2003 Edge Idle Surges and Stalls with AC ON

Rangstang · Aug 16, 2018

This started about 3 months ago. No CEL, but when truck is at IDLE with AC ON, it will start to drop and then surge after 10 seconds or so and progressively the idle will go lower each cycle and eventually stalls.

I have not changed any parts as of this point. Any ideas?

RonD · Aug 16, 2018

IAC(idle air control) valve would be my first stop, it can be cleaned

Computer uses IAC Valve to set idle RPMs, can't use an idle screw with fuel injection.
So computer can adjust this to set cold engine RPMs, 1,100, and then slowly reduce RPMs as engine warms up, to target idle, 650 manual trans, 750 automatic

Computer should also bump up RPMs by 50 or so when an automatic is put into gear
And also when AC is turned on

In 2003 computer also runs the AC, it gets a "demand signal" from the cab controller, it then turns on AC Compressor, so if compressor comes on computer should be bumping up the idle

When you have IAC Valve off, plug in its 2 wire connector
Turn on the key, engine off
IAC Valve should be open all the way for starting
Unplug the 2 wire connector, IAC Valve should close all the way, a spring pushes it closed
Plug it back in and it should open all the way again
Repeat until you are satisfied it is not sticking

Rangstang · Aug 17, 2018

Thanks Ron, very helpful info. I'll give this a shot. I've had IAC problems on Mustangs in my past and considered this a possibility, but it's the opposite problem in that the idle on the Mustangs came down really slow. I never thought of checking it that way before. Great idea!

This Ranger is a manual so I guess it only comes up when the AC kicks on.

Rangstang · Aug 19, 2018

Ron,
Looking inside the IAC, it appears to me that someone has been here before as there is some scoring on the inside of the housing.

If the IAC valve should open with the key ON, engine OFF and close when unplugged, it's either bad or no signal as it doesn't move at all.

Additional info: After reading a bit online, I tested resistance from pin to pin unplugged and I get 10.5 ohms both ways. Also checked from each pin to body and got infinite resistance. With connector unplugged and key ON, I got +12.3V between the contacts and lastly I tapped on the housing to see if it would get movement when I plugged in and unplugged with key ON and no response.

No power

Key ON, engine OFF

RonD · Aug 19, 2018

Good testing

Yes the Red wire at the IAC Valve should have Battery Voltage(12.X) with key on.
The other wire is the Ground but from the Computer, computer "pulses" the ground on this wire to control the voltage at the IAC Valves solenoid.
With key on engine off there is full ground on that wire so your voltage checks out.

Never tested OHMs on IAC's solenoid, so can't say what it should be, but.........it IS just a 12v solenoid so if you apply 12v and a ground it should move, there is no Polarity, so no 12v and ground side/pin, if it doesn't move then replace it

Rangstang · Aug 19, 2018

RonD said:
Good testing
Never tested OHMs on IAC's solenoid, so can't say what it should be, but.........it IS just a 12v solenoid so if you apply 12v and a ground it should move, there is no Polarity, so no 12v and ground side/pin, if it doesn't move then replace it

Boy Ron, you are the man! Always helping me out.

I had a spare IAC in my parts stash that I removed from another Ranger that just had the engine rebuilt when I bought it years ago. It literally looks new inside and is super clean. Got the same resistance as the other, but I plugged it in to the connector with key ON engine OFF and again, no movement at all. Should these be opening all the way with +12V and GND that's supplied from the PCM? Is all the way only like 1/8" or is it like 1/4"?

So it sounds like I have two bad IAC valves?

RonD · Aug 19, 2018

Yes, should open fully with 12v and ground, I would test both with vehicles battery, as said there is no polarity so + and - don't matter as long as there is both

Inside the IAC is a spring that holds the the valve closed, see if you can move the valve with a screwdriver, won't hurt anything, maybe both are jammed???

