Question for our Engines Expert Paul McBride: I have a Cessna 172R and the oil temperature fluctuates from minimum to maximum constantly until shutdown. Just wondering if this could be a bad temperature probe or could it be the gauge?
Tom Elms
Answer: Tom, from the information you provided, I’d suspect there may be a loose connection or a bad ground on the oil temperature gauge.
If you’ve read some of my previous articles regarding oil temperature troubleshooting, you’ve probably read about using a laboratory grade thermometer to check the actual oil temperature.
Extreme caution must be used when checking the oil temperature in this manner, but it does provide you with an accurate temperature.
Of course, the other option — and easiest thing to do — would be to check the entire wiring and gauge on your system, which could be done by any reliable maintenance facility.
My father shut down an engine on our Apache years ago because of a high oil temp reading and at near full gross weight that plane had zero climb! We diverted to an uncontrolled airfield and a normal landing nearly turned into an accident as a plane suddenly rolled onto the runway and started his takeoff roll as we were 1/4 mile out and very committed. This in spite of my dad making all the appropriate intention and position calls and watching the other plane holding short. Anyway, there was a dirt runway for sailplanes parallel to the runway and he landed there. All of this because of that oil temp gauge which had failed. Luckily nobody was hurt that day and there was no damage to the plane from landing on a bumpy dirt track!
Trace the wires. It could be a bad connection, a bad wire or even a short to another gauge. Have you noticed any other gauge malfunction.
Run a temporary wire from the sending unit to the gauge after removing the old wire. Test run. Problem gone, bad wire, problem still there , sending unit probably. Internal ground on sending unit. Use proper wire as it’s all about resistance.
Mac,
That’s a good set of tests to determine the problem.!
The resistance of the wire is minimal vs the 30-100’s of ohms for the sender.
I don’t know which sender that engine/ gauge uses.?
With the gauge going from min to max, I’d suspect a frayed wire between the sending unit and the gauge connection, that is shorting to ground, [ a max reading], and then open circuit, [ a min reading]
A loose connection anywhere would have the gauge read from min to ‘normal’, not going to max.
You’ll have to trace the wire from the sender to the gauge to find the fault.
My guess would be to 1st look at the connection to the sender on the engine accessory case.