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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

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Old 02-08-2013, 09:12 AM   #1 (permalink)
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I measured the voltage at the ECM and my readings are BAD! PICTURES INCLUDED

So for the last few days I have been trying to figure out why my computer keeps throwing code 24, which is intake air temperature. Well I finally found the yellow brown wire that is suppose to go to the THA so I hook it up to the ecm check the resistance between the wire on the connector in the MAF and it all is good. So then I went to the ECM with the Ignition on and started measuring the voltage. Here is the Chart I used: http://personal.utulsa.edu/~nathan-b...ne/44engin.pdf

Well when I measured between E2 and THA I got no Voltage and I should have got smoething.

So then I tried VTA-E2 with throttle Valve fully closed and it gave me around 5.5 volts, Well if the throttle vavle is closed It SHOULDNT have gave me any Volts maybe like around .5?

So now the question is what is causing these bad readings? Im I doing something wrong? Im thinking the ECU is shot, and that is what is giving me the bad readings, Before I started a thread about a big wire mess, with all those splices, well this is what I came down too, Im thinking I need to try and find a ECM.

This is my other thread I posted a couple of weeks ago:
http://forum.yotatech.com/f116/big-w...nluded-264393/


What do you guys think? Thanks.

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Measuring the VTA:

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Old 02-08-2013, 09:33 AM   #2 (permalink)
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I have a complete 1989 4 runner wiring harness, with Computer, and everything, Im thinking of switching that in there?
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Old 02-08-2013, 11:16 AM   #3 (permalink)
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Should have stayed in the same thread makes it easier to follow, thanks for linking it atleast, but know some of us are kinda lazy and aren't going to go read it. Anyways...

Did you verify the wire you used actually goes where it should andisn't cut somewhere inbetween, eg probe at both ends air meter and computer?

Does the meter pass the tests?

What is the battery voltage at the meter?

That's all slightly academic, 'cause yeah if you found the right harness I'd definatly swap that trash out. Or maybe pull the cut one and properly rebuild it using the good one for reference.
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Old 02-08-2013, 04:14 PM   #4 (permalink)
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Yes I verified the wires going to where they should go, and yes the meter passes the test, and the battery voltage was over 12volts, I have checked everything, So the only thing that I can come up with it being is that it is the ECM? What else can I check?
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Old 02-08-2013, 06:03 PM   #5 (permalink)
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Take a resistence reading of the THA signal wire, between the ecu plug and afm plug with both ends disconnected.

Take a short test of this wire also, disconnect both plugs and test for continuity to the body. If you have continuity there is a short so take a resistence reading to ground. The lesser resistence reading should be closer to the short, if it were a printed circuit board atleast this is a little complicated in an vehicle.

Do the harness resistance checks (FSM ecu section). Anything that is out of spec in this test is either a bad sensor or bad wire.

There is great big long post on testing wires here

And of course your meter might be out of calibration, test it against another(or two or three). We're not launching rockets to mars, it doesn't have to be spot on just in the ball park and give consistent readings.

Proper (eg clean), good (eg sharp) probes, well seated and no fingers in the reading.

You can take the wiring and ecu out of the equation. Disconnect the afm plug and supply 12v to Vb and ground on E2.. Er scratch that!

Look at the diagram/schematic for the AFM notice that the THA sensor has one side connected to ground and the other to the ECU. What this means is the voltage you see on the THA at the ecu is comeing FROM the ecu! There is either a) a voltage supply problem to the ecu B) the ecu is damaged.

Again, Voltage supply to the ECU or the ECU is damaged.

So swap in the new un cut/mangled harness. Important that you don't plug the new ecu into the mangled harness! You might damage it also then your new harness won't help untill you put in a third ecu.
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Old 02-08-2013, 11:22 PM   #6 (permalink)
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Thanks for the advice. I am pretty sure it is the ECU, Who ever did the wiring before really messed stuff up! I am not even going to mess with it anymore, I am just going to put in a new wiring harness and ECU. I have a complete Wiring harness with ECU from a 1989 4 runner that was 4 wheel drive and automatic. But I was talking to someone, and said it should not be that hard to put that in, just make sure the Ecu gets power from when the ignition is on, and the and the automatic safty switch is connected, So it will start. What do you think about putting that harness into my 2wd truck? It shouldnt throw any codes should it? Because I will not have the 4 wheel drive part of it hooked up? I am thinking I could probally do this in a weekend?
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Old 02-09-2013, 12:50 AM   #7 (permalink)
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Red face

looking at your pictures I can see how bad the some of the wires are corroded that can be some of your problem.
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Old 02-09-2013, 12:50 AM
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