SPREADSHEET: GM IAT

Airboy · **Posted:** Sat Mar 15, 2014 10:41 pm

Daemos wrote:

Airboy wrote:

Did some measurements on my 2006 USDM WRX and found that the internal resistance is closer to 940 Ohm. I made three measurements with different resistor values (3.3k, 1k, 470) and each one would give a different internal resistance if I assume a supply voltage of 5V. Then I measured the open circuit voltage at the harness and it was 4.78V instead of 5V. Using 4.78V in the calculations gave internal resistance value of ~940 for all three measurements.

Made a couple of measurements of the resistance of the MAF IAT sensor element as well. One at ~60F and the other ~35F (in the fridge for 20 minutes, with wires extended). Temperature was checked with an infra red thermometer). The results lines up with the stock calibration table really good, using 4.78V and 940 Ohm series resistance.

Have you tested the GM IAT sensors?

I installed a air temp sensor from Haltech with a 1/8 NPT fitting. It is supposed to be the same thermister as the GM unit. From ambient temperatures of ~10F up to ~50F, the air temp calculated by the ECU is quite close. The voltage vs temperature table was generated using internal resistance of 940 Ohm, 4.78V, and the published resistance values from AEM. When ambient air temps get warmer I can check the rest of the scale.

Daemos · **Joined:** Wed May 08, 2013 1:23 am **Posts:** 142

Airboy wrote:

Daemos wrote:

Airboy wrote:

Did some measurements on my 2006 USDM WRX and found that the internal resistance is closer to 940 Ohm. I made three measurements with different resistor values (3.3k, 1k, 470) and each one would give a different internal resistance if I assume a supply voltage of 5V. Then I measured the open circuit voltage at the harness and it was 4.78V instead of 5V. Using 4.78V in the calculations gave internal resistance value of ~940 for all three measurements.

Made a couple of measurements of the resistance of the MAF IAT sensor element as well. One at ~60F and the other ~35F (in the fridge for 20 minutes, with wires extended). Temperature was checked with an infra red thermometer). The results lines up with the stock calibration table really good, using 4.78V and 940 Ohm series resistance.

Have you tested the GM IAT sensors?

I installed a air temp sensor from Haltech with a 1/8 NPT fitting. It is supposed to be the same thermister as the GM unit. From ambient temperatures of ~10F up to ~50F, the air temp calculated by the ECU is quite close. The voltage vs temperature table was generated using internal resistance of 940 Ohm, 4.78V, and the published resistance values from AEM. When ambient air temps get warmer I can check the rest of the scale.

Mind sharing how you generated the table?

Airboy · **Posted:** Sat Apr 05, 2014 12:08 am

Daemos wrote:

Mind sharing how you generated the table?

Similar to the spreadsheet posted above (viewtopic.php?p=81728#p81728) I used the voltage divider equation to generate voltage vs temperature. Then I transferred the "rescaler" tab in my spreadsheet (viewtopic.php?f=32&t=957) and generate the temperature values for the table in the ROM.

This is what I am using so far.

Code:

21   0.36   0.52   0.67   0.83   0.99   1.14   1.3   1.46   1.61   1.77   1.92   2.08   2.24
0   265.9   238.6   220.3   205.0   192.3   182.2   172.6   163.9   156.4   148.8   142.3   135.6   129.1

39   2.55   2.71   2.86   3.02   3.17   3.33   3.49   3.64   3.8   3.96   4.11   4.27   4.42   4.58   4.74
2   117.1   111.0   105.3   99.1   93.3   86.9   80.1   73.4   65.9   57.5   48.7   37.4   24.4   4.2   -40.0

I'll check it against the thermocouple the next time I have it on the dyno.

ryanr · **Joined:** Thu May 12, 2011 8:36 pm **Posts:** 116

I have added a v2 of my IAT spreadsheet (page 3) where you can also create the sensor scaling table for 0-4.7v after reading a few articles/posts from Cobb and Airboy. I tested my voltage going to the IAT sensor itself and it was around 4.72v with the car on.

jon7009 · **Posted:** Fri Aug 21, 2015 2:58 pm

In prep for a Crawford AOS, I removed the PCV valve and drilled with 37/64" bit and tapped with a 3/8 NPT, then installed the GM IAT where the PCV went. I have a USDM 2003 ECU and haven't measured resistance as my multimeter battery died lol

I'll be using ryanr's v2 0 - 4.7 scaling and report back later.

I purchased Echlin ECH TS5100 temp sensor from napa for $22.50.

jon7009 · **Posted:** Sun Sep 06, 2015 1:53 pm

I tried ryanr's first and clearly was off, bcheck's post #11 is correct for me. Forgot to check back in.

terryjr · **Posted:** Thu Apr 21, 2016 11:22 am

So that i understand correctly, to wire the GM sensor its 5v source to one pin and signal out to the ECU iat signal input correct?

I'm not using ht stock IAT wiring, trying to keep it to the engine harness. IAT is in the pcv port
I planned on using a tgv 5v supply for the sensor.

jon7009 · **Posted:** Thu Apr 21, 2016 12:05 pm

on the website flexconverter you can also get a MAF adapter harness to provide clean looking/connecting power to the IAT sensor and re-routes the IAT signal in place of the MAF IAT. No cutting/splicing/extending.

terryjr · **Posted:** Tue Apr 26, 2016 8:36 am

terryjr wrote:

So that i understand correctly, to wire the GM sensor its 5v source to one pin and signal out to the ECU iat signal input correct?

I'm not using ht stock IAT wiring, trying to keep it to the engine harness. IAT is in the pcv port
I planned on using a tgv 5v supply for the sensor.

i had this wrong. I have my IAT sensor under the throttle body. And using the RH tgv wires to feed signal to the ecu and not need an extra harness. I just switched B135 B23 (old tgv signal wire) to B23 (iat signal wire) and used the ground wire and signal wire from old TGV harness to the IAT sensor. The GM IAT is just a resistor between the ecu signal to ground. ANd rescale dthe IAt in the rom to the GM iat scale floating around

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