Code P1504 IAC Circuit Fault. NEED HELP!

[joegr]

So test for any voltage on the other “brown” wire?

I guess I’m confused with how power is provided to the IAC that there would be voltage to both wires.

The yellow striped wire gets constant voltage, and the PCM controls voltage through the other wire to ground and shut off/close the IAC?

No.
The IAC is just a coil that opens a small air valve, and air from that opens a larger air valve.
The positive side of the coil always has +12V on it when the key is on.
The PCM very rapidly switches the negative side to ground. The more time it is grounded, the more open the valve becomes. It is closed when it has no power.
You measure voltage between the positive connection on the coil and the negative connection on the coil to see if the PCM is indeed ever driving it open. Measuring between the negative terminal and ground does not tell you much.

 

[Broseph]

No.
The IAC is just a coil that opens a small air valve, and air from that opens a larger air valve.
The positive side of the coil always has +12V on it when the key is on.
The PCM very rapidly switches the negative side to ground. The more time it is grounded, the more open the valve becomes. It is closed when it has no power.
You measure voltage between the positive connection on the coil and the negative connection on the coil to see if the PCM is indeed ever driving it open. Measuring between the negative terminal and ground does not tell you much.

This only works with the engine running correct? If so, means I’ll need a second body or hold the throttle lever open to keep it running.

 

[Broseph]

Checked the IAC resistance even though it’s new just because (9.5-9.7).

Key on (engine not running) I was getting 10.8 volts between terminals. What would I be looking for with engine on? Voltage drop or fluctuation?

 

[joegr]

10.8 key on, engine not running is probably about right. How about with the engine actually running, foot off gas pedal (or at least RPM below 600)?

 

[Broseph]

10.8 key on, engine not running is probably about right. How about with the engine actually running, foot off gas pedal (or at least RPM below 600)?

Just got 12.7 as it was about stall around 600. I adjusted the throttle plate slightly to keep it from stalling to test. At ~800 it was 11.8. Not sure if the voltage is rpm dependent. Seems counterintuitive to me to have lower voltage at slightly higher RPM.

Either way it’s elevated when running.

 

[joegr]

Just got 12.7 as it was about stall around 600. I adjusted the throttle plate slightly to keep it from stalling to test. At ~800 it was 11.8. Not sure if the voltage is rpm dependent. Seems counterintuitive to me to have lower voltage at slightly higher RPM.

Either way it’s elevated when running.

Why is that counterintuitive?
The higher the voltage, the more open the IAC valve is.
The lower the RPM (below target idle speed), the more open the valve needs to be to speed up the engine.
All this seems to indicate that the circuit to drive the IAC is working. (The IAC was plugged in for those measurements, correct?)

Either your IAC is mechanically blocked or stuck, or you have some other problem.

 

[Broseph]

Why is that counterintuitive?
The higher the voltage, the more open the IAC valve is.
The lower the RPM (below target idle speed), the more open the valve needs to be to speed up the engine.
All this seems to indicate that the circuit to drive the IAC is working. (The IAC was plugged in for those measurements, correct?)

Either your IAC is mechanically blocked or stuck, or you have some other problem.

I was just thinking because battery voltage increase some with RPM increase. That makes sense now that you explained it.

The measurements were taken at the plug (disconnected from IAC)

Two IACs being faulty, I dunno.

 

[joegr]

I was just thinking because battery voltage increase some with RPM increase. That makes sense now that you explained it.

The measurements were taken at the plug (disconnected from IAC)

Two IACs being faulty, I dunno.

Those measurements are meaningless without the IAC connected. With no load (from the IAC), you really can't tell at all what the PCM is trying to do.

 

[Broseph]

Those measurements are meaningless without the IAC connected. With no load (from the IAC), you really can't tell at all what the PCM is trying to do.

Haha. Told you I’m not good at this. Thanks for helping me out. I’ll give it a go tomorrow.

With the plug connected, would the easiest place to get measurements be on the back of the plug by pulling out the weather-proof grommets?

 

[joegr]

I'm not sure about the easiest way. I would probably remove a little insulation from an open area on the wires, and then seal it back, maybe with some liquid electrical tape, then cover that with real electrical tape after it dries.

 

[Broseph]

Got both on hand already, thanks.

Is there a recommended RPM or something for setting the throttle lever stop? As mentioned I tightened the bolt to keep the throttle open a little more so the car wouldn’t stall (I marked the original spot, only took 3/4 turn.

Just curious, so I don’t set it too high to throw off the readings on the IAC.

 

[joegr]

I think that the service manual just says not to change it. When you get things fixed, try to set it back to where it was. My wild guess would be about 500 RPM fully warmed up.

 

[Broseph]

I'm not sure about the easiest way. I would probably remove a little insulation from an open area on the wires, and then seal it back, maybe with some liquid electrical tape, then cover that with real electrical tape after it dries.

So I said I'd test voltage on both wires going to IAC a day after posting. Here we are 3 months later.

I am now get 0 volts on either wire grounded or across both wires. I'll note I got some ghost reading of millivolts I think (my meter auto adjusts to millivolts if detected), not sure if that's PCM or just the meter acting up, it was 20 millivolts. I confirmed the meter works and I was getting power to the IAC fuse (#10) under the hood. Actually took the box apart and tested the other side of the fuse to be sure and was getting power there. So perhaps i was getting some last remnants of power as the wire was giving out 3 months ago, and now it's completely severed.

Since I have power at the fuse box, would it be as easy as splicing a new wire under there and splicing into the orange wire close to the IAC connector?

 

[joegr]

...Since I have power at the fuse box, would it be as easy as splicing a new wire under there and splicing into the orange wire close to the IAC connector?

It should be, as long as the open isn't right at the existing connector.

 

[Broseph]

It should be, as long as the open isn't right at the existing connector.

Thanks. You mentioned that wire powers other components? Wanted to make sure if I’m looking at a butt splice, or a Y splice by the fuse box. The box diagram says Fuse 10 for is for the IAC on 02s. Nothing else (i.e MAF) is listed for Fuse 10.

 

[Broseph]

She’s up and running (at idle)! Maybe there is a problem spot in the power wire to the IAC on the Gen I. I ran a splice at the fuse and by the IAC connector. Car immediately did its initial rev for 20 seconds on start up and settled at ~700 RPM on its own. No more feathering the throttle at stop lights.

Thanks all