The International Cessna 170 Association

First flight after my annual, about 20 minutes into the flight, I noticed my CHT bouncing around at 100* or so. I paid closer attention to my oil temperature for the remainder of the flight and when I landed, I opened the cowl access doors and couldn't see anything amiss. The CHT lead appeared intact and un-kinked. I forgot to check under the instrument panel to check the wiring there, but barring that - anyone have this happen and what was the solution?

I had a very similar thing happen last Fall. The final diagnosis by the A&P was two-fold: first, the lead wire had become "mostly" disconnected from the fitting where it attaches to cylinder #2. Simple fix: he cleaned off the wire and re-attached it. That fixed issue #1 which was the bouncing around. Secondly, it was stopping around 100 degrees or so. He discovered that the glass (plexi) cover on the CHT gauge had somehow become pushed in on the right side and was pushing down on the needle just enough to limit its travel. Simple fix: he used a suction cup to pull the cover back out again.

Interesting timing: the cover moved in again last week - I suspect due to vibration, and the fact that it isn't secured very tightly - so I used the suction cup technique, pulled it out, and put a dab of silicon adhesive near the edge to hold it.

Not suggesting I have any personal knowledge that's helpful here, just passing along what my A&P found. He stated that the gauges are pretty reliable and any issues are typically related to the connectivity of the wire.

A somewhat similar issue yesterday for me. Just finished my annual and made a first flight. Noticed the cylinder head temperature (#2) was only showing 200 degrees. On the return trip no temp at all. I moved the ring on the transmitting cable 180 degrees as I was concerned the cable might ground out on the cylinder. I suspect the wire may have broken due to this action. I'll inspect and see if I might be able to solder that wire at the ring that goes around the base of the bottom spark plug. If not successful I'll have to replace the transmitting cable.

170C wrote:A somewhat similar issue yesterday for me. Just finished my annual and made a first flight. Noticed the cylinder head temperature (#2) was only showing 200 degrees. On the return trip no temp at all. I moved the ring on the transmitting cable 180 degrees as I was concerned the cable might ground out on the cylinder. I suspect the wire may have broken due to this action. I'll inspect and see if I might be able to solder that wire at the ring that goes around the base of the bottom spark plug. If not successful I'll have to replace the transmitting cable.

Frank, the wires are iron/constantan and its kinda hard to get a good connection between these two wires and then get them soldered to the edge of that ring......they really don't cost that much.

Heat and vibration in the engine compartment takes its toll on the splices between the wires and the probes. Electronics International came up with their own connector, the OLC-1. They are available at Aircraft Spruce. I've never tried them. Anybody have experience with these?

I have them OLC,s in numerous places as all my engine instruments are made buy EI. The big down side is “when” they come loose you have to carry a very small Allen key to tighten them. I must have lost half dozen of these Allen keys in the last 6 months.

That’s good to know. Sounds like EI needs to go back to the drawing room on that design. Heat-shrink butt splices might be a better way to go.

Gary
Heat shrink butt splices work good but have to be cut out to change the thermocouple, the EI connectors can be easily changed, if you have the key!

edbooth wrote:Frank, the wires are iron/constantan and its kinda hard to get a good connection between these two wires and then get them soldered to the edge of that ring......they really don't cost that much.

What's the best source? Is it any old K-type ring thermocouple?

TFA170 wrote:

edbooth wrote:Frank, the wires are iron/constantan and its kinda hard to get a good connection between these two wires and then get them soldered to the edge of that ring......they really don't cost that much.

What's the best source? Is it any old K-type ring thermocouple?

There are several sources Frank. Aircraft Spruce and Specialty, Wicks aircraft, Chief, Wag-Aero. Any 18 mm ring type thermocouple will work. Just make sure you get the right type for whatever indicator you have. "J" or "K". J's are red/white and the K's are red/yellow. Usually cylinder head temps use type J....and exhaust temps use type "K".

Depends on the instrument. My Electronics International UBG-16 uses Type K for both the EGT and CHT. As Ed mentioned, the wire color will tell you which type you have.

Think I have a spare gauge if needed

Think I have a spare gauge if needed

Think my cht & ext gauges are Westech. They were in the plane when I acquired it (probably the cheapest at the time) and I have purchased both cables in the past. My cht seems to be working now?? The last 2-3 flights its been indicating 275 or so. Occasionally 300 in a climb. Seems a bit cooler than what I remember prior to the annual. Last summer, in a climb, I'd see almost 400 until I leveled off and then 375 so the sending cable may still be inaccurately sending data to the gauge. After I go to snf I'll see what that trip shows. Has anyone switched from the spark plug washer type to the one that screws into the cylinder head itself? Might be better?

Not all cylinders have the boss for the screw-in CHT probes. I ordered my ECI cylinders with that specific feature (late 2005). I seem to remember that the instructions that came with the cylinders said to lower the manufacturer’s CHT limits by 25(?) deg F when using probes in that position.