E38 ECU Kept alive by polling CAN

[ruckmaj]

Hi All.

First time posting here,

Im an electronics engineer for a design company and i'm trying to find info on the e38 PCM specifically in regards to the ECU being kept alive by external devices polling.
The devices we develop do high resolution incident logging and use the CANbus for data acquisition and forwarding accelerometer data. I've found that these devices are keeping the ECU's alive and running down the battery over a day or two.

My question is, whats the timeout for the Vref to go low when the ECU enters a low power mode after ignition is removed. I thought i had a solution by changing some programming in our equipment to look at the chassis voltage but its not working due to the complexity in different vehicles so i'm doing some more hardware logging to determine the timeouts and voltages. My plan was just to watch and log the voltages on ignition, chassis power and the vref off a sensor to determine what the vehicle is doing.

Any help is appreciated

Adam

[antus]

You should be able to request RPM data fro the SAE pid from the computer, and when below a threshold, eg 100 rpm, shut your device off. Being an SAE PID would work with anything. I dont know what the bus timeout is, it probably varies between manufactures. PID 0x0C see here: http://en.wikipedia.org/wiki/OBD-II_PIDs

[Tazzi]

Well, Iv noticed on the VZ V6 ecu here that there is ALOT of chatter when ignition is turned on, but quickly goes *quiet* without any other devices present. Im unsure if this is normal or if the ecu goes into some sort of power saving mode or error mode, but all other requests work.

Antus has the right idea. If you are reading from the BUS, then a simple SAE PID request such as RPM will work perfectly for detecting if the device shuts off. If ignition is turned off, and the ECU no longer replies (generally there is a curtisy timer that allows further communication for ~30sec) but in worse case scenario, you can make the device retry ~3times before turning off/ waiting for ignition back on.

[immortality]

Sounds like it's a device used to monitor data in an accident situation so you would want it to continue to operate for period even after the engine shuts down as could be the case in an accident where the engine stalls but the car is still moving and bouncing off other vehicles or solid objects before coming to a rest.