Code P1504 IAC Circuit Fault. NEED HELP!

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Randy Bobandy

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I've managed to maintain and repair my 2001 Lincoln LS V8 to the point of well being, except this one issue. As stated in the title of this thread I'm getting the code P1504. My IAC valve is new (I bought a new one thinking that would fix the problem) but it is not receiving any power. I went ahead and bought a multi-meter to check for voltage coming from the wiring harness for the IAC. Behold! no voltage. So I am now confident that I've located the point of my problem, however I'm not confident in how to fix it.. What I really need is where to go from here. Are there any possible connections or wiring harnesses that could be loose? Is there a fuse or a relay for this IAC? I'm at my wits end and I've exhausted all of my options except for going to a mechanic. I figured I would try here first.

Thank you for your time and any information you have to offer.
 
How did you measure to determine "Behold! no voltage." ?

There are two wires to the IAC, a green/yellow wire and a brown wire. Measuring for a voltage between the two wires with the engine not running does not mean much.

Measure between ground (an unpainted metal part of the engine) and the green/yellow wire. It should have 12V or so when the key is turned to run.
If not, check fuse 105 under the hood. If it's not the fuse and if the fuse is getting power when the key is on, then trace the wiring between the fuse and the IAC. It starts out as a green/blue wire and goes to a splice where it connects to the green yellow wire as well as two green/red wires. The green/yellow wire goes to another splice where it splits into two green/yellow wires.

If all that is good, then trace the brown wire back to pin 9 of the PCM.
 
Connected to the IAC is a wire harness with two wires coming from it, a brown wire, and an orange wire. I tested the voltage on that harness with the key on and the engine off, as well as with the engine on. As a control I tested another wire harness of the same type, in the same manner and did get a reading.

Where is the PCM located?
 
Also, there doesn't seem to be a fuse 105 under the hood.. Is it possible that you got the wrong number? I appreciate your help, I'm just having some difficulty figuring this one out.
 
Fuse 105 is fuse #5 under the hood. (1xx = hood, 2xx = cabin, 4xx = trunk)
The PCM is behind the glove box. The wiring harness side connector for the PCM is on the engine side of the firewall in that area.

It sounds like you have a break somewhere between the fuse and the IAC connector.
 
Thanks for that information. I was just confused because going by a fuse diagram I found, fuse number 5 under the hood seems to be for the fuel injectors... I may be wrong though. I am fearing that i do have a break somewhere. In that case I may just take it to the shop. It isn't going to be fun pulling apart the wires and tracing them back. When you mentioned checking pin 9 on the PCM, is that something I can check from the engine side of the firewall, or do i have to take out the glove box?
 
Randy Bobandy said:
Thanks for that information. I was just confused because going by a fuse diagram I found, fuse number 5 under the hood seems to be for the fuel injectors...
Click to expand...

As with most fuses, it serves more than one circuit. (For example, it also feeds the fan speed actuator.)

Why are you wanting to check the PCM circuit when you have already found that the power feed to the IAC is bad? Sure, the PCM half of the circuit could be bad too, but you've already found something that is bad and will absolutely prevent the operation of the IAC (and cause the OBDII code) all by itself.

Tracing the wiring is time consuming (as in expensive if you pay for labor for someone else to do it), but it is not difficult.
 
I'll check the fuse, and the power running to that fuse. As for checking the PCM I guess I'm just not completely understanding how the IAC is wired. My thinking was that maybe the power was coming from the PCM and there could be a break between there and the IAC. If what you're implying is that the break is likely between that fuse and the IAC, and not likely to have anything to do with the PCM that makes my job easier.
 
Power to the IAC is as follows - The battery through a fuse, through a relay, through fuse 105, through a wire to the IAC. We know that power is getting to the fuse, because the engine couldn't run if it didn't. IF your measurement at the IAC connector is correct, then the fault has to be between the fuse and the connector. The PCM is not involved in that.

