[Zikelskim]

Easiest way to explain the problem is a short YouTube video (18sec)


Have tested another Gps and it works on the rev nano, but the one with the problem has Mag.
Its get even crazier when i connect mag, CPU gets red and I2C reports problems, but i get a reading on the MAG.

Both gps are Neo-M8N-0-01

[Mateusz]

it works on the rev nano, but the one with the problem has Mag.

If you tested GPS (#1) on Revo nano and that works, but then GPS (#2) does not work on the same Revo Nano. Then it's an indicator that GPS (#2) might be either faulty, or user error such as not connecting cables correctly or without external voltage. Is the cable assembled correctly ?

P.S one thing at a time, first get GPS green then care about MAG. These are two independent devices just happen to be powered together usually inside same GPS module, but that does not need to be the case. You can use Mag and GPS separately.,

[Zikelskim]

Cut all the cables and redid everything and now the GPS is working. But when i connect the MAG i still get a red CPU and the GPS loose connection.

Its not a problem because the MAG on the revo is green and i working perfect.
Got the quad in the air today and its almost rock solid in the air with positionHold. 

Tnx for the reality check 

[TheOtherCliff]

The PixHawk / APM style of GPS/mag has an I2C mag.  I2C is a communication format like NMEA, DSM, SBus, etc.  I2C mags are susceptible to cable problems.  The cable must be short and twisted.  Even then, the cheap clones sometimes have issues and the fix is to install a pair of pullup (to 3.3v?) resistors (say 3k to 5k) on both SCL and SDA lines in the I2C cable.  Best place is to just add the resistors inside the GPS shell.