Please help this idiot!!!
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Junior User
Joined: Dec 2005
Posts: 50
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Please help this idiot!!!
soo..today i do an oil change and i check what trouble codes i have on my 87 f250 5.0l efi and i have a power steering switch open circut code that ive had forever....so i come up with this brilliant idea to run a power wire straight from the battery to the power steering switch....the engine stalls and wont start and idle on its own now as if the IAC was shot....also when i hook up the cables to connect to my obd scanner if the engine is running the engine dies and will not start...with the cables connected to the scanner...if i disconnect it will start but runs like *****.. so i took it for a ride after and its bogging out ,smoking, and when i fully depress the throttle it has absoultely no power...... please help!!! i dont know where to start!!! id appricate any adivice...
Senior User
Joined: Mar 2009
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From: metro detroit area...
sounds like you fried something bro... there is a reason you have fuses and relays and things in the truck to limit the amount of current going to any one thing. As to what you fried, I have no idea, but would not recommend doing that again. All I can really say is wow... and damn... and sorry
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
You might should have thought about how the power steering pressure switch works before feeding it battery voltage. You likely shorted battery voltage through the signal return (computer 0V/ground) circuit, frying the computer. The 0V/signal return circuit is used by practically all engine sensors. With that circuit messed up, the computer does not have an accurate input from each of the sensors that use that circuit, which is why the engine is running so terribly.
There is likely an internal pull-up resistor on the PSPS signal wire to 5V so that when the switch is open there is a 5V signal on the PSPS wire and when the switch is closed between the signal wire and 0V, the signal drops to near zero. By attaching 12V, the pull-up resistor could be fried, or a resistor on the 0V circuit on the computer main board. The PSPS isn't really for anything other than to give the idle a small bump when there is load on the power steering pump, so if it doesn't work, it really isn't going to hurt anything. If you did want to try something, you should have jumpered between the two pins on the switch connector.
There is likely an internal pull-up resistor on the PSPS signal wire to 5V so that when the switch is open there is a 5V signal on the PSPS wire and when the switch is closed between the signal wire and 0V, the signal drops to near zero. By attaching 12V, the pull-up resistor could be fried, or a resistor on the 0V circuit on the computer main board. The PSPS isn't really for anything other than to give the idle a small bump when there is load on the power steering pump, so if it doesn't work, it really isn't going to hurt anything. If you did want to try something, you should have jumpered between the two pins on the switch connector.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
You will just have to look around for it.
It is a 620 ohm R-82 1 watt resister.
Here is a diagram, the PS input is pin J1-#24.
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It is a 620 ohm R-82 1 watt resister.
Here is a diagram, the PS input is pin J1-#24.
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Fleet Mechanic
Joined: Mar 2005
Posts: 1,603
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From: St Louis
I also think you will have to find a new ECU, if I remember right the computer uses 5volts for the sensor voltages. Putting 12v and that much available amperage may well have fried the circuits in your ECU. You are lucky just to even get it to run.
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Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
If you put 12V on the wire going to the computer the pull up resistor has battery voltage on the other end so it should be OK. However you may have took out the 6.2V zener diode and the main processor.
If you put 12V on the ground side wire this goes right to the battery NEG post after going through the computer plug. This could mean the wire could have open going to the computer (other sensors use this same wire), the pins in the plug or printed circuits going to the #40 & #60 pins. This also could have back feed the other sensors and took them out also or there circuits in the computer.
The PS sensor was more than likely OK in the first place. The reason you were getting a code from it is that you were not turning the wheel when you was to do so in the test.
If you put 12V on the ground side wire this goes right to the battery NEG post after going through the computer plug. This could mean the wire could have open going to the computer (other sensors use this same wire), the pins in the plug or printed circuits going to the #40 & #60 pins. This also could have back feed the other sensors and took them out also or there circuits in the computer.
The PS sensor was more than likely OK in the first place. The reason you were getting a code from it is that you were not turning the wheel when you was to do so in the test.
Lead Driver
Joined: Nov 2002
Posts: 9,557
Likes: 44
From: Concord, NC
Originally Posted by subford
If you put 12V on the ground side wire this goes right to the battery NEG post after going through the computer plug.
That's pretty cool that you have a circuit diagram for the computer. Where did you get it? I think that would be a pretty good resource to have.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
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From: Easton,Ks
IAT diagram below showing the Sig Return ground.
Email me if you would like the original and my enhanced drawing of the circuit board.
It is a 1987 drawing.
The IAT circuit:
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Junior User
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what would cause the engine to shut down after i plug in my obd scan tool?....the truck starts right up but idle surges real bad....but as soon as i connect the obd harness to the scan tool the engine dies??
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Junior User
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i tryed scanning with the engine off but the scan tool wont connect..but i removed the ecu and i found that the tag on it said 3.0l taurus/sable only? pat.3903856 pat.4116173 EEC-IV GN EFI-SD22B E7TF-12A650 ANIA *496*....im not sure if any of theses numbers mean anything but i removed the ecu cover and found a portion of the board burned..im trying attach pic's..