Garmin Hacking

This is all about Garmin hacking and seeing what a write page would be like…. blah blah blah

After some bugging of my sister, she sent me her Garmin GPS that she had been using for Geocaching so I can now use it to Wardrive with.

I knew I would need a a data cable to hook between the GPS and my laptop and had a good idea it was something that I could build myself.

Materials Needed:

  • Serial cable (An old serial mouse works perfect. Also the db9 to cat5 setup works awesome too.)
  • Wire cutters
  • Wire strippers
  • Multimeter or a continuity tester
  • Expired credit card or thin peice of plastic
  • Zip-ties
  • Epoxy

garminContactsInvestigating the eTrex, we find that on the back top is a rubber cap that folds back to reveal four pins that are used to hook up a dat acable

Page 45 of the eTrex user manual yielded the info we need to as to which contact does what. If you are looking at the the backside of the eTrex while it is verticle, the pins are Ground (-), Data Out, Data In, [SPACER], Power(+).

Standard db9 Serial Pinout
Pin Num. Name Desc.
1 DCD Data Carier Detect
2 RD Receive Data // RxD or Rx
3 TD Trasnmit Data // TxD or Tx
4 DTR Data Terminal Ready
5 SGND Ground
6 DSR Data Set Ready
7 RTS Request To Send
8 CTS Clear To Send
9 RI Ring Indicator
eTrex to db9
eTrex db9
Ground (-) 5 – Ground
Data Out 2 – Receive Data
Data In 3 – Transmit Data
Power (+)

garminContinuityTestingTo figure out which serial pins you will need, turn the female db9 connector so that it faces you and the tapered side is on the bottom. Start counting from right to left, 1-5 on the top side and 6-9 on the bottom. It took me a while to figure this out for my poor-man’s network cable kit; now it is committed to memory. For the male db9 connector you cound from left to right, 1-5 for the top row and 6-9 on the bottom.

Use your multi-meter in continuity testing mode to figure out which color wires go to pins 2, 3 and 5. The first proto-type that I made was using an old Atari gamepad extension cable that I used for years as a mouse extension. I think I damaged it by trying to shove the tips of my multimeter into the pin holes. If you don’t have a probe small enough to fit into the pin holes do not force it. Strip both ends of a workable length of wire. Wrap one end around a test probe, fold the other bare end in half and slip it into the pin holes when testing. Once you find the associated wires for pins 2, 3, & 5, snip the rest off.

garminCableFittingI cut my expired to a size that would fit, turns out that it is 11/16″. There is a little play when it is set into place on the eTrex so I think that the entry space is actually 3/4″, but close enough. Don’t bother cutting the card to length just yet, we’ll do that later. Cut a small slit in the card to accept the spacer that seperates the Data In and Power pins of the eTrex. I used the ends of my wire snips to make the small cuts. Better to take off too little and have to take off a little more then take too much off and not have enough support. Once the card is cut to shape and fits snug and secure, star by making a little road map of where you are going to make your holes for the wires to fit through.

garminHolesI used a 1/16 drill bit spun through my fingers to make the holes for the wires. I drilled two holes per wire, one just before and one just after the contacts on the eTrex for each of the 3 wires. Four more holes were finger drilled using a 1/8th bit for the zip-ties to hold the cable in place. I have a cable stripper that actions like a plier but will lock onto a wire and yank the plastic back from a wire, not totally stripping it, but getting the job started.

garminCardContactsLoop the zip-ties through the holes design for them and get them started. Do not tighten them down yet, you’ll use these to help support the wire for the next step, threading. Threaded each wire down through the first set of holes and back up through the holes on the end of the card. Flipping the card over would real real only the bare wire. These are the contacts that will match up with the contacts on the eTrex.

Once you are satisfied with what you did snug the cables down, but not too tight that you couldn’t undo everything if it doesn’t work.


Attach your new Garmin eTrex data cable to the eTrex and turn the eTrex on. Attach the other end to your computer and open up HyperTermial. Start a Hyper Terminal session on the com port that the new data cable is plugged into, set the com speed to 4800 baud, the parity, start & stop bits should be default (N,8,1).

Next press the eTrex Page Button until the MENU appears, arrow down to SETUP, press the Enter button, and arrow down to INTERFACE, press the Enter button again. Press the Enter button to select which I/O format to use, arrow down to NMEA OUT and press the Enter button again.

In hyper terminal you should get some output that looks like:


If you don’t, double check that your Hyper Terminal settings are correct. If those are correct, it is time to double check that you have the wires for pins 2, 3, & 5 and in the correct order.

If you get good output it’s time to hook it up to you laptop and take a cruise with NetStumbler [site] .

Happy Wardrving!


  1. THAN
    June 5th, 2010 at 01:30 | #1

    how to do the 25 pin port to my etrex gps?

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