IAC issues

[05xlt]

I have an 05 with the 4.0 and auto transmission. I recently swapped transmissions due to the old one losing reverse. Ever since I did the swap, when I turn the key on the idle air control valve pulses, the piston just bounces in and out really quickly, and the truck won’t start. I can’t figure out what in the world would cause it, and can’t find anything searching. Any ideas? Thanks In advance.

 

[RonD]

Welcome to TRS

Unhook either battery cable for 5 min, then reconnect, resets computer

Do you also hear a Relay Clicking?

IAC(idle air control) Valve is just a 12volt solenoid, it gets 12volts with key on, Computer(PCM) then Grounds it to open it all the way for start up
PCM then pulses the ground to lower IAC Valve voltage after startup to set 1,100rpm idle(cold start idle)
Its called PWM(pulse width modulation) control, and it pulse at least 20 times a second so not something you could hear at the IAC Valve

If IAC Valve is opening and closing rapidly with key on, then either the 12volt is the issue, or the PCM Ground is
Unplug it's 2 wire connector and test red wire for stable 12v to battery negative, Key ON
Then test the other wire for stable voltage to battery positive, the ground, should show about 10 to 11volts, not the full 12volts, that's normal
If both show stable voltage, no jumping around then IAC Valve is the issue, but very very odd coincidence
Only Motorcraft or Hitachi brand IAC valves should be used on Fords

The 12volts comes from PCM Relay in engine bay fuse box, thats why I asked about a Clicking Relay noise

The PCM has several(5 I think) external ground wires, not shared inside the PCM, so one loose ground wire could cause that symptom

 

[05xlt]

Welcome to TRS

Unhook either battery cable for 5 min, then reconnect, resets computer

Do you also hear a Relay Clicking?

IAC(idle air control) Valve is just a 12volt solenoid, it gets 12volts with key on, Computer(PCM) then Grounds it to open it all the way for start up
PCM then pulses the ground to lower IAC Valve voltage after startup to set 1,100rpm idle(cold start idle)
Its called PWM(pulse width modulation) control, and it pulse at least 20 times a second so not something you could hear at the IAC Valve

If IAC Valve is opening and closing rapidly with key on, then either the 12volt is the issue, or the PCM Ground is
Unplug it's 2 wire connector and test red wire for stable 12v to battery negative, Key ON
Then test the other wire for stable voltage to battery positive, the ground, should show about 10 to 11volts, not the full 12volts, that's normal
If both show stable voltage, no jumping around then IAC Valve is the issue, but very very odd coincidence
Only Motorcraft or Hitachi brand IAC valves should be used on Fords

The 12volts comes from PCM Relay in engine bay fuse box, thats why I asked about a Clicking Relay noise

The PCM has several(5 I think) external ground wires, not shared inside the PCM, so one loose ground wire could cause that symptom

Really appreciate the good info. I apologize for posting in the wrong space. Only have .05 volts from ground side of iac plug to the hot side of the battery.

 

[RonD]

OK, now try testing red wire to ground wire on IAC connector directly
Hook up meter first then turn key on
Should see 10-11volts

IAC valve is a Monitored system, meaning computer expects to "see" 12volts on its IAC ground wire with key on
So it may be turning off ground pulse if no 12v is seen with key on
Solenoid, relay or volt meter passes the 12v thru to its ground, so computer would "see" the 12v(key on) and activate the ground pulse

I assume you did see 12v on red wire?

IAC Valve still unplugged
If you have OHM Meter set it to lowest ohm setting, usually 200
Then test each connector/pin in IAC valve to its metal case, testing for internal short, 0 ohms means there is a short, should be no connection to metal case on either pin

Pin1------meter----------metal case, meter should not change, same for pin 2

 

[05xlt]

With the information you’ve given me it leads to a faulty ground, across the iac plug with key on I have .05. So I’m assuming there is a ground that potentially bolts to the bell housing or somewhere affiliated with taking the transmission out, since that’s the only thing I’ve done other than put a new battery in. Thank you for all the good information it helps a ton.

 

[RonD]

0.05volts DC?

Starter motor bolt usually has the trans/engine ground

But voltage should fluctuate if IAC solenoid is opening and closing like your said, so that's odd, at 0.05v it would just stay closed

 

[05xlt]

Yeah DC. I found an oxygen sensor end pinched between the bell housing and block that I missed , I’m thinking that’s causing my issue so I am going to pull the trans back and fix that up and check things again.