MacBook Pro 15 820-2850 both fans pulsing every second

[Cocobill]

Hello,,
I've got a MacBook Pro 15 820-2850 board that has a green light on the charger, both fans are pulsing every second. If I keep SMC_ONOFF shorted, the pulsing of the fan occurs at twice the speed. PPBUS_G3H is fluctuating between 12.54V and 7V. I also changed the C9560 cap, did not help much. Also measured PP3v3_S5 in diode mode and got .142. What do I fix first?

 

[dukefawks]

Check all rails for shorts. Does it charge a known good battery?

 

[Cocobill]

No, it does not charge a known good battery. In diode mode PP3V3_S5 is 0.140v. Is that normal?

 

[dukefawks]

.140 could be ok. What is diode mode with the leads reversed? Resistance to GND? History of the board? PPBUS stable?

 

[Cocobill]

Diode mode with leads reversed reads 0.515v, resistance to ground is 10.42 kOhm. History is unknown . PPBUS is NOT stable

 

[dukefawks]

Need to fix PPBUS issue first. Check resistance between pins 17/18 27/28 of U7000. Inspect for corrosion around TPS51125.

 

[Cocobill]

Resistance 17/18 is 3.5 Ohm, and 26/27 is 21 Ohm. TPS51125 looks clean.

 

[dukefawks]

Replace the 51125. This one is going to suck it looks like.

 

[Cocobill]

Replaced 51125, PPBUS_G3H is stable at 12.52V. However, it does not charge a known good battery.

 

[dukefawks]

PP3V3_S5 present now?