Jeep Garage  - Jeep Forum

Jeep Garage - Jeep Forum (
-   Grand Cherokee - WK (
-   -   IAT ohm range? (

John_R 04-02-2010 10:38 AM

IAT ohm range?
Anybody know the acceptable range for the IAT sensor? It seems a good way to check it is take a reading across the terminals 'cold', then run a hair dryer and check the temp and see what the difference in ohms is. Makes sense...

However, I don't know what is considered acceptable to be sure the IAT is functioning alright. Like an idiot, I used rubbing alcohol on that thing, as I got some chips on it as I was testing fitment on the nipple I made. Whether or not it affected the unit I have no idea. But, I did get a P0128 code (thermostat) shortly after and its been on since.

It's possible it's my thermostat, but what a coincidence. Before I go replacing parts (I know thermo is easy and cheap and will do eventually), I want to eliminate this as a possibility. The temp takes awhile to reach operating temp and it's just weird the timing (say it turns out thermo).

jeepgcoman 04-02-2010 12:45 PM

Re: IAT ohm range?
I'll take a look in my notes to see if I can find it. About four years ago I did an ohm reading of the sensor from freezing up to about 120 degrees. The reason I did this was so I could construct a manually controlled IAT device so I could manually adjust the IAT to what I wanted. I still have it, and use it, on my 05 Hemi Magnum.

I don't think you'd be getting a "thermostat" code if there were problems with the IAT sensor. It has it's own set of codes. Have you changed the thermostat? Some guys would get codes, especially in cold weather, if they put a 180 degree unit in. It wouldn't warm up fast enough for the PCM's satisfaction. I use a NAPA Auto 190 degree in both my Hemi's and have never had a problem. It'll even run without the IAT sensor being hooked up, albeit, not well. I also don't think a little rubbing alcohol on it would hurt it. Without my notes, I think it was up around 50000 ohms or more. By the way, a new sensor is about $35, as I recall. Does yours still have the black plastic shield around it?

John_R 04-02-2010 01:41 PM

Re: IAT ohm range?
Good point about other codes. I was only going off what the description for P0128 had suggested and only suspected the IAT since that was the only thing I 'played around' with, so to speak. Just odd on the timing of the CEL light and now the thermostat possibly going out. Now, I did unplug my overflow line (radiator to reservoir) and suppose there's a chance some air got into the system, but who knows. Maybe I'll open the cap and let it run, just in case.

I had read (here maybe it was) that there was a bad batch of thermostats on the 05 WK's, so it is possible this is one of them. If it's been changed, I don't know (bought the car about 3 weeks ago with 50k mi and it seemed "mod" untouched other than the magnaflow, yet well maintained judging by cleanliness and how it runs overall)...I think I'll stick with 195 degree (better yet, I like the 190 you mentioned) oem rating on the Thermo, just to avoid any potential future CEL's.

The Jeep does seem to run cool (barely gets to 1/4 and maybe 3/8 on the idiot gauge). I don't have a way to measure actual operating temp at the moment though.

I was also reading about the IAT mods and how some people put a 4.7 ohm resistor inline, to fool the PCM into thinking it's ~20 deg F cooler air. I was thinking it would be cool to tie it into a switch so it could run either the 'fooled' rating or the proper rating, for whatever reason, or to put some sort of variable resistor. However, I would want to source the oem style male and female connectors to do it right, or maybe one of those extension pieces and modifiy that. $50+ for the wiring harness (the power wire thing) is ridiculous IMO

Yeah, mine (IAT) still has the black plastic and unspliced wiring as far as I can tell, so bet it's the original. I would like to put my meter on it, just to see where it's at.

The Jeep runs fine, but the CEL bothers me.

jeepgcoman 04-02-2010 07:40 PM

Re: IAT ohm range?
The 190 thermostat usually runs the temp. on my WK at 180-195 degrees, depending on stop and go. On the highway, in moderate temps., it's about 185 degrees. On the factory gauge it's just above the 1/4 mark. I have a DashHawk, and I am sure it is pretty accurate.

