Wiring Diagram For IAC 99 GT

Have you measured the voltage across the IAC? Have you used a test light across the IAC? The brightness of the test light will be a clue as to what duty cycle the IAC is seeing. For example a 50% duty cycle should have an effective voltage of 6 volts.

Note, some VOM meters are better able than others to read a duty cycle. The best way is with a digital VOM that is capable of reading direct duty cycle. But don't discount the "low tech" method of the plain old test light.

There are no fuses to check as you have already "proved" there is power for the IAC by confirming key on +12 volts at the iAC red wire.

The other way to test for a short to ground is the voltage drop method. Measure the voltage between the IAC signal return line and battery negative. IF there's a short to ground this will show up as a very low voltage reading.

In the save vein to test for an "open" signal return line will not read 12 volts across the IAC. In the case of an open signal return line the red wire will only show 12 volts between red and a chassis ground.

Another functional test that can be done is to ground the signal return line. This should force the IAC to 100% duty cycle. This should result in a high idle condition.

If you still feel the need to ring out the IAC signal return line it's the WH/LB wire going the PCM pin #83

IMO you may be "over thinking" this. Take a VOM. Set it to what will measure 12 volts DC. Test that it works on your battery. Using a small thin needle back probe the non Red wire of the IAC. Place the other lead on battery negative. Start the motor. Read the voltage from the VOM.

Some budget digital VOM's will not read accurately a pulsed DC circuit. Repeat this test with a test light that will "load" the circuit. You should see the "brightness" of the test light change in response to the changing duty cycle.

A test light can also be your friend with regards to checking to see IF the signal return line is shorted to ground. Put the test light pig tail on battery positive. Disconnect the IAC connector. Lightly probe the signal return line. IF it's ground faulted the test light will be bright.

If interested in getting a full copy of Ford service manual with wiring diagrams I maybe able to help. PM if interested.

OBTW, IF the IAC signal return line were shorted to ground, THEN the IAC duty percent would be 100%. I don't see this "causing" a start/stall condition. Seems more likely to cause a very high idle. But it seems easy enough to rule out or in. What happens when the IAC is electrical is disconnected? Does it still start and then stall?

Are you able to keep it running by feathering the throttle? If so, that shows that the motor isn't getting enough air to idle.

EDIT: recently after doing some work on my 2000 GT, I "forgot" to reconnect the IAC electrical connector. The results were a crank, start, stall. It would run if I feather the throttle. Since my car ran fine before I started the work I figured it HAD to be something that I did. Double checking my work found the disconnected IAC electrical line.

Based upon my experience it might be more plausible to have an "open" IAC signal return line. But again this would show up with low tech "test light" and VOM tests at the IAC. Further, your grounding the IAC signal return line should result in a high idle right after start up.

Onesick99GT said:

I tested this today. When we probed the return line (White wire) it lights up and there is a clicking noise coming from somewhere in the engine compartment or firewall. This was done with the key off.

Btw when I start my mustang it revs up around 2500 rpm then instantly drops to stall it. No hesitation in the idle just drops off back to zero. I set the idle screw according to the throttle body manufacturer standards. It states to turn it in until it almost touches then one full turn which opens the butterfly just enough to allow enough air to pass through it keeping it at idle.

Click to expand...

The details matter here. Is the IAC electrical line connected? Is the motor running?

If the IAC is connected and the motor is off what is likely happening is that the test light is back feeding the motor's 12 VPWR circuit through the IAC. The clicking you are hearing is the other sensors and solenoids on the motor being powered up via the test light. The test light lights because of the draw of all the other devices. If this is true then the test results don't tell us anything.

IF your theory is there's a ground fault on the IAC signal return line (white), then the test needs to be done with the IAC connector disconnected and the test light pig tail on battery positive. But wait! How could this test give a false positive? What role is the PCM playing? We don't know what the default action of the PCM is with regards to the IAC signal return line (is it open or closed)?

It seems to me that more information can be had by disconnecting the IAC connector and placing the test light across the IAC body connector. In this way the test light takes the place of the IAC. Note the brightness of the test light at key on motor off. Now crank the motor holding the throttle partway open. Exactly what does the test light do?

The goal here is to determine once and for all IF the PCM does indeed have control over the IAC valve. For this to be the case we should expect to "see" the voltage and duty cycle at the IAC change. This changing voltage should be reflected in the "brightness" of the test light.

Real world experience story: I just did a quick T-stat repair on my Wife's Town Car. Went to start it and it started easily, revved up and then shut down (sound familiar?). Well the car ran fine before I "repaired" it. So I knew it had to be something that I did or overlooked. A visual inspection showed the MAF connected but a intake tube breather line was disconnected. This was allowing un-metered air into the motor bypassing the MAF. The motor stalled when the PCM when from "open loop" mode to "closed loop" mode. Because of the intake leak the MAF flow was soooooooo low that the amount of fuel the PCM was injecting was too low to allow the motor to run.

After re-connecting the breather line the motor started and ran fine.

Given the information in your previous threads an intake vacuum leak is suspected. Do you see HOW this could "cause" the start/rev/stall symptom?

Here's where an ODB2 scanner can be your friend. What if one were to graph:

• MAF flow
• RPM's
• fuel pressure
• PCM mode (open or closed loop)

I would be watching what the MAF and fuel pressure is doing right at start up. I would want to know IF the motor is quitting right when the PCM mode is changing from open loop to closed loop.

Also would want to know if the fuel pressure is a stable 40 PSI during the cranking/start/stall sequence.

I would then repeat the test with the MAF disconnected. What this should do is force the PCM to remain in open loop. What does the motor do now?

The amount of information that a $30 USB dongle can provide has to be seen to be understood. If given a chance an ODB2 scanner can transform the way trouble shooting is done.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/