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@2informaticos as you suggested I have replaced that chip with a new TMP423B from DigiKey.
Problem still exists, CPU fan is spinning up to maximum speed, like when pins 4 & 5 were missing, even though the 7pin ACDC data cable is connected.
This is what has changed now that I have replaced the DAFI TMP423B on the logic board: when measuring diode mode between pin 4 to ground and pin 5 to ground I measure around 0.7V even when reversing the multimeter leads (before one way was open line). I assume this is good?
BUT what I haven't measured before is when I do the same test of pin4 to ground and pin5 to ground ON THE PSU, then I get open line (both ways when reversing the multimeter leads).
What does that mean? Is something still broken but on the PSU side?
Also I have opened "iStats menu" sensor page and this is what I get so it is still missing some sensors but I don't know which ones are missing by default and which one is missing because of the fault that is causing the CPU being pinned to max.
Unfortunatelly, we don't have exact schematic for this board.
Considering 00134 as guide, U5600/50 are both connected in the same bus.
If U5600 got damaged, it can block the communication on entire bus.
CPU proximity, as PSU temp are controlled directly by U5600.
TMP423 can control 3 remote sensors, plus its internal sensor.
You should check all its adiacent resistors.
The remote sensors are simple NPN transistors, connected as diode; more temp sensitive than a simple diode.
You should check/replace the sensor form PSU; all sensors connected to U5600 in fact.
Compare the readings with other sensors, connected to other temp interface (U5650).
Remove U5600 and check again the temps reported by macOS.
Thank you very much for your detailed reply @2informaticos I really appreciate it.
Out of curiosity I've checked the "iStats menu" sensors readings after disconnecting the 7-pin PSU "data" cable and I got the same readings as in my previous comment (comment #29) when the 7-pin cable was properly connected.
Because I urgently have to figure out how to get my iMac working without the 7-pin data cable (to make it battery powered so I can work with it in the middle of nowhere), I have bought another iMac exactly like this one I have messed up so I can test how to bypass the 7-pin cable.
(I'll fix this other one later when I have time)
In comment #11 you said:
"Once you get it working correctly with all the normal connections, then disconnect J6901 and solder a diode, or NPN transistor between pins 4 & 5.
Check Q5605 in schematic, to make an idea."
Could you please explain to me in detail what I should do? I'm lost..
What kind of diode do I need? Which way should it go --> from pin 4 to 5, or from pin 5 to 4?
You don't need a diode.
The corresponding NPN transistor is connected in diode mode; B & C tied together.
Just get one from any MacBook, or iMac board.
Solder it in the same configuration, between L5610/11 lines; be aware, B+C go to P line and E goes to N line.
If I understand you correctly I solder B+C to PIN-5 and E to PIN-4 of J6901 and that should make the logic board think that the PSU temp sensors are connected as they should be even if the 7-pin data cable is disconnected?
If I don't have access to any donor MacBook or iMac boards can I buy an NPN from Digikey? If yes, could you help me find which one to buy?
@2informaticos YOU ARE A GENIUS!!!
The NPN transistor is giving the logic board the temp readings so it is not missing the 7-pin data cable, which means I can replace the original ACDC power supply with a DCDC supply so it can be battery powered!!
Thank you very much again! Battery powering it was the most important thing for me.
When I have free time I have to fix the other iMac, the one I started experimenting with and shorted out. As you suggested I'll go through the resistors on that data line to find out what is causing the "AC/DC T2 Sec Heatsink", "CPU Proximity", "CPU Backside" and "Skin" sensors to be missing.
If it is the SMC then do you think I might be able to find a replacement anywhere online?