Mechanics

EFI 4A-FE ECU HAS BURNED TRACK.

1991 Toyota Corolla • 160,000 miles

Background: I followed the Haynes manual directions to check a bad regulator, and shorted the F terminal on the regulator. Car shut off immediately EFI fuse blown. I found the wires from the alternator to the battery and engine fuse box were really bad and fixed them. I traced the EFI circuit short to the computer (ECU). Only the power in and ground terminals connected to the ECU and it blows the fuse. I took out the ECU and found a burned out track inside.
Is it likely that some component (O2 sensor, igniter, coil, air flow meter, etc. Etc) that is connected to the ECU shorted and burnt out the ECU? How can I/ should I test the sensor components to see if they have shorts in them before I put in a new ECU and burn it out? Can I hook up the wires to the ECU (power and ground first) one at a time and see if a wire/component burns it out? Is there a better way--checking resistance on each component?
Avatar
Tobiack
November 22, 2011.




Disconect the all the wiring connectors on the computer and see if the EFI fuse will blow w/o it-


Tiny
Rasmataz
Nov 22, 2011.
I have given you the closest thing to a pinout diagram that we have. It is the engine control or TCCS, which is driven by the ECU. It does however have most of the sensors runing through it and it only has some ECU inputs as I am assuming, I do not have true ECU diagram, going by what I have that the ECU is broken into several PCM like the TCCS. This is different from Hondas, that is what I am used to, which have a central ECU that handles all controls for the whole car. The service manual that we have does not have all the information that a factory service manual would have depending on the manufaturer as some do not release all information. That is why factory service amnuakls are several hundred dollars. What I gave you is a little hard to follow as you have to count the terminal number and then keep tract of the circuit by the color of the wire. You can follow each connector to its sensor.
However, testing an ECU or PCM is tough without a specific pinout map and what type of test to run between which pins is not advisable. It gets so complicated that I do have the output signal descritptions that you could use to test the ECU, but it would take 50-75 screenshots to give it to you and we do not have the gate size to accomadate that. For example the O2 sensor uses both the intake air voume temperature and the coolant temperature into account before it sends a voltage signal or not. So, you would need the proper pins and the input for the 2 sets of pins for the Intake volume temp and the coolant temp. Some circuits use potentiometers and I am not sure how you would test that, probably using a min and max input value and testing the output. So, it gets to be too tough. The best you could do is to test for continuity or resitance to see if circuit is shorted or not. Then since you have the issue with the ECU, it will have to be rebuilt anyway. If you use this site's search engine, you will find that it brings up banner ads for repair facilities that rebuild ECUs. They are generally set up to be ones that are located nearest you. So, I would call them and ask if it is worth the time to test as given the results, you are going to have to fix the circuit board one way or the other. If you are equppied to do that, you may save some money by buying the schematics for the ECU and biulding the circuit yourself. You can find people on EBAY or search the internet to find schematics for sale. Your car is old enough that the rights may be released. You could go to an Advanace Auto or Auto Zone where they carry Haynes manuals and see if they have an ECU test that is going to be helpful. Even what I have does not give values to work with. You would be stuck with working with voltage and resistance.
I am still attaching the TCCS, which is not the ECU as far as I can tell, but it may very well be as it seems to have all the controls that an ECU would have. Just goes by a different name and since I don't have a pinout chart, I assume it is an ECU whether it is the one you are looking for or not as I cannot find anything on a higher level than the TCCS.


Rasmataz, the fuse does not blow with the computer disconnected, but I have been systematically disconnecting tons of stuff in the engine compartment and leaving them disconnected. Since many sensors get power before the ECU (on my wiring diagram), if they were cooked they would blow the fuse with no ECU, right? So I will plug things in one at a time and see if it blows, right?
Thanks


Tiny
Tobiack
Nov 22, 2011.
DrCranknWrench, I will plug in sensors (until the fuse blows) as described in my response to rasmataz. If I then check each pin at the computer fro resistance (ohms) and voltage (DC 12v) will that be safe? Since many sensors are on the positive side, will I need to check between each pin and ground, and each pin and positive? Is checking for continuity with a multimeter a bad idea because I could short out something by supplying voltage when it is not supposed to get it?
Thanks


Tiny
Tobiack
Nov 22, 2011.
First, rasmataz is an expert Tech that has been at this so long information from him is priceless. You can see how eager we are to help.
Follow his advice as it is excellent advice in any case.
As far as testing the ECU, sticking with continuity test is safer and since you do not have a full pinout, well you sort of do, and a test set of instructions, it is advisable to stick with a resistance type test. Looking at the signal you would expect to see from the ECU dependant upon what the signal is for, infinate resitance or no continuity for an open circuit in a no voltage state or no resistance and continutiy in a closed circuit no voltage state. This will at least give you an idea if there is a short or closed circuit in a circuit where it should not have continuity or be open.


Does the EFI fuse blows when computer is connected when the ignition switch is turn On or it blows while connected without turning the key-

You gonna blow the EFI fuses a couple times or maybe more to get to this culprit or make a jumper using a reverse lite socket and go across in place of the EFI fuse then start disconnecting sensors the one that turns off the lite will be the trouble circuit

I want you to find the circuit opening relay-remove it, now remove EFI fuse and then connect all the wiring connectors on the Comp then put the fuse back on-did the fuse blow? Let me know

I've been here before with Toy Previa same fuse blowing and its a tough one to deal with-it turns out to be the airflow meter

A wiring problem could either be shorted, grounded, open and crossed-


Tiny
Rasmataz
Nov 22, 2011.
Rasmataz,
V Does the EFI fuse blows when computer is connected when the ignition switch is turn On or it blows while connected without turning the key- Only with key ON.

You gonna blow the EFI fuses a couple times or maybe more to get to this culprit or make a jumper using a reverse lite socket and go across in place of the EFI fuse then start disconnecting sensors the one that turns off the lite will be the trouble circuit Is this only if the fuse blows with key OFF? Computer connected or disconnected?

I want you to find the circuit opening relay-remove it, now remove EFI fuse and then connect all the wiring connectors on the Comp then put the fuse back on-did the fuse blow? Let me know
If the circuit opening relay is the EFI relay right next to the EFI fuse, (gets power from the ign circuit as well to actuate the relay) then: 1) the EFI fuse does NOT blow with the EFI relay removed, or with the ign 10A fuse removed.

It has been raining and I have not plugged in all the sensors in the engine compartment to see if the fuse blows with no computer yet.
Thanks


Tiny
Tobiack
Nov 22, 2011.
The circuit openng relay is under center of dash in console
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Tiny
Rasmataz
Nov 22, 2011.
Rasmataz, with everything connected except the computer, the fuse is OK with the key ON.
Then, with only the circuit opening relay removed, and the computer B+ and ground connected to the computer, the fuse blows.
(I did not bother connecting the other 30 wires to the computer, none were crossed)
Does this mean my components are OK? Only the computer is bad?


Tiny
Tobiack
Nov 23, 2011.
Sounds like its coming from the computer


Tiny
Rasmataz
Nov 23, 2011.

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