Scary High Oil Temps - Lycoming 540

bwevans said:
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 band :)rockon:); 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.
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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%.

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

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.
 
GeorgeC said:
I found one apocryphal post that mentioned an _82_ ohm resistor:
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Good catch there. Probably a typo on the resistance. An 82-ohm resistor instead of 820 ohms would make the problem worse.
 
The resistor could have been installed in an attempt to make a non OE sensor work. Airparts of Lock Haven has an STC that involves installing a 840 ohm resistor to make a later p/n sensor compatible with the guage, since the OEM p/n is nla.

The OE sensor was a 362 EW. The later sensor mzde compatible with the resistor STC is Rochester 3087-87. The resistance curves are significantly different.

If he Replaced the sensor with a tested, salvage 362 EW and removed the resistor and still has the same problem, I'm thinking ground as well.
 
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@SixPapaCharlie between what @Dan Thomas says here and Tyler said on Facebook, you have plenty of leads... If either one of those guys is close enough to work on your plane, that'd be money well spent! In either case, if your mechanic reads these things and they don't make complete and total sense to him, find an electrical specialist.

Do you know what the two ends of the resistor were connected to? Without knowing that, it's harder to make sense of why it was there, though it may have been due to the STC Chip mentions in post 163 above, or a possibly misguided attempt to deal with the issue Tyler was talking about with the varying sender/gauge ranges.

If I understand correctly, you have taken all of the parts out, and both old and new ones are reading 260 when in boiling (212) water *and* not in the airplane? What are you using for a 12V power source when you do that?
 
Did you guys miss that Bryan says the resistor was installed across the posts on a fuel gauge, NOT the oil temperature gauge? I think there is fixation on something that is installed in a separate circuit, unless Bryan is wrong on where it was actually installed.

As Dan mentioned, the single terminal temperature sensor completes its circuit via the engine case and the airframe. I wouldn’t be surprised if there are other electrical system problems that may not be recognized yet. Don’t assume anything is good unless it is checked.
 
He doesn't really need 12v. All he needs to do is heat the sensor to a known temp and compare the resistance between the terminal and the threads to the resistance curve posted earlier in this thread for the p/n. If it's good,the sensor is good.

Might make sense to place a variable resistor between the sensor wire and ground in the airplane, and dial in a few curve values to check guage accuracy. If all that checks good, it's gotta be a ground.

Maybe run it up, get the oil warm and clip a jumper directly from the sensor case to a good firewall ground and see if that moves the needle down off the peg.
 
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flyingcheesehead said:
@SixPapaCharlie between what @Dan Thomas says here and Tyler said on Facebook, you have plenty of leads... If either one of those guys is close enough to work on your plane, that'd be money well spent! In either case, if your mechanic reads these things and they don't make complete and total sense to him, find an electrical specialist.

Do you know what the two ends of the resistor were connected to? Without knowing that, it's harder to make sense of why it was there, though it may have been due to the STC Chip mentions in post 163 above, or a possibly misguided attempt to deal with the issue Tyler was talking about with the varying sender/gauge ranges.

If I understand correctly, you have taken all of the parts out, and both old and new ones are reading 260 when in boiling (212) water *and* not in the airplane? What are you using for a 12V power source when you do that?
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I did it out of the plane as well as in the plane.
I brought an electric kettle to the plane, and while still wired up to the plane, put the probe in the kettle and it pegged the sensor. The behavior is the same on the plane and off the plane.
Off the plane, I connected a 9v and 2 AA batteries in series.
 
mondtster said:
Did you guys miss that Bryan says the resistor was installed across the posts on a fuel gauge, NOT the oil temperature gauge? I think there is fixation on something that is installed in a separate circuit, unless Bryan is wrong on where it was actually installed.

As Dan mentioned, the single terminal temperature sensor completes its circuit via the engine case and the airframe. I wouldn’t be surprised if there are other electrical system problems that may not be recognized yet. Don’t assume anything is good unless it is checked.
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Correct it was connecting 2 terminals of one of the fuel gauges above the oil temp gauge.

In the photo, the oil temp guage is below the gauge shown with the resistor. That resistor is on 2 terminals of the same gauge.

20240712_165250.jpg
 
flyingcheesehead said:
@SixPapaCharlie between what @Dan Thomas says here and Tyler said on Facebook, you have plenty of leads... If either one of those guys is close enough to work on your plane.
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I will happily pay someone to fix this. Dan you live near me?
I read the replies here and on FB but I don't have the vocabulary or ability to understand what they are saying.
That's why I'm considering an engine monitor. Some really smart person can unwire all of this and make it right with modern equipment.
 
SixPapaCharlie said:
I did it out of the plane as well as in the plane.
I brought an electric kettle to the plane, and while still wired up to the plane, put the probe in the kettle and it pegged the sensor. The behavior is the same on the plane and off the plane.
Off the plane, I connected a 9v and 2 AA batteries in series.
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Well that makes sense, because the sensor wasn't connected to the engine, and therefore not grounded. Try the same thing, but clip a wire between the threadsof the sensor and a good firewall ground and see what you get.
 
Salty said:
I’m confused why we are talking about a resistor that wasn’t even in the circuit for the gauge in question.
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Typical for this board. The common theme seems to be to focus on the theoretically possible cause while the real problem is staring people in the face but being ignored.
 
Chip Sylverne said:
Well that makes sense, because the sensor wasn't connected to the engine, and therefore not grounded. Try the same thing, but clip a wire between the threadsof the sensor and a good firewall ground and see what you get.
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I did. I ran a wire from the engine block to the sensor.
 
