I was wondering if removing the battery cables for say 5 or 30 minutes will completely reset the ECU. Or will using a scantool do things that removing the power won't. The car in question is Toyota 4 runner 2011, but this is also a more general question.
removing the battery cables for say 5 or 30 minutes will completely reset the ECU
Nope. Not completely.
By the way, you can reduce that time to about 10s by just pressing the brake pedal. This will reset your radio and a few sensors and maybe clear out a glitch.
will using a scantool do things that removing the power won't
100%
You cannot replace a diagnostic reset. Some things will still be there in permanent memory when you're pushing up daisies.
I guess that really depends on what you're trying to accomplish. A sophisticated scan tool with bi-directional functionality is able to do a whole lot more than simply discharging your electrical system.
If you want to thoroughly deplete the electrical energy from the car's system you can accomplish this by disconnecting the battery and turning on lights, etc until all the current has discharged. This won't reset or clear codes but it can help reset the ECU.
If you search the board you'll find this kind of question has been asked many times. The short answer is that disconnecting the battery for 20 or 30 minutes will reset the computer. If you're in a hurry turn on the headlights or work the brake lights and any residual power will be cleared almost instantly.
@chucktobias I have searched the web and some say 5 minutes, others 30 minutes. So maybe if the headlights are turned on it can go faster when the cables are disconnected. Hopefully that would not draw to much current to damage something. What I want to know, is if the effect of disconnecting the battery is the same as using the scantool. Perhaps when you use the scantool it also resets other modules/controllers which removing the battery won't. I had seen a post on the web that suggested such a thing, so I hoped that someone knowledgeable could clarify that.
You're overthinking this. What specifically do you think might be damaged by turning on the headlights or hitting the brake with the battery disconnected? After disconnecting the battery the only thing left in any module would be data stored in flash or other persistent memory such as permanent codes.
I am not saying that there would be damage. If there is a capacitor on the ECU board, and it is discharged too soon then the current might be to high and damage something. This is similar to an old style TV set where voltages would be discharged with a serial resistor to prevent too much current.
@dharter If that were the case ECUs would be blowing all over the place every time someone tried to switch something on when their battery goes dead. If you like the idea of a resistor Scotty has demonstrated putting a resistor between the battery terminals but the general consensus here has been that it is not necessary. (I don't recall the specifics, you'll need to search for it.)
OK thank you.. Someone I know installed a generic clockspring which fixed a broken cruise control. But the airbag is now broken. They would not reset it with a scantool. It was done by removing the battery cables.. So if the generic part was good then, why did it not work? Did it need to have a scantool reset it which would accomplish more perhaps syncing it with the airbags. It is a $500 part and replacing it with a generic Toyota part may not be needed. It may just need a scantool reset.
@dharter It's very common for aftermarket parts to not work properly, especially cheap Chinese knockoffs. A clockspring is a critical part. You can see what Scotty has to say about it in this recent topic:
https://carkiller.com/scottykilmer/qa/clockspring-replace-issues/
@dharter old tv sets ran on thousands of volts. car ECUs don't