I was not aware the ECM read the battery temperature too.

I think if the modifications were minor (like the 1,500 RPM threshold for the APPS manual high idle), the ECM would not throw a code and go into limp mode.

I did read recently on TDR that the IAT sensor was being manipulated in the opposite direction. They adjusted it so the ECM would think the truck was at 200*F and back off the injection timing somewhat. I believe this was done to run a Comp box in the winter without the engine knocking too bad.

Perhaps controlling the AIT via an in dash potentiometer might be a good thing.

EDIT:

I'm not sure the board rules on links to other bulletins, but THIS IS THE THREAD <--- from TDR I was talking about.

 

Ph4tty, here is a circuit that may work. The resistor values will have to be tried and adjusted but it may be a good starting point.


I did a search for the IAT sensor connector and found it is a Metri-Pack 150 series, 2 pins. The problem with Metri-Pack connectors is that the plastic shell can be unique to the application. My truck uses the 2-pin connector on the IAT sensor and the coolant temp sensor, but they are different and will not interchange. Buying a generic Metri-Pack plug probably would not fit.

Phialabuster, you are probably correct. The PCM is the one looking at the battery temp. I seem to remember some code that the ECM checks for “Rationality” of the sensors, but it may have been a senior moment.

 

I'm sorry but I don't see that working. Because the resistance of the IAT is dynamic you could not have a static added resitor value work. What I'm trying to say, while sounding important and educated , is that by hooking in in series with the IAT you can make the ECM see 15* at an actual 30* temp. When the IAT reads, say, 80* or so it will see an IAT of somewhere around 65*. No workey! Now if you completely bypassed the IAT you would be taking out the variable resistor, giving consistent IAT readings regardless of varying intake temps.

 

Very crude and quickly whipped up, but illustrates my point. Relays not energized, for normal operation. Relays energized and the ECM sees a set resistance/intake temp.

 

Doh , I just realized my circuit diagram has an error in the switch labels. The second switch down for selecting High Idle and 3 Cylinder High Idle is labeled backwards. When that switch is closed you will be in High Idle and when Open you will be in 3 Cylinder High Idle. I guess the barley pop caught up with me last night.

Here is an update on the IAT sensor. The graph is drawn from the values published in my manual and I extrapolated to get down to -10*C where the 3 cylinder high idle should kick in. As you can see there are two lines on the graph. This is because my info gave a minimum and maximum resistance value. The graph is non-linear at high temperatures but gets linier at low temperatures.

To figure out what value resistor to use for 3 cylinder high idle, it looks like the total resistance of the IAT and resistor should be has to be around 50K ohm. If you want this to happen at room temperature (20*C) the added resistance should be 40K ohm. This was calculated by looking at the graph and finding the resistance of the IAT at 20*C which is about 10K ohm and subtracting that from to 50K ohm figure that the ECM would see if it was really at -10*C. In my schematic replace the 25K ohm resistor with 40K.

For high idle without the 3 cylinder cut out The ECM is expecting a temperature below 0*C. I will pick -5*C. This will require the ECM seeing a resistance of about 38K ohm. Again, if I subtract 10K ohm to make this work up to a room temp of 20*C the resistor value needs to be 28K ohms. But there is already a 40K ohm resistor in the circuit. To get a value of 28K ohm the new resistor has to be larger so the parallel combination is 28K ohm.

28K= ((new resistor)*40K)/(new resistor+40K) solving for new resistor = 93K ohm. So in the schematic above substitute the 93K ohm resistor for the 10K ohm. I was not even close on that guess,

And TheBigNasty, you are absolutely correct that the IAT is a dynamic resistance. My circuit will not work if the real outside temperature gets above 20*C, but below that temp it will. Your circuit will work and you can dial in the temperature correction.
On Edit: I fixed my schematic to correct values. Good luck.

 

The potentiometer idea is probably best but can be done more easily using one relay. I just wish someone would try something to see if the whole idea makes any sense. I am not willing to cut into my harness.

