can someone explain this code to me
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Joined: Jun 2002
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From: ma
can someone explain this code to me
p0112
iat low input circuit
I replaced the sensor, light comes back on
took it to ford who had it two days. says it need a new pcm, but can't tell me why.
oh ya, 97 f-150, 160,000 miles, plain jain xl 5-speed.
It has some symptoms of a bad o2 sensor, bad mileage, rough idle., but could a 02 sensor somehow be linked to the iat?
any thought sscully?
thanks in advance
iat low input circuit
I replaced the sensor, light comes back on
took it to ford who had it two days. says it need a new pcm, but can't tell me why.
oh ya, 97 f-150, 160,000 miles, plain jain xl 5-speed.
It has some symptoms of a bad o2 sensor, bad mileage, rough idle., but could a 02 sensor somehow be linked to the iat?
any thought sscully?
thanks in advance
Technical Article Contributor
Joined: Jun 2002
Posts: 10,511
Likes: 10
From: Under the flightpath of old ORD 22R
The dealer can't say why it needs a new PCM ?
Strange, that would be they checked the sensor, and the harness per the service manual test procedures, and it shows a hard fault. That would be a simple answer to give, as the list is not that long to check on this one.
So is this still the same problem from JAN-03 ?
https://www.f150online.com/forums/sh...hreadid=104708
After you picked it up, you said the CEL went out, and hadn't come back on.
I would start to think if it is the same chain of events, and it just came back up, that it is the wiring harness, not the PCM, that might be causing this. Don't know how the dealer ruled out the wiring harness this time or last, but replacing it is the easy way.
The IAT has 2 wires on it.
The Gray wire is the wire to pin 39 on the PCM. This is via connector C120.
The Gray w/ Red stripe is the output voltage from the PCM for the IAT, TPS, DPFE, and a host of other sensors. This wire via 2 splices is connected to pin 91 on the PCM ( which for some reason is called signal return ?? ).
If you route new wires on the harness to the PCM, and splice the wire at both ends for the Gray wire to pin 39, ( removing the wire from the stock harness, and leave enough room for resplicing later ), and for the other, remove from the IAT side of the harness and tape off, and splice with a T-Tap into the circuit on pin 91.
This would tell you if there is a problem with the harness. Remember to leave enough room to put the stock harness back on later ( i.e. don't clip them too short at the PCM side or the IAT side ).
The other TEMPORARY way to test this is to put the IAT in the sun on a black floor mat or black top driveway, let it heat up a bit, and check the resistance. Get a resistor of this size from Radio Shack or a component shop in a 5 Watt size, and put it between pins 91 and 39, with the IAT unplugged. This is not for continual use, but would let you know if the IAT harness is bad. With the resistor across pins 91 & 39 ( IAT unplugged ) the voltage drop would be right there, and the temp would be constant. Use T-Taps to make leads long enough to get the resistor on the harness ( maybe even clipping the one for Pin 39 long enough to re-splice and solder it back on later ) so you can take it for a test drive with 2 constant speeds over 48 MPH to ensure that the drive cycles are completed for all.
I am suspecting the Wire to Pin 39 on the PCM, as if you had problems with the Pin 91 side, you would have severe drivability issues I would think.
The other SWAG ( read huge SWAG ) is that there is a problem at one of the splices from the Pin 91, and maybe the low voltage condition is due to another sensor like the DPFE or TPS bleeding off too much voltage ?
Come to think of it, maybe the IAT is not getting enough voltage from Pin 91 due to a splice or harness problem, and already has a low voltage condition to start out with, then the voltage drop over the IAT thermristor is way low on the pin 39 ?? Maybe checking the voltage at 91 with the Key on, and engine off, and then check for the same voltage at the IAT connector on the Gray w/ Red stripe wire ?
Sorry for the long ramblings, just tossing out all the things I would check. As Neal said in the last post, if you do need a new PCM, get it from Mike Troyer, not the local ripe off dealer.
