Abnormally low CHT indications

[PilotjohnS]

Originally Posted by RV74ME

Yep, doing that now on 2 of them. And they are reading 180 and 161??? Ugh

Given these readings, I would look at the wiring. Are the thermocouples wires routed nicely? Are there splices or other connections in the path to the sensor that could be causing another “fake” thermocouple? Is the installation per the instructions? Or maybe the water is not really boiling:-).

You could also check the cold side by using an ice bath. As long as liquid water and ice coexist, the water is. 32 F. But i think many different types of sensors have similar temp readings in the ice bath.

[RV74ME]

I also suspect the wiring. Gonna have to wait til next week, but will recheck all the pins are correct. And no, there are no splices anywhere. Only connection is at the probe. Except i used spade connector instead of the one that came on the probe.

[RV74ME]

Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses ?professionally? built🙄.

[BillL]

Originally Posted by RV74ME

Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses “professionally” built��.

Keep looking. A thermocouple has dissimilar metals that produce electricity (voltage) above absolute zero. This DC voltage is related in a non-linear way to the temperature. The signal is small and, clearly, polarized. ANY dissimilar metal contacts in the circuit will also produce voltages. Typically this is a non issue as they are low temp and do not contribute to an error. The fact that all are off the same amount means they are either correct or both sides are miss wired in the same way. Look at your schematic for how they are connected to the instrumentation. If two thermocouples are wired in series the the voltage would be half . . . but , then, how would you get 4 readings??

A look at the complete picture of the engine performance parameters will help in the diagnosis. Fuel flow in particular. Is it running super rich? Is the timing way off, as in retarded? If retarded, one would expect higher EGT, but that can be masked with super rich mixture. I ran 700F EGT, but it was very cold ambient. Single parameter analysis limits the process.

Honestly, I think it's a high probability that your temps are being measured correctly. The data point is ambient, it would not be correct with a wiring issue.

[Captain Avgas]

Originally Posted by FLY6

. If oil temps don’t get to 180 degrees, they will not burn off the water vapour.

My IO360 Lycoming manual states a maximum oil temp of 245 F, and a “desired oil temp” of 180 F. There is no minimum, but Lycoming says: “Engine oil temperature should not be below 140 F during continuous operation.”
In other publications Lycoming recommends 165 to 200 F.
I would suggest that the comment about the oil temperature needing to be at 180 F to “burn off the water vapour” is erroneous. Water will evaporate out of the oil at much lower temperatures than 180 F....it just takes longer.

[snopercod]

Originally Posted by RV74ME

Yep, doing that now on 2 of them. And they are reading 180 and 161??? Ugh

Immersed in 212 F water (what altitude are you at?), a Type K thermocouple puts out 4.096mV. If your box was set for Type J rather than K, that 4.096mV would read out as 170.6 F, pretty close to what you saw. I know you checked this setting once, but I would check it again.

[RV74ME]

Originally Posted by BillL

Keep looking. A thermocouple has dissimilar metals that produce electricity (voltage) above absolute zero. This DC voltage is related in a non-linear way to the temperature. The signal is small and, clearly, polarized. ANY dissimilar metal contacts in the circuit will also produce voltages. Typically this is a non issue as they are low temp and do not contribute to an error. The fact that all are off the same amount means they are either correct or both sides are miss wired in the same way. Look at your schematic for how they are connected to the instrumentation. If two thermocouples are wired in series the the voltage would be half . . . but , then, how would you get 4 readings??

A look at the complete picture of the engine performance parameters will help in the diagnosis. Fuel flow in particular. Is it running super rich? Is the timing way off, as in retarded? If retarded, one would expect higher EGT, but that can be masked with super rich mixture. I ran 700F EGT, but it was very cold ambient. Single parameter analysis limits the process.

Honestly, I think it's a high probability that your temps are being measured correctly. The data point is ambient, it would not be correct with a wiring issue.

I confirmed that the wires were reversed at dsub (lo was hi, hi was lo)

[RV74ME]

Originally Posted by snopercod

Immersed in 212 F water (what altitude are you at?), a Type K thermocouple puts out 4.096mV. If your box was set for Type J rather than K, that 4.096mV would read out as 170.6 F, pretty close to what you saw. I know you checked this setting once, but I would check it again.

I?m at 800 feet. And yes i checked 3 times that in fact i have type K selected. Now that i have fixed the wiring issue, i?m expecting it to read correctly, but wont know until i get back home in a few days

[BCP Boys]

Originally Posted by RV74ME

Looks like its the wiring. The red and yellow wires were installed backwards at the dsub. What i learned is these two wires are different. One is alumel and the other chromel. So trying to correct this mistake by reversing the connection at the probe does not work.

Anyway, just finished reassembling the connector and will test everything thursday when i get back.

So much for having my harnesses ?professionally? built🙄.

Glad you found that there was an issue in the harness. I know the weather around the Southeast has bee **** for the past 2 days. I'm interested in hearing to see if that fixed your problem. The one thing that is interesting to me is that you stated in your earlier post that when you turned on your EFIS in the hangar, the temps read the same as the outside ambient temperature... why would it read correctly in the lower degrees but not read correctly in the higher temps. I'm not questioning you, I'm just curious.

[snopercod]

Originally Posted by RV74ME

I?m at 800 feet.

Water boils at 210.5 F at that altitude, so no big deal. I'm not convinced that you found the problem, though. If the wires were reversed, you would be sending negative voltage to the box and get the big red X again. Maybe they're reversed at the box and somewhere else, too?