Why does battery voltage reading need to be accurate?
These days injector behaviour at different voltages is well understood. The speed of opening and amount of fuel delivered vary with voltage. Corrections are made in fuelling calculations to account different performance for differnt voltages at the injector. If what the Megasquirt ECU understands is the voltage at the injector is different in reality, then it is likely fuelling calculations will be incorrect. This is why the battery voltage calibration needs to be correct in your Megasquirt ECU.
My experience with incorrect battery voltage calibration resulted in leaning of AFR when the charging system became heavily loaded by the electric radiator fans. The Megasquirt MS3 was reading a different voltage than what was real. This resulted in the injector pulse width being incorrect delivering less fuel. The leaning affect was most pronounced at idle when the alternator is operating in lower RPMs.
There is some advice which suggest leaving the calibration as is and alter the injector dead times to balance out AFRs as various voltage conditions. I attempted this and found the method to be time consuming and inaccurate. Injector manufacturer’s normally have invested time and provide ample information on how the injectors behave, why change inputs?
So how do I correct my Megasquirt battery voltage calibration?
These are the steps I used to verify and calibrate battery voltage on your Megasquirt ECU. You require a digital multimeter and Tunerstudio connected to your ECU. The steps are:
- With the engine not running and ECU powered, measure the voltage across the battery terminals. Now measure from a live side of an injector plug and the negative post of the battery. If you’ve wired the ECU correctly the difference between voltages should be minimal. If not you should investigate your wiring for voltage drops and correct prior to proceeding.
- Open Tunerstudio and set up a BattV gauge in your cluster. See what reading the ECU is seeing in comparison the voltge at the injector. If you are within 0.1V then that is likely good enough. If not proceed to step 3.
- Open the calibration dialogue box in Tunerstudio through Tools-> Calibrate Battery Voltage.
- I scaled the value in Max ADC Count voltage based on what my digital multimeter read at the injector and what the ECU was reading. Ie. (12.2V actual / 11.8V ECU)*29.7V = 30.7V. Burn your changes to ECU and verify the difference between ECU and battery. I used 0.1V as my tolerance.
- Verify your changes with the engine running. Fire up the engine and while measuring the voltage at the injector verify the difference to the voltage at the ECU. With luck the difference will be the same as in step four. My difference was 1% so I chalked that up as a success.
After correcting the Megasquirt battery voltage calibration for my installation, my problems disappeared. No leaning out under heavy electrical load and lower battery voltage. I also noted a slightly easier to start engine likely because the correct injector dead time was properly referenced.
I suspect if you’ve read this article you may have been in a similar situation to myself. Did this article help you? Were you able to solve your problems?
My YouTube channel: https://www.youtube.com/builtonpurpose