Since you just got yours, I would recommend you do like I did (I've had mine since Oct.) and check everything out. Mine had 48000 miles on it. The spark plugs had never been changed, it had the wrong weight oil in it (thanks to the stupid dealer), the transmission TSB had not been accomplished, and the PCM/TCM had the original programming that was causing several problems, mainly with the U-Connect, and the transmission. I've now gone through everything and did all the same mods to it that I had done to my Hemi Magnum, plus I added the Sonnax line pressure booster to the transmission. I've made my own CAI and K and N filter, installed a catch can, the thermostat, cut the EGR tube, put new siped tires on, added my MDS "ON" light, added a DashHawk, installed a new Nav. unit joystick, had a custom 3 in. Magnaflow cat. back exhaust installed, and many more items. It now runs quite respectable. My son and I are going to take both of them to the races next month and we'll see just how good it runs. I know it's not as fast as the Magnum, but it seems to be closer now than it was when I bought it.

The IAT mod. is similar to my IAT mod., except mine is totally adjustable to any temp. I choose, just like you're describing. I have found, after about 4 years of using it on my Magnum, that about 20-30 degrees below ambient seems to work the best. I think on the WK I am going to take a 30 degree below ambient resistance reading on the Magnum and then buy a fixed resistor of that value, and stick that in. Cost should be under $5. It'll then be just like the ones you've read about. Initially, the way I tested it on the Magnum was to remove it, put it in ice water and take a reading. Then I took my heat gun and ran it up to about 130-140 (I don't remember) and took another reading. Then I bought a resistor for that range. I can turn the potentiometer off and it will read stock or I can turn it on and set it anywhere I choose. The DashHawk gives me a real time temp. readout.

John_R 04-02-2010 10:28 PM

Re: IAT ohm range?
Great information, thanks a lot! I know this engine has so much hidden potential and not even counting even before any of the serious mods. One thing I like about Chrysler is they seem to understand that some people will actually dwelve into their machines and provide us with a lot of information and reasonably easy routes to go there. There are limitations, for sure, but generally speaking they've done a good job in that respect the past few years; it's a shame they have so many other problems and have their hands tied around their back by the gov't and regulations, etc...I'm all for sensible stuff, but, well that's another subject lol.

I'll have to look at your stuff to get a plan going, as it sounds like you've done some good experimentation and exactly what I'm looking for...Being able to use the V8's power as it's intended without these restrictions. I traded in a 2006 GTO 6-speed for this, and in doing so accepted (well, trying to :D ) the tradeoff of power to practicality and adventures, etc. I have other cars for the performance, but still, I want this thing to run like it should and well it would be nice to have as much as possible, within reason :)

Oh yeah, part of the fun (for me) when buying used is going thru everything and learning about it, updating all the expendibles (fluids, plugs, wires, etc.) and making things better. However, I have a wife that does not share this understanding and so I have to be creative in funds and such; that part is the biggest challenge!

jeepgcoman 04-03-2010 02:52 PM

Re: IAT ohm range?
John, the potentiometer that I used was a 0-50K. I guess I didn't keep the values for various temperature readings once I found the range. Hopefully in the next couple weeks I'll get some time to take a reading on the Magnum and figure out which value resistor I'll need to drop the IAT 20-30 degrees from ambient. Here's my writeup on the first rendition of the pot. I've since changed it so I can go both rich or lean at WOT. As I said, there doesn't appear to be much value in this system and a simple fixed resistor would work fine, unless you want to experiment like I did. You can do a search of my screen name over on that forum and see all the other "crap" I messed and experimented with over the years on the Magnum. It was fun. Before the ethanol laced fuels of today, I could routinely get 30-31 mpg with the Maggie. Now the best I can get is just over 29 mpg. I'd like to see if I can get the WK up to 22-23 mpg on the highway. My best so far is about 20.8. Kind of tough when you're pushing a heavy box down the highway.

