Intake Temp -40F - Miata Turbo Forum - Boost cars, acquire cats.

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Old 08-12-2007, 05:35 PM   #1
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Default Intake Temp -40F

I was sooo pissed off. Jerry and I were working on some ground and ignition issues, and I thought I was finally going to get a chance to really test the car out since making some modifications. Not so lucky, MegaTune reports Intake temp is stuck at -40F.

I assumed that the most likely issue would be a failure in the connections between the temp sensor wiring and the AFM harness. So I opened that up (which was a PITA due to the 50lbs of electrical tape I used to mumify the connection). But alas, the connection was sound. The other connections seemed solid, but I didn't have time to conduct further investigation; I've got family in town.

The intake temp sensor is a GM open element. I assume it's just a thermistor. Does anyone have a reference chart (V vs temp) to check the sensor for proper operation?

This never happens when it's 70F outside. Nope, it waits until it's 100 so I have to sweat my *** off.
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Old 08-12-2007, 05:53 PM   #2
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what about the wiring to the IAT sensor?
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Old 08-12-2007, 06:07 PM   #3
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seems ok, didn't have a lot of time. I had continuity between the two wires, and did not have continuity between either wire and chassis ground. did not check for resistance or voltage.
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Old 08-12-2007, 06:10 PM   #4
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My vote is on sensor are you running stock air intake sensor or the GM if the GM i think you either have a wiring issue or a bad flash on the easytherm for stock AIT sensor maybe well given your last quote it seems like a shitty ground to me
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Old 08-12-2007, 06:20 PM   #5
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i could buy failed sensor, but doubt MS software/firmware issue. That's why I was hoping someone had temp vs voltage data for the thermistor. Becuase then I could put it in a pot of hot water and compare voltage output to a thermometer.

I should have clarified, with the 2 wires from the sensor not hooked to anything else, I had no continuity to ground. Meaning I didn't have a short to ground.
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Old 08-12-2007, 06:27 PM   #6
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do you have a multimeter? That would help quite a bit. As for the resistance you are looking for, why not open up easytherm and look at the GM default values. Also, I cant verify that it is correct, but it probably is, I found this after a quick google search. Looks like just what ya need: http://www.innovatemotorsports.com/f...php/t-988.html
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Old 08-12-2007, 06:33 PM   #7
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cool, thanks ryan. that was what i need. so in ambient 90-something I should get around 2.3-2.4V.

Quote:
F Volt
304 0.000
260.6 0.016
226.4 0.312
199.4 0.468
179.6 0.624
163.4 0.780
152.6 0.936
143.6 1.092
136.4 1.248
129.2 1.404
123.8 1.560
118.4 1.716
113 1.872
109.4 2.028
102.2 2.184
96.8 2.340
91.4 2.496
84.2 2.652
77 2.808
71.6 2.964
64.4 3.120
59 3.276
53.6 3.432
48.2 3.588
42.8 3.744
37.4 3.900
32 4.056
24.8 4.212
17.6 4.368
5 4.524
-9.4 4.680
-29.2 4.836
-58 5.000
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Old 08-12-2007, 06:44 PM   #8
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ya just turn your car on and test that **** right now
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Old 08-12-2007, 06:48 PM   #9
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Quote:
Originally Posted by akaryrye View Post
do you have a multimeter?

I lol'd at work. Would be a bad thing if anyone with a DIY based (or just plain DIY) ECU did not have a multimeter.

The ultimate, and basically only, tool needed when it comes to diagnosing electrical issues.
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Old 08-12-2007, 07:12 PM   #10
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Pulled sensor and probed. 0V
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Old 08-12-2007, 07:16 PM   #11
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for sure, i actually thought about editing that out. I am pretty sure that Ben is competent when it comes to cars and whatnot, but Ive been surprised by people in the past.
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Old 08-12-2007, 07:17 PM   #12
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Quote:
Originally Posted by Ben View Post
Pulled sensor and probed. 0V
well theres your problem right there, I bet there are available sensors for like 10 bucks at the dealership. Wait, make sure that you are getting the 5v reference voltage from megasquirt first, but its kinda hard to screw up a temperature sensor wiring.
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Old 08-12-2007, 07:38 PM   #13
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I should have 5V between the 1st and 6th pins on the AFM harness, right? (those are the ones that the temp sensor hooks to)
I have 0V there.
The thermisistor does change resistance with ambient temp. I had 4000k ohms outside and 3600 ohms inside
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Old 08-12-2007, 07:54 PM   #14
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Wait... why are we talking voltage and temp sensor? They are resistance/ohm based...

<-- confused.
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Old 08-12-2007, 08:35 PM   #15
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Old 08-12-2007, 08:47 PM   #16
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Right, but it's a circuit so it needs power. Should be 5V goes in, the thermistor adds resistance based on temp, so less than 5V comes out. Resistance decreases as temp goes up.

My sensor does change resistance based on temp, it seems. R should decrease as temp goes up. However I'm getting some mixed readings, but probably due to the fact I have to hold it in my hand.

MY MS shows -40F which is the bottom of the scale. Which would mean 0V/infinite resistance.

I have 0V at the AFM harness. There is a connection somewhere. I do not know which pin at the ecu suppies 5V to the AFM, but suspect (hope) that's where the problem is.
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Old 08-12-2007, 09:16 PM   #17
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Quote:
Originally Posted by Ben View Post
Right, but it's a circuit so it needs power. Should be 5V goes in, the thermistor adds resistance based on temp, so less than 5V comes out. Resistance decreases as temp goes up.

My sensor does change resistance based on temp, it seems. R should decrease as temp goes up. However I'm getting some mixed readings, but probably due to the fact I have to hold it in my hand.

MY MS shows -40F which is the bottom of the scale. Which would mean 0V/infinite resistance.

I have 0V at the AFM harness. There is a connection somewhere. I do not know which pin at the ecu suppies 5V to the AFM, but suspect (hope) that's where the problem is.
NO based on the inputs you have post earlier you not getting a resistance...

IF 5V is been supplied power... based on the scale you posted... MS is reading 4.9XXXXX. Should then you voltmeter been reading say 5v across since that is what -40 means? It sounds to me like you where probing the wrong pins. 0V should read 300F not -40.

Maybe I am getting everything flipped in my head, I am still ******* dead, but that was logical pops first.
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Old 08-12-2007, 09:25 PM   #18
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the scale below is wrong. web info isn't always right, as that's not how a thermistor works. its resistance approaches 0 as temp increases, and resistance apporaches infinity as temperature decreases. The MS is supposed to supply the thermistor 5V, the thermistor will drop the voltage, and the MS calculates delta V to determine temperature.

I think the thermistor works, but it's not receiving power.
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Old 08-12-2007, 11:06 PM   #19
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I dont remember how I wired it for sure, but I think I hooked one wire up to the megasquirt and the other wire to either a ground or a 5v source on the ecu harness.
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Old 08-12-2007, 11:12 PM   #20
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Jerry, If/When you read this, could you please post which wires at the ECU harness go to the pins at the AFM connector that we plug the AIT sensor to? I'd like to probe out the +5V source for functionality, and then just directly wire from the MS to the AIT sensor without going through the AFM wiring. Assuming there's no type of filtering circuit in the AFM harness--which I doubt is the case.
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