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Old 01-11-2010, 10:25 PM   #1
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Default GM IAT sensor / Easytherm

Hello guys,

I've been battling an ever-changing tune due to IAT temperature.

I finally took the time to do some empirical testing and it appears that at least in colder temps (around 28 to 50 degrees IAT), my MS thinks its around 12*F colder than it really is.

Due to this, I decided to remove the IAT from the car and test it as per DIY's website by immersing in cold water, medium temp, and boiling water. I took a few extra data points and came up with the attached graph. The higher values were nearly spot-on compared to the expected value, but there was some deviation in lower temps.

So my questions are:

1) does that look 'off' enough to create the issues i'm seeing?
2) if i were use easytherm to generate a new curve based on these measurements, does it matter that my lowest temp is 19*F vs the -4*F used in the provided .inc file?
3) has anyone else used the standard GM IAT sensor but re-easythermed for accuracy?
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GM IAT sensor / Easytherm-gm_iat_graph.jpg  
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Old 01-11-2010, 10:31 PM   #2
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look about half way down the first post, which settings did you use?

https://www.miataturbo.net/forum/t7000/
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Old 01-11-2010, 10:37 PM   #3
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I dont mean to sound dense, but which which settings?

I got the MS from braineack, he burned the HiRes10G firmware onto it, which is also the project i open when running megatune.

I think you may be asking about "Step 3", in which case i guess braineack chose option 2 (gm iat, mazda clt). He did not change the CLT range though as suggested there.
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Old 01-12-2010, 12:48 AM   #4
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I have the same problem. When I program the IAT with the diy settings in tunerstudio it goes from being spot on with the clt sensor to reading about 15 degrees cold. When it is warm outside it seems to read correctly. I've thought about leaving the default tunerstudio settings for the IAT, but I'm afraid it might not be correct when the temp warms up.

Let me know if these IAT settings fix your problem. I'll try it tomorrow
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Old 01-12-2010, 09:51 PM   #5
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I haven't tried to re-burn yet, I'm waiting to hear from anyone else that's tried this and had some success.
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Old 01-12-2010, 11:06 PM   #6
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damnit. i tried to flash the MS but i see now that i screwed it up.. I created new INC files using easytherm and then just hit the download to MS button... it downloaded 350 lines then quit..

UHOH now I can't seem to get it to respond on the COM port, it looks pretty dead.

Now that the original hires firmware is gone, do i have to get in there and set that boot jumper?? ( i really dont want to, its buried w/the stock ECU and its cold outside)
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Old 01-13-2010, 12:34 AM   #7
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i got the MS apart, shorted the boot jumper, and was able to re-flash the original firmware i had on there.

Then i got adventurous after reading about another person who pretty much made the same exact mistake as me (not miata though..) I selected *custom*, not hires10g in easytherm, and was able to generate a new S19 file combining the original firmware with my new sensor values.

I burned it to the MS and, ............ it's no better and maybe worse than before so i'm a little at a loss, as i used the values i measured by hand :(

Anyway, i'll try running the car tomorrow and repeat my tests to see what's going on.

-Tomaj

P.S. i think i'm going to get one of those IR thermometers .. any thoughts?
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Old 01-13-2010, 09:33 AM   #8
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Quote:
Originally Posted by ctxspy View Post
So my questions are:

1) does that look 'off' enough to create the issues i'm seeing?
2) if i were use easytherm to generate a new curve based on these measurements, does it matter that my lowest temp is 19*F vs the -4*F used in the provided .inc file?
3) has anyone else used the standard GM IAT sensor but re-easythermed for accuracy?

1. no.
2. no.
3. no.
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Old 01-13-2010, 02:47 PM   #9
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Quote:
Originally Posted by Braineack View Post
1. no.
2. no.
3. no.
In that case, my only other thought is that i should measure the resistance at the megasquirt vs. at the sensor, to see if the wiring is introducing any additoinal appreciable resistance.

Does that sound reasonable?
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Old 01-13-2010, 03:09 PM   #10
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I doubt it is; the bias resistors for the AIT and CLT only have a +/- error value of 1%, BUT, you never know how much that also offsets the curve, so a true reading might help.

did i build you a parallel or standalone harness?
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Old 01-13-2010, 04:02 PM   #11
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Quote:
Originally Posted by Braineack View Post
I doubt it is; the bias resistors for the AIT and CLT only have a +/- error value of 1%, BUT, you never know how much that also offsets the curve, so a true reading might help.

did i build you a parallel or standalone harness?
You built it as parallel.

Regarding the bias resistor, what would the effect be if i had the wrong value in easytherm? I used the value you had provided in your .inc (and also confirmed in another thread), but is it possible that you put a different physical resistor in?

in the same vein, is it easy to find this resistor on the board and do a visual check?