Good luck, and let me know if you have any questions on this rather long posting.
Strange, that would be they checked the sensor, and the harness per the service manual test procedures, and it shows a hard fault. That would be a simple answer to give, as the list is not that long to check on this one.
So is this still the same problem from JAN-03 ?
https://www.f150online.com/forums/sh...hreadid=104708
After you picked it up, you said the CEL went out, and hadn't come back on.
I would start to think if it is the same chain of events, and it just came back up, that it is the wiring harness, not the PCM, that might be causing this. Don't know how the dealer ruled out the wiring harness this time or last, but replacing it is the easy way.
The IAT has 2 wires on it.
The Gray wire is the wire to pin 39 on the PCM. This is via connector C120.
The Gray w/ Red stripe is the output voltage from the PCM for the IAT, TPS, DPFE, and a host of other sensors. This wire via 2 splices is connected to pin 91 on the PCM ( which for some reason is called signal return ?? ).
If you route new wires on the harness to the PCM, and splice the wire at both ends for the Gray wire to pin 39, ( removing the wire from the stock harness, and leave enough room for resplicing later ), and for the other, remove from the IAT side of the harness and tape off, and splice with a T-Tap into the circuit on pin 91.
This would tell you if there is a problem with the harness. Remember to leave enough room to put the stock harness back on later ( i.e. don't clip them too short at the PCM side or the IAT side ).
The other TEMPORARY way to test this is to put the IAT in the sun on a black floor mat or black top driveway, let it heat up a bit, and check the resistance. Get a resistor of this size from Radio Shack or a component shop in a 5 Watt size, and put it between pins 91 and 39, with the IAT unplugged. This is not for continual use, but would let you know if the IAT harness is bad. With the resistor across pins 91 & 39 ( IAT unplugged ) the voltage drop would be right there, and the temp would be constant. Use T-Taps to make leads long enough to get the resistor on the harness ( maybe even clipping the one for Pin 39 long enough to re-splice and solder it back on later ) so you can take it for a test drive with 2 constant speeds over 48 MPH to ensure that the drive cycles are completed for all.
I am suspecting the Wire to Pin 39 on the PCM, as if you had problems with the Pin 91 side, you would have severe drivability issues I would think.
The other SWAG ( read huge SWAG ) is that there is a problem at one of the splices from the Pin 91, and maybe the low voltage condition is due to another sensor like the DPFE or TPS bleeding off too much voltage ?
Come to think of it, maybe the IAT is not getting enough voltage from Pin 91 due to a splice or harness problem, and already has a low voltage condition to start out with, then the voltage drop over the IAT thermristor is way low on the pin 39 ?? Maybe checking the voltage at 91 with the Key on, and engine off, and then check for the same voltage at the IAT connector on the Gray w/ Red stripe wire ?
Sorry for the long ramblings, just tossing out all the things I would check. As Neal said in the last post, if you do need a new PCM, get it from Mike Troyer, not the local ripe off dealer.
Good luck, and let me know if you have any questions on this rather long posting.
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Joined: Jun 2002
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From: ma
Thanks,
yes this is the same problem from jan 03. Gas mileage is killing me. I will try all of your suggestions this coming week. What are your thoughts on the o2 sensors? they are the originals. not connected to my problem in any way?
thanks again
Dewey
yes this is the same problem from jan 03. Gas mileage is killing me. I will try all of your suggestions this coming week. What are your thoughts on the o2 sensors? they are the originals. not connected to my problem in any way?
thanks again
Dewey
Technical Article Contributor
Joined: Jun 2002
Posts: 10,511
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From: Under the flightpath of old ORD 22R
The O2 sensors with 160K miles, yes those would need to be replaced, but under a scheduled maintance thing. They are not listed in the Ford schedule, but it has been noted a a normal maint item, like changing the fuel filter.
