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how to tell if ECU is bad

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Old 11-10-2011, 12:40 PM   #1
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how to tell if ECU is bad

i have been driving around with my CEL on for about 1 month. the last i had checked i had the "STA signal etc" code and i replaced the distributor and rotor. that seemed to fix it and everything was fine.

however, a new code came up, something about the EGR sensor and the air intake something or other (the little white box on top of the sensor on the intake). anyway...everything was working fine. then the idle started getting higher upon starting and staying at this high idle for a longer time then usual.

yesterday while driving the car was running along fine and then just shut down. all the lights on the dashboard lit up but there was no engine. all the accessories like the radio and headlights all still worked but the engine wouldnt start.

so i tried to check for codes while i was waiting for the tow truck. but i noticed the CEL doesnt even turn on when the key is in the ignition. its as if the bulb just burnt out, but im almost positive it didnt.

is there any way to check the ECU itself? how can i tell the ECU is bad if i cant even check codes on it?

ps: truck is a 91 3vze 5 speed.
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Old 11-11-2011, 01:35 PM   #2
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i went to autozone and bought a multimeter.

then i read around the board some more and found some people with similar problems. see here....

so i checked the gauge fuse and it appeared good, i took the dash all apart and checked the bulb just to be sure and it was fine.

Then i checked the W wire. The FSM says to make sure the "batt" plug on the ECU says 11v. mine read 0. so i checked the battery directly under the hood and got like 12.5-14.

going on, i checked the W wire port and got 0.

im not sure if maybe im not measuring this right (because this is the first time ive used a multimeter). These were my steps....
1. put key in ignition and turn truck to "on" (but not start).
2. put multimeter (innova 3306) to DC Volts (20...i think).
3. put negative cable to a wrench i had next to me.
4. put the positive wire in the W port squishing it in to try and reach a contact.

and thats how i got my 0 reading. actually...sometimes it reads negative and then moves to 0.

any help will do. just trying to see if my ECU is just bad...or if im just using this machine wrong.
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Old 11-11-2011, 01:51 PM   #3
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ECU (ECM) testing procedure below. Keep in mind all tests must be performed with the key in the 'on' position (engine off).

You can test problem circuits individually or you can take the extra time and check them all (recommended).

Good luck.

Last edited by BMcEL; 11-11-2011 at 02:03 PM.
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Old 11-11-2011, 07:51 PM   #4
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yeah thats the page in the FSM I was looking at that told me how the voltage should read 11+v when the positive is hooked to "batt" and i got a 0 reading.
i left all the connections plugged in (like it says) and turned the ignition to on. 0 readings all around when putting the probe into the connection.

am i supposed to be sticking the probe into the connection where the wire meets on the inside? or am i supposed to be piercing the rubber shielding of the wire?

im not yet convinced that im using the multi right.
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Old 11-12-2011, 10:49 AM   #5
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if it looks like this
Name:  Cnv0184.jpg
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Size:  86.8 KB
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