No Backlight Macbook A1990 820-01041

[Comp_Pro]

Hello all,

Am trying to figure out this backlight issue with this A1990 Macbook Pro.

Tests performed:

There is no backlight even with a good screen.
Backlight IC is receiving the correct support voltages.
Fuse F8400 ok
Current sense resistor R8400 ok
Backlight IC is receiving 3.3V on enable pin.
Backlight IC receiving voltage from PPBUS_G3H, aprox 12.5V.
Backlight IC receiving 5V from PP5V_G3S
Backlight IC BKLT_SCL and BKLT_SDA both measure 0.545V diode mode to ground
I2C_BKLT_SCL and I2C_BKLT_SDA both measure 0.546V diode mode to ground.
I2C_BKLT_SCL and I2C_BKLT_SDA both measure continuity between LCD connector to R8452 and R8453 although show 4 ohms resistance each.
Replaced ribbon cable from motherboard to LCD board.
Replaced backlight IC U8400

I also measured every single resistor and checked every cap related to IC U8400 and did not find any bad components. I also replaced U8400 just in case with no luck. The only thing I can think of so far which may be the issue is that when measuring continuity of I2C_BKLT_SCL and I2C_BKLT_SDA from the LCD connector to R8452 and R8453 near U8400 measures about 4 ohms each. Probably due to the trace size and distance between the two measurement points but not sure if this could be a factor.

Any ideas you guys could throw at me would be appreciated.

[reformatt]

What about resistance to ground at cathode of D8410? Most common issue with these machines is a shorted tantalum cap on the output.

What voltage do you get on the backlight output? If you have 0, then either Q8400 isn't being enabled or the output is shorted. If you have 12.6V, then Q8400 is being enabled, but the inverter is not switching. A lot of issues on the older A1466 for me (apart from dead IC's) are corroded traces on the feedback path so check for continuity in that area (from D8410 cathode back to the IC via voltage divider R8431/R8432).