The more I look at it, I would guess it is the wire either from the splice to the IAT, or the return wire.
If it was a problem with splice S135 or S138, then there would be a whole list of things reporting errors.
This is the list of items I could come up with.
S135 ( this is the one that is connected to S138, which is connected to Pin #91 on the PCM )
- TPS
- DPFE
- HO2S #11
- HO2S #21
- IAT
- Cylinder Heat Temp Sensor
- S138
S138 Connections
- Transfer Case Speed Sensor
- Camshaft position sensor
- Fuel Tank Pressure Sensor
- S141
- output shaft speed sensor
- Turbine Shaft Speed Sensor
S141 Connections
- Digital Trans Range Sensor
- HO2S #12
- HO2S #22
- Transmission
The bad MPG might be from the O2 sensors, but not part of the DTC or problem. Best to up the money and replace them first. Normal maintenance items should be fixed prior to working on the problem. Replace them, reset the PCM with a battery disconect, and then get the code(S) and start on the problem. The IAT is kind of stand alone as far as feedback dependant on another item ( like say the DPFE or the O2 sensors ), so if the IAT is reporting the problem, chances are the IAT is the cause, but I would never say 100% on any of this. If the O2 sensor was causing the low voltage in to a device ( like the IAT, causing a low voltage over the IAT ) then I would also suspect that a second code would be set, and the DPFE would be having problems as well as the Cylinder Head temp, and TPS, which could be 2 or 3 more DTCs set, and other drivability problems aside form the rough idle, which without a DTC is usually indicitave of a dirty IAC.
Take the time on the O2 sensors first, then try the checking of the voltage at the splice and at the IAT for the input voltage before moving on to the "eBay performance chip" of the resistor over the IAT connections at the face of the PCM.
Good luck let us know what happens next.
The more I look at it, I would guess it is the wire either from the splice to the IAT, or the return wire.
If it was a problem with splice S135 or S138, then there would be a whole list of things reporting errors.
This is the list of items I could come up with.
S135 ( this is the one that is connected to S138, which is connected to Pin #91 on the PCM )
- TPS
- DPFE
- HO2S #11
- HO2S #21
- IAT
- Cylinder Heat Temp Sensor
- S138
S138 Connections
- Transfer Case Speed Sensor
- Camshaft position sensor
- Fuel Tank Pressure Sensor
- S141
- output shaft speed sensor
- Turbine Shaft Speed Sensor
S141 Connections
- Digital Trans Range Sensor
- HO2S #12
- HO2S #22
- Transmission
The bad MPG might be from the O2 sensors, but not part of the DTC or problem. Best to up the money and replace them first. Normal maintenance items should be fixed prior to working on the problem. Replace them, reset the PCM with a battery disconect, and then get the code(S) and start on the problem. The IAT is kind of stand alone as far as feedback dependant on another item ( like say the DPFE or the O2 sensors ), so if the IAT is reporting the problem, chances are the IAT is the cause, but I would never say 100% on any of this. If the O2 sensor was causing the low voltage in to a device ( like the IAT, causing a low voltage over the IAT ) then I would also suspect that a second code would be set, and the DPFE would be having problems as well as the Cylinder Head temp, and TPS, which could be 2 or 3 more DTCs set, and other drivability problems aside form the rough idle, which without a DTC is usually indicitave of a dirty IAC.
Take the time on the O2 sensors first, then try the checking of the voltage at the splice and at the IAT for the input voltage before moving on to the "eBay performance chip" of the resistor over the IAT connections at the face of the PCM.
Good luck let us know what happens next.
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Joined: Dec 1997
Posts: 9,417
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From: Windsor,Ontario,Canada
There is a wiggle test thet the dealer has to perform before replacing the PCM. If the other sensors are not setting codes then it is either the IAT sensor, the wires between splice 135 and pin 39 (circuit 259 & 743) or the PCM. I also, don't understand why pin 91 is called a return.
JMC
JMC