Dan Thomas
Touchdown! Greaser!
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Dan Thomas
That first stripe is gray, which is an 8. It's an 820 ohm resistor. And a 10% tolerance would mean 82 ohms on either side of 820. Your 5 ohm tolerance for a 520 ohm resistor is 1%.I don't think that's an 820 ohm resistor, its 520 ohm. Bands are read left to right with these older resistors the banding is grouped to one side. You never start with a metallic bandrockon
; the metallic is the tolerance band and is always on the right. So the green is 5, red is 2, the brown is the multiplier so x10, with the silver being the tolerance of ±10% for the total resistance of 520 that could read as little as 515 to as much as 525 ohms.
Judging by the size of the resistor compared to the crimp terminals on it, it's a half-watt resistor.
So much advice on this problem, and none of it enlightened by reference to the wiring diagram in the service manual for the airplane. Here, for instance, is the gauge wiring for the PA-24, serial numbers 24-2844 and up:
![1720889803994.png 1720889803994.png](https://www.pilotsofamerica.com/community/data/attachments/123/123505-87c87fa2990f8b77182b766ffe681747.jpg)
You can trace the power from the bus and breaker to the fuel gauge and oil temp gauge. Then you see the sensor line out of the oil temp gauge, passing thru wire Q5A to that little symbol with a "B" in it. t is not a resistor; totally the wrong symbol, so it's likely a connector, suggested by the change in wire number to Q5B to the oil temp probe. There is no resistor anywhere in the system, not in this S/N range nor any of the others shown in the service manual, so if that 820-ohm resistor is in the gauge line, it's been put there later, maybe by some mechanic trying to correct an overreading gauge.
Which then begs the question: If it was overreading in times past, what was causing it? Has this system been checked as I recommended way back in post #13? An excerpt from that:
A poor engine ground to the airframe can do it.
The alternator causes electron flow from the bus, through the alternator to the engine ground (or to airframe ground via a dedicated alternator ground cable), thence to the negative battery post, through the battery (which charges it, since this flow is reverse to normal battery discharge flow), and out of the positive battery post to the bus.
Now, the oil temperature sensor is a brass fitting that screws into the oil filter housing. It has one terminal, which leads to the oil temperature gauge. The other side of the oil temp gauge goes to the bus. The bus power pulls electrons through the gauge, which gets them from the engine crankcase vie the sensor, which has a resistor in it that lowers its resistance when it gets hot.
If the alternator is not well-grounded to the airframe, those electrons from it like to find other paths to the airframe, and one of those paths is through the oil temp sensor and to the gauge. This increases the electron flow through the gauge, spiking it....
....The fix was a length of 20-gauge wire and some terminals to attach it to the engine crankcase as close as possible to the temp sensor, usually at an accessory case stud and nut, and then to the gauge's metal instrument case. This removed the voltage differential caused by bad grounding and the gauge would (usually) smarten up.
The PA-24 service manual:
http://www.aeroelectric.com/Reference_Docs/Piper/pa-24-18--250-260-260t-400_smv1998.pdf
Wiring diagrams are in section 4, near the end of the manual.