IAC testing help - low/erratic idle. engine dies. PCM not sending signal?

[Dustin_89BII]

Hi folks. Just rebuilt the 2.9 from my 1989 b2 4x4 and fired up. Still having a low and erratic idle. Got koeo codes 12 & 13 at one point.

I am going through the diagnostics for the iac. Here what I’ve done so far:

1) multimeter test of iac unit : 10.7 ohms. (7-13 is spec).

2) multimeter test of iac connector : 11.58 volts with key on engine off. (~12 is spec, of course).

3) Test of iac unit connected straight to battery to test function of “plunger solenoid” or whatever it’s called. It jumps about 3/8” when power goes to it. (From what I’ve read, that’s the travel to expect).

4) koeo and iac detached from plenum. I plugged connector into iac — no visible change/movement in the solenoid/plunger.

Shouldn't the plunger move if I plug in the iac unit with the key on engine off? (Cold engine) .

Do these tests imply that there’s some kind of issue with the EEC/PCM not sending a signal? (Pin 21 gry/wht wire).

Do I need to test continuity on that wire? I’ve never gotten into the PCM, and don’t know how to do that. Any assistance on this would be great.

Thanks!

 

[RonD]

There is no IAC Valve "signal", its just voltage

The IAC Valve has a 12volt wire, 14v engine on, that's on with key on
The IAC Valve's Ground wire comes from the PCM
The PCM uses PWM(pulse width modulation) to lessen ground potential on that wire, which, in effect, lowers the voltage the IAC Valves solenoid has.

PWM is used on many systems to control a solenoid valves "open" status", i.e. EGR solenoid, EVAP solenoid, anytime you don't want just open and closed, PWM is used

Day light running head lights also use PWM to run the head lights at 60-70% brightness, but it uses the 12volt side instead of the ground side

11 to 12v key on engine off is normal, but it would be Battery Voltage specific, i.e. if you have a new battery with 12.8v you would see higher voltage than an older battery with 12.3v

You can tap the 2 wires with engine running and should see 7 to 8volts, but this is IAC Valve specific(and alternator voltage specific), the PCM "learns" what voltage sets what RPM on this one system, so not universal
And PCM varies the voltage constantly to keep RPM stable
Also higher voltage when engine is cold to set the higher idle a cold engine needs

Motorcraft and Hitachi brands are the only True Solenoid IAC Valves
Other brands use Dual solenoid operating system
They can use PWM but also have Digital stepper motor control, this does use a "signal", a voltage signal
Using these on a Ranger that just has PWM can cause odd issues, as the 3rd Party IAC Valve can wrongly interpret PWM as digital signal so responds oddly

 

[Dustin_89BII]

Hi Ron!
Thanks for the reply. So I’m terms of trying to fix the issue, what do you think I should do next?
It sounds like I need a Motorcraft or hitachi iac— I definitely have a cheaper one that Must have the dual solenoid.

To clarify, based on my tests already, should the IAC open when I plug the connector into it with key on like in test 4 above? If it doesn’t open, what are the reasons why? Computer issue, connection of the connector itself, like broken pins, etc., or an iac that isn’t the correct singe solenoid?

thanks!

 

[RonD]

I can't speak to 3rd party IACs as far as what they should do when sent PWM
They for sure should open 3/8" with direct battery voltage, but thats not what you get via the 2 IAC Valve wires, it doesn't have a full Ground

Wrecking yards are full of Motorcraft IAC Valves, Ford only had a few models, it was easier to have throttle bodies machined to fit current IAC Valve models than to make new IAC Valves for different engines, lol

An old Motorcraft IAC valve will be better than any new 3rd party IAC valve

Also pull the computer and have a look at the circuit board, there are 2 or 3 barrel Capacitors that will leak and corrode, and will cause odd issues like poor idle and stalling
Under $5 to fix

 

[Dustin_89BII]

Ok thank you! I’ll check it out. Happy 4th!