Fuel gauge issue has me stumped

[redacted]

Hello all,

Aerospace logic fuel gauge with McFarlane senders. Senders and gague all installed in the past 5 or 6 years. RH fuel gauge started showing only about 7 gallons when full on the ground. In flight it would quickly drop to 2 or 3 gallons and just stay there.

Fuel gauge has a 10k resistor in the sender wiring. Rang the wires on both sides and all is well. Hooked up a black box to the sender wire and simulated a sender - gauge worked normally so...bad sender.

New sender installed in the RH tank and calibrated. All works normal. Come to the plane 4 days later and as soon as I hit the master, RH fuel gauge showing 7-8 gallons and stayed there for 1.5 of flying. Tank had near 20 gallons in it. I could kick the rudder and move the fuel around, it would drop and rise as you'd expect, but never showed more than 8 or 9 gallons. Tank had at least 15 in it at the time.

I am stumped as to why the RH side isn't working. Isn't failed in the same manner, and was working normal after install and calibration.

My next thought is to pull a new sender wire, connect a new 10k resistor, and wire it up. Hate to have to calibrate again only because I need help getting the tail in the air. I'm happy to play with 2 gallons of fuel at a time, just hate to bother folks for help. Can't imagine it's the wiring since it calibrated and worked perfectly fine.

Why no CiES senders? I'll tell you why. I'd rather spend $600 for one new McFarlane sender than $2100 for 2 CiES and a new gauge. Cuz the part number of the gauge that uses CiES senders is different and I can't use my current gauage for those senders.

Tongue in cheek I'd love to just punch two holes in the tank at the wing root and install a clear tube like a Supercub. Not moving parts, probelm solved. Any ideas here? Thanks

[n2582d]

Other than the CIES units, I don’t find a fuel transmitter for the 170B on McFarlane’s website. What’s the part number? To rule out a bad ground connection run a jumper wire from a mounting screw on the transmitter to the negative pole of the battery. See if that affects your fuel level reading.

[redacted]

Sorry I didn’t say, I have 175 wings on the plane. Checking grounds today at the hangar. The fact it worked fine, then not, head scratcher. But bad grounds do weird things.

It’s this one https://www.mcfarlaneaviation.com/produ ... 5iEALw_wcB

[n2582d]

Sounds like you’re on the right track checking grounds. It’s what McFarlane emphasizes for troubleshooting in their FAQs in the link you gave.

[redacted]

Yeah it was a bad ground. Ran a bond strap from sender body to a good ground. Fixed it immediately.

Thanks.

[n2582d]

Good to hear you figured it out! It’s important to know what one’s doing when troubleshooting fuel senders. Here’s the caution McFarlane gives in their FAQ’s:

Caution! Never short the battery power to the transmitter wire! It will take only seconds before the stainless steel resistance wire in the Cessna transmitter will glow red hot in the fuel tank. The hot wire could explode the tank! Never have power on when trouble shooting the fuel gauging system.