ECU Stability vs Fluctuating Voltage

addo · Post by **addo** » 30 Jan 2010, 21:06

One for the "brains trust".

How much flexibility (tolerance) is built into typical automotive ECUs to allow for transient changes to voltage? Or is it more that current can be the problem when affected by a heavy external draw?

It comes back to my recent dead battery, and in the months prior, a succession of error (fault) codes occurring.

What I'm suspecting - without anything more than peasant cunning - is that a dying battery's "shallow" capacity for accommodating power drawdown spikes, causes the errors. Culprit #1 to me, is cooling fans - such a heavy wattage.

Looking forward to informed commentary on this topic!

Thanks, Adam.

HDI · Post by **HDI** » 30 Jan 2010, 22:44

Basically ECU's are generally designed to cope with a supply voltage as low as 9.5 volts. Reference voltage sent to sensors is 5 volts , low enough below the minimum envisaged ECU supply voltage to prevent error readings.
If the supply voltage is below the lowest acceptable to the ECU the engine will just fail to start.
ECU's are designed with filters to suppress spiked supply voltage to some extent but some supply conditions can cause ECU errors or even damage.