If you had found that power was getting to the IAC, then we would look at the PCM. The PCM controls the IAC by rapidly switching ground on and off to the other side of the IAC coil.
 
I just went back and checked the fuse, and where the fuse connects, both are in working order. I also rechecked the IAC connector, the way you suggested. checking the side with the orange wire, and using the engine as ground. So going by what you've said, we can be confident that the break must be between the fuse and IAC connector... It doesn't seem like the wires coming from the fuse box are easily accessed, you also mentioned in a previous comment that there are two splices along that path, those are the things that make this seem like a difficult job. Just looking at where the wires go, from the IAC, leads me to think I'm going to be doing a lot of digging to follow the wires.
 
I guess it's all in how you define difficult. I will agree that it involves a degree of effort and will take some time.

My gen I diagrams are not nearly as detailed as the gen II ones are, so I can only give general locations. The first splice is somewhere at the front on the passenger side. The second one is somewhere at the back of the engine compartment.
 
Thank you for all of the help. I haven't been able to find much information on this problem. Do you have a a link or PDF for the wiring diagram that is specific to my year and model? Or a place I could go to find one.
 
LS4me said:
Just curious. Can you use an aluminum engine block as a good ground (does the block even need to be grounded)? Wouldn't a part of the body be better?
Click to expand...

Aluminum conducts just fine, in fact better than steel. (Look at the power lines overhead.) Parts of the body can be better, but most of them are painted (making them very poor ground points). The engine block is grounded for the starter, the alternator, and the ignition (spark plugs). Note that the two noise suppression caps for the COPs ground through two of the intake manifold bolt (on the V8, at least).
 
I have an idea. Instead of searching for the short, or the break in the wire, couldn't I splice I new wire to it, from the MAF? That's also running from the same fuse, and should have the same power. I'm tempted to do this, but I'm not sure of that would make the code go away. If this fixes the power supply to the IAC would the code go away, even though I'm wiring it differently?
 
In order to test this, before I emplace it, would it be ok to run the engine with the MAF disconnected? I just want to do it for a moment to see if the power will transfer sufficiently and the IAC works.
 
joegr said:
Power to the IAC is as follows - The battery through a fuse, through a relay, through fuse 105, through a wire to the IAC. We know that power is getting to the fuse, because the engine couldn't run if it didn't. IF your measurement at the IAC connector is correct, then the fault has to be between the fuse and the connector. The PCM is not involved in that.

If you had found that power was getting to the IAC, then we would look at the PCM. The PCM controls the IAC by rapidly switching ground on and off to the other side of the IAC coil.
Click to expand...

How would you go about determining a PCM issue with this, or could the other wire be a factor?

With the key on run, I’m getting 12 volts at the plug to IAC on what is an orangish/red with yellow strip wire. Tried checking with engine running before it died and was still getting power to that wire.

BTW, fuse for the IAC on the 02 is “10” in the engine compartment. At least from what I found. Was getting power there first before checking at the plug.
 
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Is the PCM grounding the other wire to the IAC? What is the resistance of the IAC coil?
 
joegr said:
Is the PCM grounding the other wire to the IAC? What is the resistance of the IAC coil?
Click to expand...
Truthfully I’m a novice at electrical diagnostics. For testing if the PCM is grounding the brown wire, would it just be a continuity test between that wire and any ground (engine bolt etc)? Key on, or engine running?

I can test the IAC resistance but it is new (I replaced mine like the OP once I saw the code before realizing what the code really was).
 
Broseph said:
Truthfully I’m a novice at electrical diagnostics. For testing if the PCM is grounding the brown wire, would it just be a continuity test between that wire and any ground (engine bolt etc)? Key on, or engine running?

I can test the IAC resistance but it is new (I replaced mine like the OP once I saw the code before realizing what the code really was).
Click to expand...
No. The test is a voltage reading across the IAC.
 
joegr said:
No. The test is a voltage reading across the IAC.
Click to expand...
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?
 
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