Sorry to hear your wife is not a supporter. Mine is the same way, but when I got the WK she allowed me to get it all done at once, and then quit messing with it. I spent about 4 years, off and on, tinkering and experimenting with the Magnum. I don't really blame her, since it takes away from our "us" time. It's been 6 months now, and I am about 97% done.

John_R 04-03-2010 10:09 PM

Re: IAT ohm range?
Well, good news is she let me buy a thermostat and plugs today, lol. I changed the thermostat and yeah, wow the oem unit sure was messed up. Not sure what happened to the rest of the rubber, but it's somewhere in the cooling system. That bottom hat piece was off, as well. I can see how it acted like a lower temp being there was a bypass hole from where the rubber was. I ended up getting one from Napa, but the only one I could fine was an oem 203. Do you have a p/n for the 190 unit? (maybe down the road)

As for the IAT...What a PITA to try and test that thing, being the terminals are so close together and trying to keep a probe on it and one on the other, while then getting the blowdryer going. I thought about fabbing something, but didn't want to alter the resistance levels with additional wire/fittings, etc.

Here's the weird part...I get ~27k ohm at ambient, ignition on, but as I apply the heat, the resistance goes UP (to like 32k ohm)...that sounds opposite of what it's supposed to do, right? Then again, the way I did this test I'm not putting a lot of weight on it. I did get 5V from the harness though, so the circuit's working....I noticed there are several IAT sensor P codes and none of them are coming on, so just going to assume it's OK for now.

Another thing I sort of overlooked in my placement on IAT is that the blue thing is sitting at ~45 degree to airflow vs. being perpendicular, if that makes sense? Not sure that would really affect anything, but thought I'd throw it out there, anyway.

I changed half my plugs (got dark; other half tomorrow) and took it for a quick drive. CEL still on of course (thinking it takes at least 3 or 4 correct cycles) but temp seems to get closer to 1/2 mark. So far the 'odd' bank of cylinders' plugs were .053"-.055", so it was time.

jeepgcoman 04-04-2010 03:29 PM

Re: IAT ohm range?
Sorry, John, I meant to post the Napa thermostat number. They had to order it for both mine. Here's the number: Napa 381-190. It doesn't come with the rubber seal. The 203 you got is basically the stock setting. When installing make sure the tiny bleed hole in the thermostat is in the "up" position.

Yes, getting readings on the IAT sensor can be a pain. I have a special pin adapter for my multimenter that made it easier. Yes, you are correct, the resistances are inverse of what you'd think....higher temp.= lower resistance and vice-versa.

The sensor sitting at a 45 instead of 90 degrees to the airflow might make a difference because you're not getting the full volume of airflow across it. The shop manual states that it must be mounted as indicated in their picture or it won't work correctly, whatever that means??? They are indicating more the positioning (clocking) of the plug. If it's mounted 180 degrees from stock it won't work correctly, again probably because of airflow across the sensor. I think it has to do with the little plastic shield around the element. I broke that off on one and ran it without any shield and it didn't seem to make any difference.

Three good cycles should turn the light out.

John_R 04-04-2010 05:32 PM

Re: IAT ohm range?
Well, I hope I installed that tiny bleed hole correctly then, as I'm not real sure what you're talking about :) There was this copper 'thing' that went thru a hole, and able to move about. Or do you mean up as in 12 o clock position? Hmmm, wondering if I should've waited to get the 190 then. We'll see how this goes. Maybe once I get a tuner and such, I'll revisit this and I'm going to note that part number down for future reference. I take it that it's the same as oem otherwise. I was reading some stuff on a motorad 192 thermo (416-192), but it didn't have a jiggle valve? Don't know what that is...the hat thing on the end? if so, could be transferred, I guess (mine was off sitting in the housing, lol)

electrical/electronics are my weak point, and perhaps I did not read the IAT correctly. A higher numerical value of ohms would indicate a higher resistance, correct? So, if I measure 27k ohms at a lower temp, then the number increases to 32k ohms as it gets hotter, would that mean it's getting higher resistance and working the opposite of how it should?

