AIH delete question

[mthisguy]

Does anyone know what the pcm is looking for? Is it looking at the resistance of the heater coil, or voltage from the relay, or both. The coil essentially shows a ground so if the pcm is just looking at that all a person has to do is ground the small wire thats attached to the solenoid. With a multimeter it shows .001 ohms basically a short so if thats what the pcm is looking for a resistor is not needed as long as you take that wire off the relay and ground it.

[QtrHorse]

You need to use a resistor but not exactly sure why. If it was this simple, people wouldn't have to buy the code eliminator.

[MrPlowJr_EZ_V]

Knew a guy who tried to ground the power lead to the heater, that didn't work out too well [img]/ubbthreads/images/graemlins/smokin.gif[/img]. but not sure on the small wire [img]/ubbthreads/images/graemlins/shrug.gif[/img].

[mthisguy]

I meant literally take the wire off the relay, not hook the hot wire to ground, that would be bad, the small wire on the relay is the wire the pcm is using to monitor that circuit, just dont know what its is looking for. Reading just the heater circuit will just show a ground because the heater coil has such low resitance, until it gets hot then the resistance will go up until it reaches a stable point where the current stops increasing. Basically all a heater coil is, is a short circuit.

[Larry M]

[ QUOTE ]
Does anyone know what the pcm is looking for? Is it looking at the resistance of the heater coil, or voltage from the relay, or both. The coil essentially shows a ground so if the pcm is just looking at that all a person has to do is ground the small wire thats attached to the solenoid. With a multimeter it shows .001 ohms basically a short so if thats what the pcm is looking for a resistor is not needed as long as you take that wire off the relay and ground it.

[/ QUOTE ]

A couple of comments I just measured my AIH out of the vehicle and it measured .2 to .3 ohms after I removed the reistance of the leads and even then the accuracy is +_ 1.2% plus 3 digits. My leads alone measured .4 ohms and the actual reading was .6 to .7 ohms so about all I can feel comfortable with is saying it's probably under .5 ohms.

I'm not aware of any DVM that can measure resistances now into the thousandth of an ohm. Now mine is an autoranging DVM and I can only get reading to the tenth of ohms. and it appears the high end flukes only take that down to the hundreths of an ohm. As you will see below simply measuring the resistance might not tell you much with the AIH in there and I would not ever recommend hooking any sort of "ohm" measuring device to things like the PCM unless you really know what you are doing.

Anyway you might find the following interesting or completely useless since it's a true WAG on my part ....

I will first say that I don't know exactly what sets the code, but I can give a way that it could be done. Realize that with the AIH relay open there are only to electrical paths so this following test would be one for the PCM to determine that the AIH element is not open or burned out. One is from the outlet of the AIH relay to the heater element to ground. Lets call that R1 and for a 58 amp draw at 12 volts lets just say its .2 is the value of R1. Now there is a second circuit from the outlet of the AIH relay to pin 3 of the PCM. Now I have no idea how it would do this but if somehow the PCM can measure the resistance at PIN 3 just like you would say hook up an DVM between pin 3 and ground that is basically measuring the resistance of the AIH. Stay with me here since doing this in words is tough. Lets just for purposed here that the PCM will look at that resistance and say everything is fine as long as the resistance is less that 175 ohms which is a made up figure so these next values along with the resistance we put in place of the AIH will work. Instead of measuring the resistance from PIN 3 on the PCM to ground lets put say a 200 ohm resistor between PIN 3 and ground and call that resistor R2. Now this circuit would only draw like 60 ma which shouldn't be a problem. Now because of what I call the parallel resistor forumla the new resistance between PIN 3 and ground is (R1xR2)/(R1+R2) or in this case (200x.2)/(200+.2) = 40/200.2 or just under .2 ohms and since that is less than our 175 ohm value I picked above then the PCM o.k. the AIH element is not burned out. If the AIH burns out then R1 is out of the ckt and the resistance rises to the value of R2 or 200 ohms and that exceeds the 175 trigger point that I picked above. Now let's do the AIH mod and remove that .2 ohm resistance of R1 and replace it with say a 500 ohm resistor and call that R3. Now by that parallel resistance forumla for the new resistance between PIN 3 and ground is (200x500)/(200+500) = 100,000/700 = 142.8 ohms which while it's risen a lot it's still under that 175 ohm figure.

You might be able to simply remove the AIH and that wire going to PIN 3 on the PCM, but about the only way to get that resistance at PIN 3 down below is to ground it and I would not recommend that since the PCM might well be applying a very small voltage to measure the resistance at PIN 3 expecting the path to go thru R1 and R2 and not directly to ground and you might burn something out inside the PCM effecting not only that circuit, but something totally unrelated.

I don't know exactly how the PCM checks for a ground in the ckt, but it should blow a fuse somewhere and when the AIH relay is closed then there will be NO VOLTAGE at PIN 3.

There are probably other ways to do this, and unless we know exactly what is inside that PCM that is hooked up electrically
PIN 3 we can only guess. I'm no electrical engineer, but there are probably ways with resistors, solid state devices to design a check procedure to determine the state of the AIH itself. My ohm method is just one if done that way illustrates why a code is set by just removing the AIH and not when you put a certain value resistor in there. For my example that added resistor has to be less than 1400 ohms.

I don't think there is a good answer to your question, but was only trying to give you a feel that there is probably a way for the PCM to test the condition of the AIH electrically via that wire going to the PCM.

