Exhaust gas temperature troubleshooting, cylinder head temperature thermocouple troubleshooting guide.

 Lightsport & Light Sport & Ultralight Aircraft Information.


Click here for more

Excalibur experimental amateurbuilt light sport aircraft.

Hear what our customers have to say! 


Your new AdDesigner.com ad! 

When was the last time you did maintenance on your K & N Airfilter? Is you filter safety wired? Click here to see how to clean and safety wire your airfilter!

EGT and CHT troubleshooting guide.

A thermocouple is a device formed by the junction between two special dissimilar metals which transforms heat energy into electrical energy. Aviation EGT and some CHT systems use thermocouples, lead wires composed of the same two dissimilar metals, and an gauge which is basically a millivolt meter.

Most aviation EGT and CHT Systems are based around three types or thermocouples. The most common type K, is used on most EGT systems. It uses chromel and alumel wires in the element and lead.

Many CHT Systems use a type J system or iron and constantan metals.

Thermocouples used in aviation are simple to troubleshoot with a good quality digital multi-meter The meter should be fitted with alligator clips on the test leads.

Integrity check: Inspect the entire system for loose connections, broken wires or connectors, or mismatched color codes.
Color codes must match throughout the entire system from the indicator to the probe, including the lead wires.

Resistance check: Care should be taken to be sure the meter and or the probe have been un-hooked before checking for loop resistance. A grounded type K probe Will normally show from O.6 to 1.0 ohms. The same goes for type E or red and brown probes. Type J CHT probes will show a reading of about 0.24 ohms. NOTE: Many CHT Systems do not operate on the thermocouple principle. A CHT probe with no color codes or only one wire works on resistance changes as opposed to millivolt output. Consult the manufacturer for details.
CAUTION - voltage from a multi-meter can damage an EGT or CHT indicator.

Millivolt output check, (EGT ONLY): Using a propane torch, carefully heat the tip of the probe until it glows cherry red. At this point, the probe should put out between 20 and 30 millivolts.

CAUTION - Always wear gloves and have a fire extinguisher handy when using an open flame.

If your probe passes these tests, check with the manufacturer of your indicator for calibration or repair information.

EGT System Troubleshooting Guide

No Indication Open Thermocouple Check Probe: Heat Resistance 1
Shorted or Open Circuit Check Continuity 2
C/A Probe on C/C System Check Color Codes 3
Bad Pot Repair or Replace 4
Friction in Movement Repair or Replace 4
Low Indication
at Peak
C/A Probe on C/C System Check Color Codes 3
Probe - High Resistance Check Probe Heat or Resistance 1
High Indication
C/C Probe on C/A System Check Color Codes 3
Induced EMF Through Lead Check Harness Route 5
Erratic Reading Bad Potentiometer or
Friction in Movement
Repair or Replace 4
T/C Wire Insulation
Worn at Radiator
Check T/C Resistance While Wiggling Wire 1
Erratic Reading at Altitude Lead Chafed-Grounding Check Continuity to Ground While in Fit 2
Indicator Change when Tapped Faulty gauge Repair or Replace 4
No Indicator at Run-Up Indicator Begins at 1200 F
Not All Engines Reach
1200 F at Run-Up
System Normal  
Indicator Works on Bench,
Not in Flight
Probe, leads Repair or Replace 4

1. The best way to troubleshoot EGT probes is to check the loop resistance through the probe at room temperature. Use a good quality multi-meter fitted with alligator clips on the test leads.

CAUTION: Voltage from a multi-meter can damage an EGT gauge.
Care should be taken to be sure the probe has been disconnected from the system before checking for loop resistance. A grounded red/yellow or red/brown probe will normally show from 0.6 to 1.0 ohms resistance.

To check millivolt output, remove probe from aircraft. Carefully heat the tip of the probe element with a propane torch until it just begins to glow dark cherry red. At this point, the probe should put out from 33 to 36 millivolts (1500 to 1650 degrees F) for red/yellow, or 61 to 67 millivolts (1500 to 1650 degrees F) for red/brown.

CAUTION: Always wear gloves and have a fire extinguisher handy when using an open flame.

2. To check continuity of an EGT extension lead, first inspect entire length of wire for chafing or broken insulation. Inspect terminals for loose crimps or connections. With indicator and probe disconnected, check to be sure that there is no continuity from red to yellow or from red or yellow to thermocouple housing. Next, clip red to yellow at either end and check for continuity at opposite end through the entire lead.
3. The latest Rotax engines use a bayonet style probe, and the exhaust manifold has been manufactured for probe placement at the correct distance from the piston.

Older units used a clamp on style probe which has to be installed at the proper distance from the piston, has to be centered on the manifold and in the exhaust pipe. It cannot be allowed to touch the inside of the exhaust manifold pipe, it must sit in the center of the exhaust gasses.

4. Calibration on EGT or CHT indicators MUST be checked against spark plug colour.
See How to read your spark plugs.
5. To prevent indicator error due to induced EMF, the EGT or CHT extension lead wires must not be bundled up or routed with any high current bearing wires such as alternator, avionics, lighting, heaters etc.

Make yourself visible to others when your flying...... Strobe lights make you visible! Click here for more information!

Ultralight News



Web UltralightNews.ca

Ultralight Aircraft News.ca Web Magazine Resource Site . No part of this publication may be copied or distributed, transmitted, transcribed, stored in a retrieval system, or translated into any human or computer language, in any form or by any means, electronic,  mechanical, manual, or otherwise,  without written permission . By copying or paraphrasing the intellectual property on this site, you're automatically signing a binding contract and agreeing to be billed $10,000 payable immediately. Copyright UltralightNews.ca EMAIL  
Return to Main Index for this section