-tomaj
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Old 01-13-2010, 04:06 PM   #12
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Remove both r4 and r7.

Redo the easytherm firmware using 2625 for the CLT bias and 3475 for the AIT bias.

They are the two blue resistors, cant miss them.
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Old 01-13-2010, 06:26 PM   #13
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I took off the two blue resistors (identical) from r4 and r7.

i reflashed with the bias resistor values you supplied and the result is pretty much that nothing changed..

my car hasn't been on in a day and a half and shows: CLT 31/32 and IAT shows 24*F. Local weather is reported at 33 on google.com

-Tomaj
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Old 01-13-2010, 06:32 PM   #14
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did you copy the new .inc files into your project folder's mtcfg folder?
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Old 01-13-2010, 07:03 PM   #15
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i let easytherm do it, verified by checking file mod date in the folder.

FWIW i checked the resistance with the sensor in the car, it read around 9500 ohms at the current temperature. On the 'official' resistance curve chart i found, it lists 32 degrees as 9450, so it seems completely accurate up to the AFM harness.

I will next try to figure out which inputs on the MS harness are for the IAT and test the resistance on the DB37 to see if there's some issue there.

My current thinking is that the easytherm et al was correct all along but i have a wiring problem leading to increased resistance... a bad solder joint at the harness? ( i had to **** with my harness a bit a while ago)

-Tomaj
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Old 01-13-2010, 09:03 PM   #16
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i tested the resistance at the AFM harness as stated before, to be correct... however as soon as i plug into the AFM harness, and test the resistance again (either there or at the Db37 connector) i get around 2625 ohm... is that a coincidence that this number is the same as the bias resistor value you mentioned for my CLT sensor?

i'm not really sure what's supposed to happen once i hook into the AFM harness, as i guess at that point it's sharing the IAT sensor value with the stock ECU??
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Old 01-13-2010, 09:23 PM   #17
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well hardy harr harr (in a pirate voice... not sure why, i guess i'm just excited)

on a hunch i figured that 2625 wasn't just a fluke so i plugged that into easytherm as the bias resistor value for the IAT sensor.

i flashed, and........ drumroll please....... 100% correct!!!!

Brain, Joe, anyone -- please confirm if this statement is correct:
You can get your bias resistor value (if running parallel? and with no r4 / r7?) by unplugging the MS and testing the resistance across the IAT at the MAF or the DB37.



Forgot to add -- thanks for the help Braineack!

Last edited by ctxspy; 01-13-2010 at 09:38 PM. Reason: to give credit where credit is due.
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Old 01-14-2010, 09:54 AM   #18
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That didn't work for me. To get the values I gave you took a long process of trial and error a few years back when we were first figuring out the values.

I know for absolute certainty that using 1280 for CLT and 1450 for AIT for the bias with R4 and R7 installed on a parallel install gives you accurate values. My CLT and AIT values would be within 1-2F with the car sitting overnight on a 20*F morning or 80*F afternoon.

Therefore, if you do the math for parallel resistors and you know the value for the one 2490 and need to find X, it's simple algebra.

solve for x:
1280 = 1 / (1/2490 + 1/x)

a:
x = 2625


Regarding fining the resistance with a multimeter:

When I took the resistance reading of the stock ECU plugged into a boomslang harness and read the resistance between ground and pin 20 (AIT) I got a value of 3630.



If I plugged in a MS with R4 installed, the value now read 1840ohms.



Using those numbers gave me no where close to accurate values. The numbers I gave you do and are what I personally use.

I'll ask again, once you flashed your new easythermed .s19 file, did you move the .inc files that easytherm created, based on these new bias values, into your /mtcfg (MT) or /inc (TS) folder?


What you can do, without having to flash the MS over and over, is a somewhat easy process. Open easytherm and create a new .s19 and .inc files. Changing the bias of the incorrect reading up/down 200-300 ohms at a time. Each time you create a new file, copy the .inc files into your /mtcfg folder. Now turn the car to ON and read the values with your laptop. There is no need to flash the MS as your laptop is converting the resistance values straight off your .inc files not the MS unit. Once you come up with a bias that gets the AIT and CLT to read alike, you can then finally flash the MS with that .s19 based off those values and the MS unit will read the values your happy with.
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Old 01-14-2010, 02:49 PM   #19
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Scott,

I will try to digest the content of your e-mail tonight.. Regarding your question of moving the inc files, i just verified that they are in the mtcfg subfolder of the project that I opened with megatune.

-Tomaj
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Old 01-14-2010, 02:55 PM   #20
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and it says:

Quote:
; Generated using EasyTherm4.exe,
;
; Air Temp Thermistor Comment Field
;
; ***** CAUTION - NON STD BIAS RESISTOR REQUIRED *****
; File generated for use with 1280 ohm resistor at R4
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