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No OTG Equals AOG: Replacing an Inop Oil Temp Guage

Written by Hiram Lopez
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Identifying, removing and replacing the oil temperature gauge in 100 series Cessna aircraft with 24 volt electrical systems can be challenging. Here is a step-by-step survey of the process.

June 2015-

The basics
Cessna 100 series aircraft with 24 volt electrical systems are equipped with a Rochester model 90213 Oil Temperature Gauge (OTG).
The OTG shares the instrument case with a CHT gauge. The pair is known as an “instrument cluster case,” and is identified as Cessna part no. 624700081 model C669561-101. This type of instrument cluster is found in Cessna aircraft with Lycoming engines.
A five-amp circuit breaker powers the dual CHT and OTG cluster. The breaker also feeds the fuel gauges. In the absence of CHT, OTG and fuel gauge indication(s)—first check the breaker.

“R” is for “required”
A Cessna 172RG Cutlass presented with the OTG gauge needle remaining inert on the left side during flight. For this model, the Equipment List contained in Section 6 of the POH (“Weight and Balance/Equipment List”) describes equipment, radios, gauges and instruments originally installed by Cessna. Some of these must be operational before flight.
In item D41 on this equipment list, there’s an “R” next to the OTG entry. You guessed it: the oil temperature gauge is required to be operational before a flight.

FAR 91.403 is very clear: “The owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition.”
Just as important is FAR 43.13 (b). In short, this regulation states a person performing maintenance shall do the work in such a manner and use material equal to its original condition. This means you must fix an aircraft to the standard of its original approved condition, either with parts from the same manufacturer or through STC—regardless of who does the work.
Equally important is Part 43 Appendix A and the definition of “authorized persons” under Part 43.17. Replacement of an oil temperature gauge is not preventive maintenance and must be completed (or supervised) and signed off by an A&P mechanic. (Per the author, the testing, removal and replacement of the OTG in this story was performed under IA supervision. —Ed.)
Identification of OTG components
The OTG is composed of a thermocouple (sensor or “send” unit) model 3080-37, which is usually mounted in the rear center/upper part of the engine in front of the oil filter.
The thermocouple is a variable resistor of 34 to 900 ohms encapsulated in a threaded brass probe that changes resistance value when the engine oil temperature climbs from ambient to near 200 degrees F.
A wire from the top and hot side of the thermocouple runs through the firewall and connects to the send post of the OTG 90213 instrument.

First, ensure engine and master switch are off. Place an electrical short to ground on the thermocouple hot lead wire, and have an observer turn the master switch on briefly. (Remember, briefly!)
Have the observer look at the OTG needle movement. Full scale deflection shows a working OTG.
During the shorted wire lead, the OTG white needle should move across the un-numbered green scale to under the red marked 245. This test separates thermocouple from OTG operationally.
With engine and master switch off, disconnect and tape the sensor wire from the thermocouple and place an ohmmeter on the 3080-37 sensor. At room temperature, a reading of near 900 ohms is acceptable. If the test is performed after engine shutdown, expect a reading of less than 100 ohms.

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