Thanks Kurt.
The LED pin is conn to VIN thru a 100Ω resistor, which works fine on the smaller display. The first thing I tried was upping the resistor to 100k, but that made no difference to either display. Tried conn to 3.3V - with and without...
I have two different TFT ILI9341 touch displays, running on a teensy 4.0 using the ILI9341_t3n library.
The smaller 2.8 display runs exactly as expected and it's back lit correctly.
The larger 3.2 display runs ok, but it's like the back light is...