ECU battery?

[jp455]

Hello all...was watching Jay Lenos garage the other day when they had the Koeniseggngng on. When they were doing the walk around they mentioned that it had a cut off switch, to which Lenos asked how they took care of the ECU not deleting the programming. Now, I've had a completely flat battery many times, as well as disconnecting the battery so it didn't go flat and I never had an issue like this. I did have a VW tech tell me this once also but I never had any issues with my VWs from doing this. Granted, I'm talking about OEM ECUs....does this sort of thing actually happen with atermarket ECUs? If so, would it be possible to wire it so that theres a small battery that is only connected to the ECU and a cut off switch that only disconnects the main battery?

[Samckitt]

The memory that is programmed into the ROM will stay, the memory that it "learns" from driving that is stored in the RAM will erase if unplugged. Not sure if there is a battery on the board or not.

[jp455]

I assume thats the case for ECUs that have fuzzy logic. The ones that are programmed manually I assume keep the program in ROM. In any case, how about wiring a small battery just for the ECU so its never out of juice?

[LS1NOVA]

I think you wire a 3 amp circuit accross the cutoff switch so the ECM keeps enough juice to save its learned memory.

[jp455]

Didn't think about that. Is this a common problem though that people work around? I just never seem to hear about it when aftermarket ECUs are being talked about.

[GetMore]

When you program an aftermarket ECU the info is stored in "non-volatile" RAM, so it will not be lost when power is disconnected.
I know the old GM ECUs had the engine program on a chip (which could be replaced). This chip would keep the information even if the battery was disconnected. However, the adjustments that the ECU would make to fueling or timing was stored in "volatile" memory, and would be lost when power was lost.
On modern ECUs and PCMs I don't know how much is stored in non-volatile memory. The programming definitely is, and the emissions monitors definitely are not, but beyond that I don't know. I have seen problems with lots of Chrysler vehicles and some GMs with them "forgetting" how to idle after the battery has died or been disconnected. (BTW, it just takes them a few minutes to relearn the IAC setting, but it's a pain if you are trying to do it right after jump starting the vehicle. All you really need to do is hold the throttle open a little bit, to keep it from stalling, and slowly let off the throttle, until it idles on its own, and then wait a minute or so before reving it to see if it'll idle or drop.)

[jp455]

well definitively learning some new stuff! Wasn't aware the idle was stored like that. I figured it would be a fixed setting. Ok, so maybe I'll go with the bypass of the shut off switch to the ECU. Guess I'll have to finally pick one and ask them about it.

[CarlC]

Yes on deleting the learned part if the ECU power is lost. The 3A bypass works when a master power switch is used but it will blow if you try to start the engine with the master off. ECU memory lost again.

Think you will always remember to turn the master on? Nope. Ask me how I know.

[jp455]

I can imagine that happening! Although on that subject I have been wanting to do a ingnition sequence for starting the car when it gets rewired. Meaning I would have to manually switch on main power, fuel pump, ignition, and then push the start button. So all the switches would be in a row next to the start button.

So is having a small independent battery to power the ECU even an option?