What is adaptive memory?
As powertrain and engine control modules (PCM/ECM) incorporate feedback from a network of sensors, the computers change fuel delivery requirements based on engine RPM, load, and expected versus actual engine performance. The PCM/ECM then calculate adjustments to be stored in long term memory for future use. This long term memory is referred to as adaptive memory since it’s really a learned modification based on actual conditions.
Once a PCM/ECM accumulates these modifications, it refers to them on subsequent engine starts and refers to “lookup tables” so it doesn’t have to constantly relearn every modification for every condition.
For example, the PCM/ECM may have stored a modification for an engine operating at 1,000 RPM with a 30% load. It stores that information in a cell, similar to an Excel spreadsheet. The next time the PCM/ECM sees the engine at 1,000 RPM with a 30% load it refers to the lookup table, find that particular cell and applies the specific long term fuel trim (LTFT) modifier.
This article discusses adaptive memory. To learn more about adaptive memory, fuel trim, short term fuel trim and long term fuel trim, read these posts:
What happens when you disconnect a car battery?
PCM/ECM contain capacitors that store a certain amount of power
to “keep alive” adaptive memory. However, the capacitors drain after about 30-seconds and adaptive memory is lost. When power is restored by reconnecting the battery cables, the PCM/ECM no longer have access to the cells and all LTFT values are gone.
If you don’t own a scan tool and disconnect the battery cables to erase a trouble code and turn off a check engine light, you should EXPECT the engine to run poorly until the PCM/ECM can relearn adaptive values.
How to force an adaptive memory dump
Many technicians force an adaptive memory dump after replacing major fuel related components to speed up the relearn process. To force an adaptive memory dump, simply disconnect both battery terminals and touch them together for about 5-minutes. This battery terminal “short” drains all PCM/ECM capacitor power which clears all current, pending and history trouble codes as well as adaptive memory.
What does my car run poorly after I’ve installed a new battery?
As mentioned above, disconnecting the battery wipes out all adaptive memory. On the first start after reconnecting battery cables, the PCM/ECM can’t refer to any previously learned data. On an engine with high miles, this situation can cause rough idling, poor acceleration and even stalling.
How long does it take to relearn adaptive memory?
It takes approximately 10 cold starts with a running time of approximately 15-30 minutes for the PCM/ECM to completely relearn LTFT values. During this time, the engine will run progressively better with each cold start.
©, 2018 Rick Muscoplat
Posted on by Rick Muscoplat
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