I was going to wait until the issues with the fuel sender were finished to write an entry on it, but it’s going on for a very long time and people are wondering what’s up with my plane. Here’s the current story:
The right-hand fuel sender, a unit made by Rochester for Cessna, is notoriously poorly made. It’s a pretty standard unit, quite similar to one that you’d find in your car. It has a float that operates a potentiometer (variable resistor). The range is 5 ohms to 90 ohms with higher being less fuel. The pot in this design of the Rochester part is what’s known as a film pot: there’s a carbon film deposited on plastic, with a wiper that sweeps across it. The float operates the wiper. The traditional method of using windings of wire is much more durable, but also much more expensive and heavier. The quality of that carbon film is terrible and quickly wears away. This is what happened earlier, but Instrument Rebuild was able to clean it up and make it work again.
It didn’t last. The rebuilt unit started failing at about six months time in service. It turned out that rebuilding the unit was a false economy for a rental. It may have made sense for a personally owned aircraft that didn’t fly much, but on a rental it turned out to be wasted money. It was $400 for the rebuild, and 2-4 hours of labor for each swap. When the unit failed again, I sent it to Instrument Rebuild, hoping they’d honor the warranty.
They attempted to. Unfortunately, the owner of Instrument Rebuild, Paul Malkasian, has passed away. His son, Bill, is minding the shop. Bill attempted to revive the unit, but the pot is too far gone. He’s currently attempting to get a replacement pot from Rochester, but he’s getting no response from them. He’s attempting to honor it, but it’s not working.
Meanwhile, my plane is sitting. And I am giving up on the Rochester part.
Instead, I’m replacing it with a CiES digital fuel sender. The CiES unit is half the cost of the Cessna unit ($400 vs. $800). It not a pot: it’s a magnetic sensor attached to the float. It’s sealed and enclosed. It should last longer than the plane does. It should be far more accurate than the old Cessna parts. And it’s STC’ed for 172S.
The downside is that the plane will be in repair for a bit longer. The fuel gauge is not compatible with the digital units, so a digital fuel gauge is on order. That’s $675. Of course, I also need to replace both fuel senders, so the cost is still $800 for the senders, not just the $400 to fix one. And the kicker is that both the new gauge and the new senders are active. That means they need to be supplied with power. For the new gauge, that’s not a big deal, as it’s in the panel where power is readily available. For the new senders, it’s more of a problem, as a new wire needs to be run to each tank to supply the sender with power.
All-in-all, it should be about 8 hours labor to get the new system in, or about $2000 total. Add to that the lost revenue from having the plane down for so long, and this has been a very expensive and hugely annoying failure. Once the CiES unit is in, though, that should be the last of these issues!
I’ll post up again when this is all resolved with the final numbers!