G3X RPM display

How are you checking the diodes? If you are measuring them resistively then they will measure very high resistance in one direction and open circuit in the other direction. Ideally you need a multimeter with a diode test function and you should measure about 0.7V in one direction.

Looking at the photo you have a signal diode, a zener diode and a resistor. If the diodes are genuinely open circuit then you should be able to read off the part number from the side of the diode. You may have to remove them to see it.

The components come in a bag from Garmin in the “Rotax engine sensor kit”, the installer then supplies the wire & assembles the cable.

Here are some images from the G3X installation manual. Others on this forum may be able to provide alternate part numbers from suppliers such as DigiKey, etc.

Al,

The Garmin part numbers for the resistor and two diodes are shown under "Note 12" in the documents posted by Des Howson.  The resistor is simple, its 1/2 watt and 300 OHMS, however the values for the Zener and Signal Diodes are not shown.  If you can't get the part numbers from the parts themselves as Kevin suggested, then I would call Garmin support and follow the phone menu to the G3X product support line.  I find these folks very helpful, and they will either tell you the values of the diodes or possibly sell them to you a la carte. Have your Garmin System ID or Serial Number available before calling. Also, call midweek, and not on a Monday, unless you like to hold.  

Garmin Support:

According to page 21-9 of the G3X installation manual (page 278 of the PDF), the Zener diode is 7.5V (a 1N5236B) and the other one is a standard 200V 1A rectifier diode (a 1N4002, but could be replaced with any 1N4003, 4, 5, 6 or 7).

As Kevin mentioned above, the easiest way to check the diodes is with a multimeter that has a diode test function.  If you don't have one, but you do have a voltmeter and a small battery (a standard 9V will work fine), you can test the diodes as follows.

For the standard rectifier diode (the black one):

1. Connect positive from the battery to the anode of the diode (the end opposite the white ring).

2. Connect negative from the battery to the grounded end of the 300-ohm resistor (the end farthest from the diode).

3. Put the black lead from the voltmeter at the connection point between the diode and the resistor.

4. Put the red lead from the voltmeter at the anode of the diode (where the battery positive is connected).

5. You should measure something around 0.6 to 0.7V.

For the 7.5V Zener diode (the brown glass one):

1. Connect the positive from the battery to the grounded end of the resistor.

2. Connect the negative from the battery to the anode of the Zener diode (the end opposite the black ring).

3. Put the red lead from the voltmeter at the connection point between the resistor and the diode.

4. Put the black lead from the voltmeter at the anode of the diode (where the battery negative is connected).

5. You should measure something very close to 7.5V.

In both cases, the resistor is limiting current through the test setup, so there's no urgency to work fast and nothing will get hot.  If you don't have a 9V battery snap, just have a helper hold wires on the battery contacts; there's obviously no shock hazard.

If you need new diodes, it'll be cheaper to buy a few on eBay than singles from Digi-Key, due to their minimum $6.99 shipping fee.

1N5236B: https://www.ebay.com/itm/154219432979

1N4002: https://www.ebay.com/itm/303034440283

Thanks so much for all your replies. I have a couple meters with the diode test feature but until now I've been using a continuity test only. Isn't that all the diode test is doing? It will be interesting to try Eric's voltage test. (I still dfon't want to believe that both diodes somehow got wiped out) I agree that Garmin's customer support is without equal. I've been a fan and customer for years. I wish I could say the same about Rotax. Engine is bulletproof though. This is my 4th install...3 912s, 1st 914. Thanks again

Al