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. :td:

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 ass off. :vash:

what about the wiring to the IAT sensor?

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.

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 :dunno: well given your last quote it seems like a shitty ground to me

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.

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

cool, thanks ryan. that was what i need. so in ambient 90-something I should get around 2.3-2.4V.

ya just turn your car on and test that shit right now

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.

Pulled sensor and probed. 0V :td:

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.

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.

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

Wait... why are we talking voltage and temp sensor? They are resistance/ohm based...

<-- confused.

:inout:

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 fucking dead, but that was logical pops first.

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.

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.

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.