Background story:
I was completing my first time custom watercooling rig, it was a full EKWB set. EK Velocity DRGB cpu block, EK-XRES 140 Revo D5 RGB PWM pump res combo, EK Vardar ER fans, and EK CoolStream PE 240 radiator. Simple loop, with soft tubing. The reason why I choose D5, just because I may expand this loop in the future.After the setup was complete, completed the leak test and everything else, I hooked it to my system. Everything works perfectly. PWM control and RPM readings are all good on both the Vardar fans and D5 pump.
Then I decided to tidying up the cables a little bit, and reconnecting all the fans/pump PWM to the motherboard. One thing I noticed, that suddenly my D5 pump run at full speed, and the RPM reading was gone.
0 RPM reading
The Problem:
After a closer inspection I saw that my PWM/tach connector from my D5 pump was connected to the motherboard in an offset position. I quickly reconnect it to the proper position, but alas, the RPM/tach reading was already dead. The PWM control still works though, but I have no RPM reading from the D5 pump anymore.I quickly googled it, and found out that many people also reported that the RPM reading from their D5 pumps (through various brands) were gone. But I found no good answer and/or fixes for this issue. Furthermore, many of the results were from quite old forum posts.
I also asked my local EK distributor about this issue, they tested their pump, and also have no RPM readings. Which then got me thinking, "is this quite a widespread issue?"
Then I begin to diagnose it myself. At first, I noticed that the 4 pin PWM connector from the D5 is not the same as a common PWM fans. On normal PWM fans, the 'clip guide' / 'notch' or whatever it called (edit: it's called "Polarizing rib"), it is positioned on between the 3rd and 4th pin, while in D5 connector, the rib is all the way over the connector's width. This shows that D5 is using the more common 4-pin small molex connector, instead of the more standardized version which have that rib in the middle (like Molex 47054-1000 part number which mentioned on Noctua PWM Specification White Paper).
Left: Proper PWM fan connector | Right: D5 PWM connector
This connector, even though works, will have a chance to be connected on an offset like I already did. Now here's the catch, if the pump is powered by MB fan header, it will probably just fine (it just won't turn on). But on D5's case, when the pump is powered by external molex connector, it is just get messy.
Proper connector, minimal risk of plugging offset
Improper connector, it is possible to shift/offset the connector like this
As per Intel's standard for PWM (also mentioned on the Noctua White Paper), the RPM/tach output (pin 3) is an open collector (or open drain for a mosfet), it needs to be driven with a pull-up resistor. However, if by accident the connector is on an offset, this tach output will be connected to a low-impedance, 12v voltage source, to the transistor, straight to the ground via external molex power. Although I didn't smell any magic smoke escape.
Improper open collector driving, creating a low impedance path, thus a large amount of current is dumped into the transistor
Proper driving procedure of tach out signal by Noctua.
Noctua even have current limit to tach output on their PWM white paper
The Fix:
After searching around for posts where people disassembled their D5 pumps, I took a closer look at where the RPM cable goes. And sure does, it goes straight into a tiny SMD transistor inside the D5. Then I bought some generic NPN SMD transistors (picked up MMBT3904LT1 from my local electronic store, but any NPN signal transistor should work I guess, since RPM is not a high speed signal), cracked open my D5, and start probing around that transistor.
The suspecting transistor (red circle), and the RPM/tach signal cable besides it (blue cable)
(Credit to someone who shared this internal pic of his D5 pump, sorry I've been unable to track back the source, I think it was from overclock forum. Let me know for the credit! I totally forgot to take a pic of my D5 internals when I fix it, but it's pretty similar.)
(Also, yes, sorry for the confusion, D5 have this color flipped between PWM and tach cable.)
I don't have any schematic for D5, and I don't know how Laing/Xylem implement their RPM signal circuit, but after probing out, I was certain that the RPM cable runs straight into the collector of that transistor. Probing further, I found out that the base and emitter of that transistor was shorted, even though there is a 100k resistor between that pins.
MMBT3904LT1
So then I pulled out that smd transistor and replaces it with the new one. Reassemble it, put a short tubing, fill it with distilled water, plug it in to my RPM/PWM tester, and voila, the RPM signal is back! I quickly drain it and reassemble back to my system, fill it with EK cryofuel, let it fill, turn my system on, and there you go, now I have my RPM signal back!
RPM is back on, hooray!
Summary:
It is quite possible that I fried that transistor by plugging the connector on an offset. That small transistor is only capable of sinking several hundred milliamps of peak current, while modern MB fan header can source current for around 2A or more.I don't know if it's a brand-specific or not, but EK might have a habit of putting wrong connectors to their products. My local EK distributor told me many times to be careful when plugging that Velocity block RGB header, as EK use 4 pin female connector while in fact it uses 3 pin addressable RGB. Well, I think the same goes for their D5 connectors, although I can't really blame EK for this, since I saw some D5 from the other brands have this same connector.
As for the warranty, well, I'm sure I have voided my own D5 because I've been disassembled it. But I'm all in with self-repair movement! 😁 Anyway, if you happen to blow the tach/RPM circuit by plugging in the wrong way like this, I'm pretty sure you should be covered by warranty. Especially if they used incorrect connector like this.
However, I think this is still a rather serious issue. If the MB's fan header don't have any current limiting capabilities, it is also possible that not only you'll fry your pump's tach circuit, but also your MB's fan circuit.
Edit:
After digging deeper into image search, I saw some EK D5 pumps that already have a proper PWM connector. I don't know if this is a batch-related or not, but takeaway on this issue is do plug your D5's PWM connector very carefully, especially if your pump comes with this kind of PWM connector. Also if you want to be safer for future uses, just replace the connector with a proper one (steal it from PWM fan or something 😁).
Links:
- Noctua PWM Specifications White Paper:https://noctua.at/media/wysiwyg/Noctua_PWM_specifications_white_paper.pdf
- Intel 4-Wire Pulse Width Modulation (PWM) Controlled Fans:
https://docplayer.net/23812689-4-wire-pulse-width-modulation-pwm-controlled-fans.html