 

I like the idea of adding resistor in series to make truck high idle at slightly warmer temps. I would be happy with around 40-45* and all automatic and simple, I love it.

Jed

 

My truck high idles anytime I want.

 

I understand what you are getting at with this diagram but, with the pot still being run inline with the IAT you will have to reset it for every temperature change. Using two relays cuts out the IAT completely and only has to be set once. And also remember that the Smarty high idle doesn't kick on instantaneously. The best way IMO would be to use a pot that bypasses the IAT, take a meter to find the values for 10* and 20*, and mark them on the pot so for high idle or 3cyl idle you just twist to the mark.

 

TheBigNasty, your circuit may be a good answer but one thing bothers me. The ECM is looking at the IAT to adjust fueling and timing. If you switch the relays in your circuit there will be a “glitch” when the relays break contact with the IAT and connect with the resistor or pot, Relays typically need about 50 mS to switch. Will the ECM detect that disconnect and set a code and put the truck in limp mode? If I unplug my IAT it will. By staying connected to the dynamic resistance of the IAT and adding a resistor in series, there is no disconnect and the ECM should be happy. One fix to your circuit might be to add a 0.1uF capacitor across the wiring harness to the IAT to hold up the voltage while the relays are switching.

I still have lots of questions about how the ECM will respond to the artificial Intake Air Temperature signal. I am thinking about just disconnecting the IAT and shoving some wires into the connector and connecting to a pot. If I can get the whole concept to work we can take it from there. Right now I am recovering from shoveling snow.

 

Seems like if I was to put a 5K ohm resistor in series it would raise high idle temp to around 42*. Do you guys see your trucks idle up at 32* or does it normally have to be colder than 32*?

 

Give or take....its pretty close to 32.....my temp gauge at my house says 32 and it dont idle up....but at 30 it does.......just depends......I think there is some margin of error......

 

Here is an update. This morning it was 11*F (-12C) I let the grid heaters cycle once, backed out of my parking space and sat with the 5 speed in neutral, parking brake on and let it idle. About a minute later the idle speed ramped up from 800 to 1200 RPM. Releasing the parking brake, pushing in the clutch and shifting gears made no difference in the high idle. Touching the brake pedal dropped the idle. I let it sit for another minute to get the high idle back. I then shifted into 1st gear, let off the parking brake and let the clutch out to roll forward. It stayed in high idle for about ˝ revolution of my tires or about 1 MPH (1.6 KPH).

And some info about how the ECM and IAT interact.

P0112, IAT voltage below minimum acceptable voltage. This happens if the IAT resistance is below 175 ohms and will turn on the “Check Engine Light” and put the engine into “Limp mode”.

P0113, IAT voltage above maximum acceptable voltage. This happens if the IAT resistance is above 244K ohms and will turn on the “Check Engine Light” and put the engine into “Limp mode”.

P1110, IAT above engine protection limit. This will turn on the “Check Engine Light” and put the engine into “Limp mode”.

P1291, No temperature rise seen from Intake Air Heaters, (Grid heaters). This will turn on the “Check Engine Light” but will not put the engine in “Limp mode”. This code will happen if a fixed resistor is used in place of the IAT sensor and is another reason to just add resistance in series with the IAT.

It is just too cold today to be comfortable working on my truck. I disconnected The IAT on the warm engine and saw that it will be easy to plug some wires into the connector. At about that time I was hanging on to the warm valve cover to get feeling back into my fingers. So I reconnected the IAT and will wait for a warmer day.

 

Linetrash75, If the high idle starts at 32*F (0*C). The IAT would be reading about 30K ohms and about 22K ohms at 42*F (5.6*C). You will need an 8K ohm resistor. This will also raise the 3 cylinder high idle from -9.4*C (15*F) to about -4*C (24*F). this *F and *C stuff gets confusing. Almost all engineering stuff uses *C.

 

I guess I need to go down to dealer and have them enable high idle with the star tool. I' never had it done had it enabled because it rarely gets below freezing more than a couple days a month here with exception of this year. thanks for the good idea.

Jed