Strange voltage on 3.3V pins

Question

I have bought a new Raspberry Pi 4 and while troubleshooting connection to a DAC module I measured the voltage at 3.3V power pins. With reference to the pin header number as shown on the diagram below, the pins I tested are:

pin-1 (3V3) & pin-6 (GND) pin-17 (3V3) & pin-39 (GND)

I am using the official R-Pi supply and I measured strange voltages at 3.3V pins. Mostly it was 5.14V and sometimes it was < 1V. This is without any external peripheral connected to the Pi. I am using a Fluke 117 DMM and I checked it against a known 3.3V source to confirm it was working properly.

I don't think this is normal? Is it?

The pi itself boots up and works fine. What could be causing this?

I have read about under voltage warnings but since I am running it in headless config so how can I check if Pi indeed is having low voltage issues?

Answer 1

Unless your RPi is a counterfeit with a completely different pinout, or badly broken, the problem is with your measurement. If you connect the + lead of your voltmeter to physical pin 1, and the - lead of your voltmeter to pin 6, you should measure 3.3 volts.

Your question, "how can I check if Pi indeed is having low voltage issues?" is confusing. 5V on the 3V3 bus is not "low voltage issues", and without further explanation, detracts from the quality of your question. It causes me to wonder if I'm wasting my time here, but on with the show...

The RPi hardware design is proprietary, but you can still get a copy of a partial schematic. Note the Power Management Integrated Circuit (PMIC) in the schematic. There was also a recent answer discussing the PMIC that may be useful to you.

Looking at the schematic, note first that the 5V supply is not regulated by the PMIC (it's regulated by the external power source), but the 3V3 supply is regulated in the PMIC. The regulated 3V3 appears on pin 27 of the PMIC, fed back to the PMIC on pin 25, and can also be accessed on TP11.

Hope this helps. If you really do have 5V on your 3V3 bus, I suggest you return your board to the source & ask for a refund.