A-ha, so maybe the rotation of the IAT on my homemade intake is what triggered this light afterall. But, how wierd the light didn't come on before, as the thermo was certainly toast. Could just be coincidence, too. Regardless, I appreciate the information, as I want to be sure everything is done right and to avoid 'mystery' problems down the road!

jeepgcoman 04-04-2010 07:48 PM

Re: IAT ohm range?
The bleed valve that I am talking about is probably your "jiggle valve". It's just a small hole or check valve close to the thermostat opening. You may want to check your old one to see. The twelve o'clock position is correct. I think it has something to do with "burping the system to get all the air out. Apparently if it is say at the 6 o'clock position it could leave an air pocket in the system and a potential for overheating.

Yes, higher reading = higher resistance. As I recall (I could be wrong, as it was about 4 years ago) the temp. and resistance were inverse to each other. Higher resistance means a lower temp. reading. Here's what the shop manual says about it:

The 2–wire Intake Manifold Air Temperature (IAT) sensor is installed in the air inlet tube with the sensor element extending into the air stream.
The IAT sensor is a two-wire Negative Thermal Coefficient (NTC) sensor. Meaning, as inlet air temperatures increase, resistance (voltage) in the sensor decreases. As temperature decreases, resistance (voltage) in the sensor increases.

The IAT sensor provides an input voltage to the Powertrain Control Module (PCM) indicating the density of the air entering the intake manifold based upon inlet air temperature. At key-on, a 5–volt power circuit is supplied to the sensor from the PCM. The sensor is grounded at the PCM through a low-noise, sensor-return circuit.The PCM uses this input to calculate the following:
Injector pulse-width

Adjustment of spark timing (to help prevent spark knock with high intake manifold air-charge temperatures)

The resistance values of the IAT sensor is the same as for the Engine Coolant Temperature (ECT) sensor."

Also, I noticed in the manual that it apparently has a different mount setup for the IAT sensor than my 05 LX. It talks about putting the sensor in and rotating it clockwise until past the lock tab. Apparently that insures proper positioning. For this you may want to check your stock silencer tube and see the correct clocking position.

John_R 04-04-2010 09:23 PM

Re: IAT ohm range?
I'll check out to see how I installed it soon, as I don't recall the position. It would've been nice if they keyed or indexed it somehow, but guess it's just one of those things that comes up later on after production.

I filed down my tabs on the IAT housing a little more to get it closer to 90 degree and it helped, but not quite there. I did try to replicate the orientation of the sensor, although it really should've been clocked another 5 degrees or so. Hoping this will suffice, or will need to make another. I'm glad you mentioned that about it not working in an incorrect position; or at least throwing off the reading...Everything is starting to line up (make sense) more on this now, as far as the code and timing. Thanks a lot for your help! and yeah, I like learning about the way this vehicle/computer operates..helps do things right next time, lol :)

John_R 04-05-2010 03:17 PM

Re: IAT ohm range?
Ugh, well another update...I am still getting a P0128 and now a P0112 (IAT low voltage) and P0113 (IAT high voltage). This thing is out to get me, lol. It seems they don't sell the IAT separately for 5.7 owners, which is messed up. The 3.7 IAT is less than $10 at napa. From the pic it looks different, so not gonna experiment. I'm gonna double check the connector and make sure it's not wet or K&N oil, or something, then button up the stock air intake back and take er into the dealer; let them replace the sensor if it's bad (under the 3 mo/3k warranty).

All times are GMT -5. The time now is 02:06 AM.

Powered by vBulletin® Version 3.8.8
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Copyright 2012 - JeepGarage.Org
The Jeep Grand Cherokee Owners Community