SDS fuel flow pull up resistor

Has anyone added a pull up resistor to get their SDS fuel flow output to work with a Garmin GEA24? I've reached out to Garmin to help size it, and after 3 emails they're still asking me who makes my fuel flow transducer. Garmin did confirm they're looking for square wave falling edges on the pulses, and SDS has said they pull that pin to ground for a pulse, but suggested it may need a pull up resistor to get the voltage high enough for Garmin to see the falling edge. I think this is the right answer, but I'm not good at circuit design.

Can any electrical guys chime in on proper resistance and power rating for this application? Thanks!

You may need a scope to see what it's doing but lacking any specs I'd probably start off with 10k and see if that works.
Hard to imagine they have been asked that question before.

agent4573 said:

Has anyone added a pull up resistor to get their SDS fuel flow output to work with a Garmin GEA24? I've reached out to Garmin to help size it, and after 3 emails they're still asking me who makes my fuel flow transducer. Garmin did confirm they're looking for square wave falling edges on the pulses, and SDS has said they pull that pin to ground for a pulse, but suggested it may need a pull up resistor to get the voltage high enough for Garmin to see the falling edge. I think this is the right answer, but I'm not good at circuit design.

Can any electrical guys chime in on proper resistance and power rating for this application? Thanks!

Click to expand...

As this is a 12V circuit, a 5KΩ, 1/4W resistor will work well. It's also the common resistor size that Garmin suggests for pull-down/up in their drawings for other sensor installations.

I didn't install a resistor on my fuel flow installation from the SDS ECU to my GEA 24.

I'd defer to Ross and Garmin for your application; he or Barry are pretty quick to respond.

Data Signal

You didn't ask this, but it just occurred to me to say that both the RPM and Fuel Flow signals from the SDS EM-5 ECU are digital pulses, which can be affected by current carrying conductors or equipment located nearby.

If you're experiencing any signal fluctuation, such as an RPM for fuel flow signal that floats around or is different than you'd expect, it's likely related to grounding.

I highly recommend using a single conductor shielded cable from the ECU to the GEA 24 for both of these signals. As this is a data signal carrying wire that we're trying to keep noise out of, I recommend grounding the shielded cable at the GEA D-Sub ground block. It's not necessary to ground the shield at the sending end (ECU), but there's no harm in doing so in this application.

The rpm signal has been rock solid since the beginning. The fuel flow and rpm wires are routed directly next to each other in the bundle from the SDS to the GEA24, if both were affected I would suspect interference, but with the 2 rpms being solid and the fuel flow being solidly not working, I think it's something more definitive than EMI issues.

I'll grab the suggested resistor, double check the physical wiring and see if I can't get this working next weekend. Thanks. Wanna post up a schematic of your diode isolation system? Might be a nice addon after phase 1 is done.

Just to close this out, after getting all the info from Barry and Ross, I headed to the airport and started to troubleshoot. It was an easy fix, as I had the signal wire in pin 22 instead of pin 23. Once I swapped it over, everything worked fine without any additional resistors.

SDS was super helpful, providing a lot of troubleshooting hints and specs. Our phase 1 airport is an hour drive from the house, so I wanted all the info so I could bring everything with me in one trip. I emailed Garmin a few times as well, and I don't think they ever understood that I wasn't using a red cube transducer. Eventually they sent me the page from the installation manual and told me to contact continental with any further questions, as that was the make of my engine and they would know better.