Hopefully I didn't make a stupid math error somewhere since I have no one to check my work except you guys [img]/ubbthreads/images/graemlins/tongue.gif[/img]


Larry, keeper of the <font color="red"> New revised </font><font color="blue">"little secrets" </font> CLICK HERE [img]/ubbthreads/images/graemlins/smile.gif[/img]

[reg2862]

Just A thought why not take a spark plug wire cut to lenth to get the resistance you need, one end to relay the other to a gound

[gottarace]

I pulled mine about a month ago.
I just taped up the (RED)hot wire and zip tied her out of the way.
Tied up the ground also.
Seems to start better now. JMO.

[mthisguy]

Your correct my meter didnt measure .01 ohms but its limit of measurement was exceeded as far as its ability to measure low ohms, in other words a short to ground. If the pcm is reading the impedance of the heater coil then it has to be sending a voltage and comparing the voltage loss between the input and ground, thats exactly what a multimeter does in the first place, that how they measure ohms, parralel resistors are added to older style meters to change the scale.
If the pcm is reading voltage output from the relay then all thats necessary is to leave it hooked up to the relay. If the pcm is reading both the relay output and impedance of the heater coil then the code eliminator sold by the vendors is necessary. If I was an engineer desiging the circuit I would just look for a path to ground within certain set of ohms, just as You stated, and not look for voltage from the relay because that entails alot more circuitry to see both and not burn out, to read ohms and be protected from the voltage of the relay just takes a simple diode. Just the addition of that diode could account for the margin of error that would cause the pcm to accept a range of voltages say from 12v to 11v as acceptable, being the diode itself will cause a .3-.6v loss. Assuming 12v is directly to ground.
It sounds to me like someone just recreated a safe way to meet both conditions and is selling it and not really knowing if its even necessary. Dont take it as an attack or anything I just like to know why I should spend my money on something other than "they sell it and it worked for me". I currently dont have access to a code reader so I cant check and see what works and what doesnt. I have bought things from the sponsors so dont get me wrong Im not against them at all. But I dont like spending money on things or hooking up resistors from a 100amp relay to ground that have a possiblity of causing a fire, resistors can and do short out on occasion and catch on fire, without a good reason.

[TheHammer69]

[ QUOTE ]
Your correct my meter didnt measure .01 ohms but its limit of measurement was exceeded as far as its ability to measure low ohms, in other words a short to ground. If the pcm is reading the impedance of the heater coil then it has to be sending a voltage and comparing the voltage loss between the input and ground, thats exactly what a multimeter does in the first place, theres no other way to measure ohms. If the pcm is reading voltage output from the relay then all thats necessary is to leave it hooked up to the relay. If the pcm is reading both the relay output and impedance of the heater coil then the code eliminator sold by the vendors is necessary.
It sounds to me like someone just recreated a safe way to meet both conditions and is selling it and not really knowing if its even necessary. Dont take it as an attack or anything I just like to know why I should spend my money on something other than "they sell it and it worked for me". I currently dont have access to a code reader so I cant check and see what works and what doesnt.

[/ QUOTE ]

Without debating the why's and how's of the AIH resitor mod, all I know is that you will get a AIH code if you don't use the resistor mod. This code can be a pain in the butt if trying to do injector contribution tests because many times the test can't be performed if there is an active code. But for $1 for a pack of 470 ohm 1/2 watt resistors from Rat Shack and some spare wire. You can solder a length of wire to each end of a resistor, wrap the resistor in electrical tape or heat shrink, and then connect one end to the post on the AIH relay and the other end to ground and the code will be gone and world peace will exist.

Hammer

[rlarkin]

The code does not show up on all years. I have a 2000, and see no code.
I removed the ground wire from the drivers side head, and the lead from the solinoid. Did nothing else, no code.

[Larry M]

[ QUOTE ]
The code does not show up on all years. I have a 2000, and see no code.
I removed the ground wire from the drivers side head, and the lead from the solinoid. Did nothing else, no code.

[/ QUOTE ]

I even have a code reader and didn't even mess with checking since I had resistors laying around and just made my own. My AIH is very hard to get at on the Van so I just took the easy way out. Also, the PCM program is obviously different from the F-series and even with those just like the TC lockup strategy might vary between all the PCMs out there.

As mentioned the best way is to see if you're particular vehicle will set a code or not, but not everyone has a code reader and as mentioned some tests evidently can't be done if this code is there.


Larry, keeper of the <font color="red"> New revised </font><font color="blue">"little secrets" </font> CLICK HERE [img]/ubbthreads/images/graemlins/smile.gif[/img]

[mthisguy]

Thanks for the info, I think I will just use a small resistor and put it between the pcm wire and ground and not hook it up to the relay, and then check codes when I get a chance

[Larry M]

[ QUOTE ]
Thanks for the info, I think I will just use a small resistor and put it between the pcm wire and ground and not hook it up to the relay, and then check codes when I get a chance

[/ QUOTE ]

Just remember that for trouble shooting and maybe code reasons the voltage at PIN 3 on the PCM should be either 0 or B+ per the Service Manual under various conditions and you'll be removing or modifying that B+ value at PIN 3. [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]


Larry, keeper of the <font color="red"> New revised </font><font color="blue">"little secrets" </font> CLICK HERE [img]/ubbthreads/images/graemlins/smile.gif[/img]