need help w/scan tool reading volts open circuit stuff...

[Jwernatl]

Guys - Im pretty ignorant when it comes to auto voltage. Im attaching what the manual says below…..

I need to see if CKT 419 is open - Scan tool?

I need to test the TPS and the O2 sensor volatage. (mainly the TPS) And, I realize this is a rookie question -** How do I test the wires when they are insulated? I have a nice multimeter scan tool that's real pretty.

And last -

when I turn my ac on, the idle LOWERS! It didn't always. A week ago it was perfect. SO,I'm trying to figure out how to see if a circuit is open or not. (How can know (sing my scan tool?) if a circuit 59 is open or closed? as stated on the last upload (below)

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[Jwernatl]

and yes - I realize I'm an idiot.

 

[gmbellew]

best advice I can give is to get aldldroid on your phone or tablet and get a blue tooth aldl reader. it will show you the sensor voltages and much more and you are only into it about 100 bucks.

 

[Camar068]

can get a bluetooth scanner on amazon for around $25 bucks for a decent one. Then get Torque plus on your phone ($10 or so). Then look at live data and compare your O2 sensor voltages to each other. If one is off from the others, change that sensor (RockAuto best place if you can wait).

As far as your TPS, in that data look for throttle position. Give it gas slowly and make sure the readings slowly increase. Look for drop outs.

When using torque, setup a guage that will give you a wave form for all the above and go through it.....makes it easier to compare o2 sensors over a period of time, and look for TPS drop outs.

youtube torque on how to setup gauges, I'm sure there's plenty.

 

[Camar068]

another thing to look at just for your knowledge is STFT and LTFT. (short and long term fuel trim).

differences between 1 bank and the other of STFT or LTFT may give you an indication of an issue as well. They should be relatively the same or hover around each other in readings.

just a quick google search, there are plenty more out there:

https://www.thedrive.com/cars-101/38138/what-should-long-term-and-short-term-fuel-trim-be

 

[Camar068]

sorry flooding you with info. Pic of the graph gauge I was talking about....so you could see it through a period of time.

Anyway, the below pic was before an O2 sensor replacement. See how they look different? Honestly, this may be fine with the condition of the engine. But after I changed the bank 2 sensor, the truck ran better and they looked more the same graph wise.

[edit] sorry i changed both cuz they were used o2 sensors from when I did the swap. They looked like the left one when I was done.

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[Matt69olds]

I’m trying to read the schematic you posted on my phone. if I blow it up to where I can see individual circuits, the image gets fuzzy.

I think circuit 419 is the check engine light? Easiest way to test for a open circuit? Will the check engine light come on with the key on, engine off? Check the bulb. Check all the fuses, make sure the ones that are installed are good, and none that should be installed are missing.

You didn’t specify what model/year your working with, but all GM cars from model year 1980 and up used something called a Remote Lamp Driver. It looked for a signal from the ecm, if the signal isn’t there, it was designed to turn on the check engine light. The purpose of the lamp driver was to illuminate the check engine light in case the ecm isn’t capable of doing so. If the ecm lost power, or for some reason couldn’t turn on the light, the lamp driver would, alerting the driver that something is wrong.


I only mention this because the odds of both the ecm and the lamp driver (assuming the trucks had them) failing at the same time are pretty remote.

Once again, I don’t know what year your working with, so this might be worthless info. The ecm doesnt control idle speed, there is a stepper motor or solenoid that mechanically raises idle position with the A/C on. If the idle is messed up with the A/C, you need to look at whatever control idle, and how it’s wired.


The TPS is a simple 3 wire sensor. One wire will have a 5 bolt reference, one will be ground, whatever is left is the signal to the ecm. Before buying any scam tools, make sure the 5 volt reference and ground is good at the sendor plug.

 

[Snoots]

Use a straight pin to go into the back of the TPS or O2 connector.

The idle should drop (a little) when the compressor kicks on. If the AC works fine, carry on.