There is a coil of wire inside that becomes an electro-magnet, same as a relay works, when 12v and ground are applied, this "magnet" pulls the valve open against the spring pressure, if voltage is varied, i.e. 8volts then spring gets the upper hand a bit and pushes valve closed a bit.
Computer pulses ground to vary the voltage and this is a very precise way to control this valve, when working it will be +/- 5rpm at most, +/- 3RPM is usual
Computer "Learns" IAC Valve voltage settings which is stored in KAM(keep alive memory), if battery is unhooked KAM is cleared, like Pre-sets in Radio, so computer will re-learn IAC settings, this will cause wandering idle for the first few drive cycles after battery is reconnected, or if new IAC Valve is installed

Rangstang · Aug 19, 2018

RonD said:
Yes, should open fully with 12v and ground, I would test both with vehicles battery, as said there is no polarity so + and - don't matter as long as there is both

Okay, the spare part with +12V and GND applied gets around 3/32" of movement total and the one from the truck gets aroun 1/16" of movement. If they should get more than that, I guess they are both bad. I will say that by pressing on each with a screwdriver I can get a smidgen more of movement, but still not what I would consider "fully" open. I do find it odd that neither would move at all with key ON engine OFF and I verified +12V from pin to pin...

Your money still on them both being bad?

RonD · Aug 19, 2018

Open IAC Valve should put engine at about 2,000RPM, which requires a bit of air flow, obviously it would depend on the tube diameter of IAC Valves opening.

Yes, not sure why IAC is not responding to wiring harness 12v and Ground????
That part is very odd

Try testing voltage with one of the IAC Valves hooked up, could be draw of device will lower voltage, so 12v is a phantom voltage, no amps
But same voltage/wire runs to fuel injectors

Rangstang · Aug 20, 2018

Using my friend's Ford break out box, I checked PCM pin 83 (sink GND from PCM) to 97 (voltage across IAC on a 98 Ranger EVTM) with key ON engine OFF and sure enough it looks like it could be a phantom GND. I also verified using a Chilton online schematic to verify that the 2003 Ranger 3.0 is still pins 83 to 97.

When I check pin 97 (PCM Power relay +12V) to battery GND, I see the +12V. Either looks like I have a signal problem on the part of the PCM or a GND is not supposed to be supplied with key ON engine OFF and it only pulses when running.

Thoughts?

RonD · Aug 20, 2018

Yes, pin 83 at PCM is the Ground for IAC valve, it is also the monitor circuit for IAC wiring
PCM expects to see 12v at pin 83 with key on, or it will set P0505, IAC system malfunction

Pins 71 and 97 at the PCM get 12v with key on from PCM Fuse, via PCM Relay
IAC Valve also gets 12v from PCM Fuse and PCM relay, as does MAF, Coil pack and Fuel injectors.
They share the 12v but no connection except for the common wiring between IAC and PCM 12v

PCM pins 24, 25, 51, 76, 77, and 103 are PCM Grounds, they are not "shared", each is for specific circuits
Make sure all are good grounds

Rangstang · Aug 21, 2018

RonD said:
Yes, pin 83 at PCM is the Ground for IAC valve, it is also the monitor circuit for IAC wiring
PCM expects to see 12v at pin 83 with key on, or it will set P0505, IAC system malfunction

Pins 71 and 97 at the PCM get 12v with key on from PCM Fuse, via PCM Relay
IAC Valve also gets 12v from PCM Fuse and PCM relay, as does MAF, Coil pack and Fuel injectors.
They share the 12v but no connection except for the common wiring between IAC and PCM 12v

PCM pins 24, 25, 51, 76, 77, and 103 are PCM Grounds, they are not "shared", each is for specific circuits
Make sure all are good grounds

Put the breakout on the truck again.

With DMM - on battery negative, I saw +12V on both pins 71 and 97 with key ON engine OFF. Also saw +12V on pin 83, so maybe it's got a pull up resistor on it?

With DMM + on battery positive, I saw GND on pins 24, 25, 51, 76, 77, and 103 with key ON engine OFF.

Are you absolutely sure that I should be getting a GND on 83 with key ON engine OFF? I'm thinking it would be best if I could check out the signal when running. That being said, I just ordered a Hantek o-scope kit to interface with my laptop so I can check signals next week.

Rangstang · Sep 23, 2018

Rangstang said:
That being said, I just ordered a Hantek o-scope kit to interface with my laptop so I can check signals next week.

Here are the signals at idle with a cold engine, but like 85 degrees outside. I am a bit confused by the 10Vpp signal centered at 0V on pin 83, but I'm guessing it has something to do with not grounding the probe. I was in direct sun and could hardly see the laptop display at all and the battery lasts like 15 minutes, so I was in a hurry. As you can see, pins 71 and 97 are ground with a little noise.

I imagine that everything here is normal, so I go with a new idle air bypass at this point?

Pin 83

Pins 71 and 97

2003 Edge Idle Surges and Stalls with AC ON

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