Cessna Cluster Gauge Issue

[SoCal 182 Driver]

Friends -

Cessna 182K.

On my last two flights, while rolling down the runway at full power on take-off, and on climb-out, the CHT and Oil Temp gauges in my gauge cluster have fluctuated wildly back and forth. When I start pulling the power back, they settle down and act normally, and continue to act normally in all other phases of flight. Once the the CHT and Oil Temp gauges settle down, a cross-check against my JPI engine monitor reveals that the CHT and Oil Temp readings are consistent with the JPI. The other gauges in the cluster are acting normally in all phases of flight -- no wild fluctuations.

I'd appreciate suggestions and ideas about what's going on here, and ideas for a fix.

Thanks!

 

[Dan Thomas]

Engine grounding. At full power, the propeller torque reaction twists the engine and can put stress on ground straps and make them intermittent. That not only messes up the gauge currents, but it makes the alternator or generator current try to find other paths to ground, and those paths include through the sensors and into the gauges, driving them crazy.

Bad engine grounds are too common. They cause hard starting sometimes, and all sorts of issues the rest of the time. Loose ground strap bolts on the engine, or paint under the strap lug. Same with the other end at the engine mount. Paint, oil, grease, dirt, loose hardware, even broken straps. There's usually another ground cable from the alternator to firewall ground, and it needs checking, too.

Edit: I just noticed that it's a 182K. I was thinking 172. Same problem though, engine grounding. On the O-470 there are metal ground straps between the engine mount lugs and the shockmounts, and their other end is between the shockmount and the engine mounts. Worn mounts let the engine torque lift the pressure off those mounts and grounding gets ugly. I have also found those ground straps broken. They flex all the time.

 

[Bell206]

a cross-check against my JPI engine monitor reveals that the CHT and Oil Temp readings are consistent with the JPI

Are you saying the JPI readings fluctuate as well? If so, are the JPI wires spliced into the OEM wires or does the JPI have is own separate sender?

 

[SoCal 182 Driver]

Are you saying the JPI readings fluctuate as well? If so, are the JPI wires spliced into the OEM wires or does the JPI have is own separate sender?

I apologize for the confusion. The JPI is not spliced into the OEM system. It has its own sensors. Once the gauge cluster settles down, it shows the same readings as the JPI. The JPI readings do not fluctuate abnormally in any phase of flight.

 

[SoCal 182 Driver]

Engine grounding. At full power, the propeller torque reaction twists the engine and can put stress on ground straps and make them intermittent. That not only messes up the gauge currents, but it makes the alternator or generator current try to find other paths to ground, and those paths include through the sensors and into the gauges, driving them crazy.

Bad engine grounds are too common. They cause hard starting sometimes, and all sorts of issues the rest of the time. Loose ground strap bolts on the engine, or paint under the strap lug. Same with the other end at the engine mount. Paint, oil, grease, dirt, loose hardware, even broken straps. There's usually another ground cable from the alternator to firewall ground, and it needs checking, too.

Edit: I just noticed that it's a 182K. I was thinking 172. Same problem though, engine grounding. On the O-470 there are metal ground straps between the engine mount lugs and the shockmounts, and their other end is between the shockmount and the engine mounts. Worn mounts let the engine torque lift the pressure off those mounts and grounding gets ugly. I have also found those ground straps broken. They flex all the time.

Great explanation! Thank you!

 

[Tantalum]

Engine grounding. At full power, the propeller torque reaction twists the engine and can put stress on ground straps and make them intermittent. That not only messes up the gauge currents, but it makes the alternator or generator current try to find other paths to ground, and those paths include through the sensors and into the gauges, driving them crazy.

Bad engine grounds are too common. They cause hard starting sometimes, and all sorts of issues the rest of the time. Loose ground strap bolts on the engine, or paint under the strap lug. Same with the other end at the engine mount. Paint, oil, grease, dirt, loose hardware, even broken straps. There's usually another ground cable from the alternator to firewall ground, and it needs checking, too.

Edit: I just noticed that it's a 182K. I was thinking 172. Same problem though, engine grounding. On the O-470 there are metal ground straps between the engine mount lugs and the shockmounts, and their other end is between the shockmount and the engine mounts. Worn mounts let the engine torque lift the pressure off those mounts and grounding gets ugly. I have also found those ground straps broken. They flex all the time.

This is awesome. It is because of morsels of knowledge like this that I love this community

 

[Dan Thomas]

The JPI sensors don't rely on engine ground paths. They have their own, so they're not affected by stray currents trying to find their way to airframe ground. From the JPI 900/930 manual:

 

[Dan Thomas]

The engine mount ground straps are like this:



The bolt goes through the small hole in the strap (#10) first, then through the lower shockmount, through the spacer, though the big hole in the strap, through the upper shockmount, then into the nut. The strap is located by a roll pin in the mount going through a small hole in the strap. The engine grounds via the bolt, into the strap, and into the engine's airframe mount (#2). The bolt moves with the engine, and the engine mount doesn't, so you can see that the strap gets flexed a lot.

