NB MX5 Alternator control

[Chris250]

Just looking for a little bit of help regarding the alternator control on an MX5, it uses PWM and I've got it dialled in so the battery shows 14.5 as per the target but the "flywheel supply" is showing over 18 volts.

Has anyone else got this matching up?

NB MX5 Alter test.pclx NB MX5 Alter test.llgx

[Adamw]

Flyback voltage from the idle solenoid etc is designed to be dumped back to the battery, on the NB1 ecu this goes out of pin 53 (1A) which should be a constant 12V live supply.  If this is lost the flyback has nowhere to go so "pumps up" the internal voltage to something higher than vbat.  

So start by checking pin 53 still has a constant 12V on it - possibly a fuse blown or something.  

[Chris250]

You were right on the money Adam, broken wire right at the ECU header on pin 53.  With the wire fixed the idle valve was a lot quieter as well.

The flywheel voltage is within 0.3-0.5 volts of the target alternator control voltage but is still showing as a fault on the alternator control.  How far out from target will it throw a fault?

 

[Adamw]

The aux reporting a fault on these is normal.  One of the tests the ecu does when the aux is off is an "open short test", it expects to see >2/3 bat voltage on the pin when off (due to the pull-up).  These alternators however have a clamping diode in them which holds the aux voltage below about 7V so the test fails.  It will still be functioning correctly.