Hi guys. I was messing around with the testbench I'm building and I did some experiments with the afm offset resistors. Here's the deal.
The afm offset resistors are in place to create a voltage offset on the incoming afm voltage. In other words the ecu actually reads the afm voltage minus X offset voltage. In the case of my 88t ecu (and probably most z31 ecus) the offset is 2.0volts. So if the afm actually reads 6.5volts at the pin the ecu is only seeing 4.5volts.
The purpose of the afm offset voltage is to bring the usable voltage output range of the afm down to the usable ecu input voltage range. The stock z31 afm outputs around 2.5 volts at idle and can read up to at least 7 volts. The ecu reads between 0 and ~5 volts. Without the voltage offset the ecu would only be using a voltage range of the afms min voltage (2.5v) and the ecus max (5v). Obviously this combination isnt good for high power air flow measurement and the lower half of the ecus voltage range is simply wasted. With the voltage offset in place the usable afm voltage is now expanded. 2.5volts out of the afm actually reads .5v on to the ecu and 7 volts at the afm reads 5 volts at the ecu. Now both the ecu and afm are using the majority of their measuring range.
The reason for removing/altering the resistors when switching afms is to remove or alter the offset. If the cobra or z32 afm only reads .5 volts at idle and you introduce a -2.0v offset the ecu wont read any voltage. This of course causes limp mode. By altering the offset so the new afm voltages read close to between 0 and 5volts you can get as much of the usable metering range of the afm as possible.
There are three resistors that we can play with that make up the afm offset circuit. There is R201 which is the small surface mount resistor and the two standard resistors mounted on the posts next to it. Based on my testing it appears R201 introduces an offset of roughly 1.58volts by itself. One of the resistors on the posts introduces an additional .4volt offset and the second resistor does about a .03volt offset. The two resistors on teh posts are likely there to trim the offset voltage for any variation encountered through the rest of the circuit. By playing with these resistors the offset voltage should be easy to modify.
The afm offset resistors are in place to create a voltage offset on the incoming afm voltage. In other words the ecu actually reads the afm voltage minus X offset voltage. In the case of my 88t ecu (and probably most z31 ecus) the offset is 2.0volts. So if the afm actually reads 6.5volts at the pin the ecu is only seeing 4.5volts.
The purpose of the afm offset voltage is to bring the usable voltage output range of the afm down to the usable ecu input voltage range. The stock z31 afm outputs around 2.5 volts at idle and can read up to at least 7 volts. The ecu reads between 0 and ~5 volts. Without the voltage offset the ecu would only be using a voltage range of the afms min voltage (2.5v) and the ecus max (5v). Obviously this combination isnt good for high power air flow measurement and the lower half of the ecus voltage range is simply wasted. With the voltage offset in place the usable afm voltage is now expanded. 2.5volts out of the afm actually reads .5v on to the ecu and 7 volts at the afm reads 5 volts at the ecu. Now both the ecu and afm are using the majority of their measuring range.
The reason for removing/altering the resistors when switching afms is to remove or alter the offset. If the cobra or z32 afm only reads .5 volts at idle and you introduce a -2.0v offset the ecu wont read any voltage. This of course causes limp mode. By altering the offset so the new afm voltages read close to between 0 and 5volts you can get as much of the usable metering range of the afm as possible.
There are three resistors that we can play with that make up the afm offset circuit. There is R201 which is the small surface mount resistor and the two standard resistors mounted on the posts next to it. Based on my testing it appears R201 introduces an offset of roughly 1.58volts by itself. One of the resistors on the posts introduces an additional .4volt offset and the second resistor does about a .03volt offset. The two resistors on teh posts are likely there to trim the offset voltage for any variation encountered through the rest of the circuit. By playing with these resistors the offset voltage should be easy to modify.
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