positive lead.
Two wires will read Vref (about +5 volts)
The other wire will read about 0 volts. That's the sig-rtn (ground) wire.
Connect the sensor and backprobe.
One wire will be +5 volts, one will be some other voltage.
The +5 volt wire is VREF. the other is the sense line.
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Fleet Mechanic
Joined: Jul 2004
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From: va
Map ground has voltage???
Thanks, I unplugged the MAP plug and got 7 volts to the black wire which is the Signal Return (ground) ?????, the middle blue wire the Map BP Signal i got 4 volts and the VRef. orange wire i got 5 volts. Is that correct to get voltage to ground, its the ground i checked in the wiring diagram in the Haines manual.
The middle wire BP Signal goes to the ECC. The orange wire VRef goes to the evp and tps sensors.
The middle wire BP Signal goes to the ECC. The orange wire VRef goes to the evp and tps sensors.
Lead Driver
Joined: Nov 2002
Posts: 9,555
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From: Concord, NC
Mike, it might be more beneficial to keep all these posts together rather than starting a new thread every time you update things. Here, we have the same question in three places, and it would be easier to reply and easier for you to see the responses if you just asked once. I posted up some things you can try in the other thread, just to make sure you see it.
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