Nate99

Just saw this. Can you explain? Are you saying I should jumper the sensors to 3C/3F as a test? Sorry if this is a noob question...

Akina_Downhill

iTrader: (1)

IAT sensor wiring : One pin to ecu (2B if your diagram is right), one pin to ground(you can use 3C/3F).

Nate99

OK got it. So I should be able to measure 5V to engine block ground on one of those two pins (presumably 2B?). I'll check that and see what I get.

Akina_Downhill

iTrader: (1)

What you need to do is measure the resistance across the sensor at a given temp and check if the reading is close to the gm iat values in tunerstudio.
This will tell you if you have a faulty IAT sensor.I think you ve done this already.
You should see 5V measuring 2B to ground.

Nate99

Bringing this back from the dead... I was out of the country for a bit and just now had some time to get going on this again. Good news is I think I'm close to the problem:

I measured the IAT voltage at ~0.16V (car on, IAT sensor unplugged) at three points: between the two pins on the IAT connector, between pins 2B and 3C on the car side of the ECU harness, and between 20 and 2 on the MS3 (DB37 connector) side of the harness. That should be 5V, not 0.16V, based on the posts above.

I also measured my IAT resistance over temperature and got the following:
~33F 8.6 kohm
~81F 2.21 k
~210F 230 ohm

Which looks about right. Based on these two data points, I think the IAT sensor is fine and the ECU is not sending the correct 5V to the sensor. I'm guessing the next step here is to track down whatever part blew up in the ECU and replace it - anyone have advice on which parts to look at? Braineack?

SpartanSV

If you have 5v at the TPS then the 5v output from the ecu is fine.

Nate99

Good point. I measured 5V at the TPS and it works just fine too. Maybe there is a short in the harness on the IAT wire? I'll check on that. Any other suggestions?

SpartanSV

You confirmed the 5v output is working on the ecu. If you don't have 5v at the IAT the only explanation I see is an issue with the wiring from the ecu to the sensor.

If you only measured for 5v at the sensor while using the sensor ground then I would re-measure for 5v at the IAT connector with a lead to chassis ground instead of what should be the connector ground to ensure the the issue is on the wiring from ecu to sensor instead of sensor to ground.

Nate99

So I checked on this, and can't find any issues. I measure no shorts between the IAT and sensor ground, no shorts between the IAT pins in the harness, and no opens on any of the sensor pins either. To me it still seems like there's something going on in the ECU. Is there anything that could have failed in the ECU which would allow the TPS to get 5V while the IAT doesn't?

If I can get a bench power supply, can I hook the MS up to 12V and then measure the sensor voltages? Or will I blow it up by doing that?

SpartanSV

Somewhere along the way you measured incorrectly. Literally the only possibility.

You don't need a bench supply when you have a car you can plug it in to.

https://trubokitty.com/#/ms3xassembly

2I is the 5v output from the ecu. Measure for 5v there (you'll have it cause you measured 5v at the TPS and your log shows a functional TPS). If you have 5v there and not at the iat sensor then run a wire from the ecu output to the sensor because your factory wiring is dicked.

You should actually be able to run a wire from the TPS to IAT instead of ecu to IAT. Again, the 5v source for each should be the same 5v output off of the ecu.

For the 3rd time, the ecu is fine.

Nate99

Well it turns out the ECU was not fine after all... I pulled up the schematic here: V3 Main Board and went through the whole MAT circuit with a buddy who's an EE. We found one bad solder joint each on R4 and R5 (no solder fillet on one side of the thru hole). Added a bit more solder to each joint, reassembled, and right away had 5V at the TPS sensor. The car now starts normally, IAT readings look good, idles well, and doesn't stall when I give it gas. I won't know if the power issue below 5500 RPMs is solved until I can drive it somewhere, but working on a plan to check that out soon.

Huge thanks to everyone in this thread that helped me out! You all are seriously awesome

SpartanSV

And this is the problem with troubleshooting over the internet. You said you had 5v at the TPS. You said you didn't have 5v at the IAT. The 5v reference for those sensors come from a single output pin on the ecu and is split off in the vehicle harness to each sensor. You gave us bad info. You had 5v at the TPS and IAT all along.

The R4 and R5 you're talking about are on the input side. If they were your problem then you had 5v at the IAT and failed to measure it properly. Those resistors wouldn't have prevented the 5v reference from getting to the sensor they just would have prevented the ecu from reading the IAT signal correctly.

Glad you got it figured out though.

Nate99

I'm not sure how I measured it wrong - I measured the TPS and the IAT exactly the same way (between two pins in the harness, and between the pins and engine block ground). I got 5V from the TPS and 0.17V from the IAT. What did I do wrong there?

Either way, thanks for the help, I really do appreciate it. Sorry if I screwed something up and made it more difficult...