SixPapaCharlie said:
I did. I ran a wire from the engine block to the sensor.
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This is the point where you beat your head against the wall and go for your glider rating. There times where the sparky fire burning thing and all its paraphernalia just don't work in harmony and all the King's horses and all the King's men can't make it better until Merlin (or a close approximation) makes an appearance.
 
SixPapaCharlie said:
I did. I ran a wire from the engine block to the sensor.
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Engine block. Now try firewall. If the ground strap from the engine to the firewall is no good, that would tell you. If it were me, I would probably run a jumper to the negative post of the battery and work forward, but I'm a little OCD about grounds.
 
Chip Sylverne said:
Engine block. Now try firewall. If the ground strap from the engine to the firewall is no good, that would tell you. If it were me, I would probably run a jumper to the negative post of the battery and work forward, but I'm a little OCD about grounds.
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Ok I'll try that
 
I think I still have the old gauges out of my lance. I think the oil temp was integrated with other stuff, so I’m not sure it will help….
 
Chip Sylverne said:
Well that makes sense, because the sensor wasn't connected to the engine, and therefore not grounded. Try the same thing, but clip a wire between the threadsof the sensor and a good firewall ground and see what you get.
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The sensor doesn't know what it's connected to. If it exhibits the issue when it's not connected to all the other stuff it's connected to in the airplane, then none of that stuff matters. "Ground" in the case of the test circuit is the negative side of the battery series.

The only real issue would be if the voltage of the out-of-aircraft test circuit was so different that it caused an erroneous reading... But it doesn't sound that way.

Maybe later I'll have to read this thread through again start to finish and talk myself through it... Maybe I'd be more helpful that way.
 
flyingcheesehead said:
The sensor doesn't know what it's connected to. If it exhibits the issue when it's not connected to all the other stuff it's connected to in the airplane, then none of that stuff matters. "Ground" in the case of the test circuit is the negative side of the battery series.

The only real issue would be if the voltage of the out-of-aircraft test circuit was so different that it caused an erroneous reading... But it doesn't sound that way.

Maybe later I'll have to read this thread through again start to finish and talk myself through it... Maybe I'd be more helpful that way.
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My understanding of what Bryan had done at that point was to have put the sendor in a container of hot water, without connection to anything. If that were the case, there would be infinite resistance and no return to the negative post of the battery.
But I was mistaken.
He put the sensor in water and connected a jumper from the threads of the sensor to the engine block, which should return to the negative post of the battery via a ground strap to the firewall and airframe. But since he still got an erroneously high reading, there is resistance somewhere between his engine ground and the negative post of the battery.

How he performed the test with the dry cells, no clue.
 
Chip Sylverne said:
He put the sensor in water and connected a jumper from the threads of the sensor to the engine block, which should return to the negative post of the battery via a ground strap to the firewall and airframe. But since he still got an erroneously high reading, there is resistance somewhere between his engine ground and the negative post of the battery.
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A resistance between the engine and battery negative will give a low reading, if the alternator isn't operating.

That sensor has a thermistor in it, a type of resistor that behaves the opposite to most conductors. Its resistance decreases as it warms up, allowing more current through it and to the gauge, which interprets current as temperature. The resistance between the sensor's threads and the airframe or battery negative will hold the reading down, not raise it. If the resistance was all that bad, the starter wouldn't work anyhow.

But when the alternator is operating, everything changes. That alternator charges the battery by raising the bus voltage higher than the battery voltage, and the electron flow through the battery reverses, changing water and lead sulfate back into lead, lead peroxide and sulfuric acid. So the alternator's electron flow is into the "output" terminal, through the alternator and out the ground terminal or alternator case. The airplane should have a ground cable directly from the alternator to the airframe, but some don't. They rely on the alternator being bolted to the engine, which is grounded to the engine mount via a cable or ground strap, and the mount is bolted to the airframe, where the electrons find their way back into the battery negative during charge.

So bad engine or alternator grounding cause problems, as the charging flow tries to find alternative routes to the airframe, and one of those is into the temp sensor, through it and to the gauge, from the gauge to the bus, and from there to the battery positive and through the battery to ground. This path results in a small boost to the gauge flow, making it spike. As I pointed out earlier, Cessna had an issue with this, with electric temp gauge kits to replace the old mechanical gauges that were getting scarce. (Those used an oil-filled bulb and sealed line to the gauge, which was just a pressure gauge, and when the oil warmed up it expanded, increasing the pressure in the system. The gauge was calibrated for temperature rather than pressure.) In the old airplanes, poor engine ground was a problem, and gauge overreading was common, so they revised the kit instructions to include a dedicated ground wire from a stud on the engine near the sensor (such as one of the oil screen housing or filter adapter studs) to the steel case of the gauge, removing the voltage differential from the gauge. This affects the ballast coil in the gauge, making its ground at the same potential as the engine case, strengthening its pull to hold the indication down to where it should be.

It's a band-aid fix. The real solution is to get all those engine and alternator grounds clean, all their contact points clean of oil or dirt and paint, all the strap and/or wire terminal crimps tight and free of corrosion and oil and dirt, engine mount points on the firewall clean and free of corrosion and paint, too. You'd be surprised how I often found nicely repainted engine mounts bolted to the firewall without the paint being cleaned off the bolt boss faces that mated with the firewall, and the faces under the bolt/washer heads.

Poor grounding can screw up any engine temperature or pressure readings with electric gauges. CHT, too. EGTs have two wires and aren't usually affected. Poor grounding makes for harder starting, alternator headset noise, and engine controls that can get cooked as starter current finds its way to the panel though them. Gremlins everywhere in old airplanes.
 
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