 

[SoCal 182 Driver]

The engine mount ground straps are like this:

View attachment 116309

The bolt goes through the small hole in the strap (#10) first, then through the lower shockmount, through the spacer, though the big hole in the strap, through the upper shockmount, then into the nut. The strap is located by a roll pin in the mount going through a small hole in the strap. The engine grounds via the bolt, into the strap, and into the engine's airframe mount (#2). The bolt moves with the engine, and the engine mount doesn't, so you can see that the strap gets flexed a lot.

This is thread, with Dan Thomas' input, has become a Master Class on engine grounding. Thank you!

 

[bnt83]

I would not be surprised if the instrument was not installed per the installation manual which clearly states the instrument must be grounded at the engine, not an airframe ground, not a local ground.

 

[SoCal 182 Driver]

I would not be surprised if the instrument was not installed per the installation manual which clearly states the instrument must be grounded at the engine, not an airframe ground, not a local ground.

I'm assuming you're referring to the cluster gauge and not the JPI. It's an interesting theory. The cluster gauge has been operating fine since I've owned the plane (8-plus years). It was even working fine after my panel upgrade in January. Then, out of the blue, the problem started about two weeks ago.

 

[Dan Thomas]

I would not be surprised if the instrument was not installed per the installation manual which clearly states the instrument must be grounded at the engine, not an airframe ground, not a local ground.

The OEM cluster was only grounded at the panel. When Cessna came out with a kit to convert the old mechanical oil temp gauge to electric, gauge readings gave some problems due to poor grounds at the engine and in the panel structure. Their fix was to run a small wire from a crankcase bolt near the sensor to the gauge's case to eliminate the grounding differential. But that's a bandaid; the real fix is to clean up the engine grounding.

 

[Dan Thomas]

I'm assuming you're referring to the cluster gauge and not the JPI. It's an interesting theory. The cluster gauge has been operating fine since I've owned the plane (8-plus years). It was even working fine after my panel upgrade in January. Then, out of the blue, the problem started about two weeks ago.

I wonder if your panel has come loose. Vibration could do it too. Put some pressure on it with your hand and see if the gauges settle down.

You could also try shutting the alternator off when the gauges are jumpy; if they settle down, it's likely engine grounding.

 

[SoCal 182 Driver]

I wonder if your panel has come loose. Vibration could do it too. Put some pressure on it with your hand and see if the gauges settle down.

Good idea. Will try to do that on the next flight.

You could also try shutting the alternator off when the gauges are jumpy; if they settle down, it's likely engine grounding.

No separate alternator switch on my 182K. And the breaker is not one that can be pulled...only reset.

 

[Dan Thomas]

No separate alternator switch on my 182K. And the breaker is not one that can be pulled...only reset.

OK. You have the single pull-on/push-off switch high in the panel. It has two sets of contacts to control the master contactor and the generator as well. That's unfortunate. And it's rare. Hope it doesn't quit.

 

[NordicDave]

Worth mentioning, many new engine frames are full painted which acts as an insulator between the mount and the plane.

 

[Lndwarrior]

I'm gonna apologize in advance for the implication here.

I just can't wrap my head around your sentence, "On the last TWO flights while rolling down the runway...."

 

[SoCal 182 Driver]

I'm gonna apologize in advance for the implication here.

I just can't wrap my head around your sentence, "On the last TWO flights while rolling down the runway...."

As a friend used to say, "subtle as a kick to the crotch."

Given that I had the JPI to cross-check and ensure the engine temps were in the normal range, I wanted to do some more troubleshooting to see if the first time was an anomaly or whether the problem would repeat itself. Since it repeated itself, the plane will be grounded until we can figure out what's wrong.

 

[camorton]

I'm gonna apologize in advance for the implication here.

I just can't wrap my head around your sentence, "On the last TWO flights while rolling down the runway...."

I don't see what is so hard to understand. He noticed the issue twice. Are you implying his second flight was somehow unsafe? It was obviously an instrumentation issue and he had the JPI as a secondary.

C.

 

[Dan Thomas]

I don't see what is so hard to understand. He noticed the issue twice. Are you implying his second flight was somehow unsafe? It was obviously an instrumentation issue and he had the JPI as a secondary.

C.

I believe the JPI is STCd as a replacement for the original primary instruments. The ones I installed were. We could get rid of the old stuff altogether.

 

[Tom J]

Depends on the JPI, not all are certified as primary so the OEM stuff must be kept.

 

[PaulMillner]

The JPI sensors don't rely on engine ground paths. They have their own, so they're not affected by stray currents trying to find their way to airframe ground.

Actually, JPI's thermocouples are grounded at the probe end. That makes it important that the JPI box's ground, the brown wire, also be grounded at the engine, not the airframe. That's not the issue here, but it's wrong to say JPI doesn't rely on engine ground paths... accurate results very much depend on the grounding being done correctly.

Paul