EGT & CHT not working

[Soleair]

I ran the engine again today, as a final break-in before I prepare to trailer my plane to the airfield. Well, it was meant to be a break-in, but I had a major problem with both EGT & CHT gauges.

 

These are both dual instruments, with two needles on each for #1 & #2 cylinders.

 

The only deflection I saw on any of the needles was one side of the CHT. This rose very quickly to an indicated 550F. The other side didn't move.

 

I closed the throttle to run at idle, and the needle slowly dropped to 375, where it stayed, so then I stopped the engine. I took out a spark plug from each cylinder, both were very dark brown, so over rich if anything. I have a non-contact infra red thermometer, and I pointed this down the plug holes into the combustion chamber. Readings were 187 on #1 cyl, 168 on #2.

 

I tried changing the under-sparkplug probes round, & the same side of the gauge showed the same reading. I changed the connections over at the back of the gauge, and this time the (false) high reading showed on the other side, with the other side showing no movement.

 

The EGT gauge showed no movement on either needle. I removed & replaced the connections to the back of the EGT gauge, checked for broken wires - nothing found.

 

The weird thing is I last ran the engine in November last year, when all the gauges worked. I have not changed any of the gauge wiring since, although I have done a permanent wiring of the engine/cockpit wiring harness instead of the temporary job I had in November as a test run. But I don't see that engine wiring should affect the instruments.

 

So - can anybody cast any light on this problem? I don't even know how these instruments work. I assume they measure a resistance at the sensor which varies with temperature change. But what actually causes the needle to move? Or are they bi-metallic probes that generate a tiny voltage, which moves the needle?

 

Can the probes and instruments be checked in any way, without substituting on a different installation?

 

Any help gratefully received.

 

Cheers

 

Bruce

 

 

[Nobody]

EGT probes are therocouples and are sensitive to methods of joining/splicing the wires as well as the wire type. Have you changed the wire type and the splices since the last time you ran it?

 

 

[Soleair]

No - I haven't changed any of the instrument wiring - it is just as it was last time I ran the engine 4 months ago.

 

 

[Yenn]

With none of them working I would suspect broken continuity in the power circuit somewhere. Having been left since November I would look first at the ground connections. Moisture could have caused problems there. Remove and re connect all power connections. As a wild card check the engine earth.

 

 

[Soleair]

Thanks, Yenn. But the gauges are independent of the power circuit: they are not supplied with power from the 'busbar' nor are they earthed (other than via the probes). Since the rest of the electrics are taken to a common earth, & all function ok, i'm guessing the engine earth is good. It certainly looks ok.

 

 

[Bruce Tuncks]

Thermocouples generate a tiny voltage from the junction of 2 dissimilar wires. This tiny voltage can operate a gauge by itself, or it can be amplified electronically. What sort is yours?

 

 

[Soleair]

With no external power supply I assume it is the former. So even less to go wrong?

 

 

[spacesailor]

Perhaps a little corrosion on the push-on terminals at back of gauge,

 

If so, a very little "Lanolin grease" would help.

 

spacesailor

 

 

[Soleair]

Ok, I'll try that. But I did remove & replace the connecting pins individually to try to ensure they were making good contact.

 

 

[Bruce Tuncks]

Try putting a digital meter on the thermocouple ends. ( where it connects to the gauge) . When the probe is warm, you will see a few millivolts if it is working. Then you will know if it is the probe wire or the gauge.

 

If no millivolts there, you have a broken wire or bad connection in the probe.

 

 

[Soleair]

Thanks Bruce I'll give that a try, when I get back from Easter visiting duties.

 

 

[spacesailor]

Please keep us posted.

 

spacesailor

 

 

[Soleair]

But I don't see that engine wiring should affect the instruments.

Wrong!!

 

I posted on another forum , where it was suggested that if the instrument sensor wires are in close proximity to current carrying wiring, that this will affect the instrument readings.

 

I thought about this, and it became logical that the magnetic flux generated by current in a wire could induce a small voltage in an adjacent wire. And evidently these instruments are measuring millivolts. Also, the only difference from when the instruments worked was that I had re-done the wiring into a big umbilical. D'Oh!

 

So yesterday I separated the instrument wiring from the rest, & made a second penetration through the firewall for these, about 80mm away. I routed the sensor wires away from the others, & ran up the engine.

 

Success! All needles showing credible deflections. Hurray! Although the CHT's read about 70F lower than the temps given by my infrared non-contact thermometer.

 

So I did the approved engine break-in. This basically comprises running the engine up in increments of 1000rpm, holding for 10 seconds at each step, then holding max rpm for 60 seconds. The revs are then reduced in 1k steps, holding for 10 seconds, down to idle. That's it - engine is run in.

 

The propellor I made for this engine was initially 65" diameter, which I hoped would be a bit too much, given the minimal ground clearance this provided. On my first run, max rpm was 5820. Nominal max is 6200. So I lopped an inch off each end (giving me the preferred extra ground clearance). Max rpm after the tipectomy was 5930, which equates to about 62hp. I'm happy with this, as I expect an improvement as the engine loosens up.

 

So I'm now assembling the plane in my front yard, prior to doing a W&B. If all good, a 1/2 day modifying my little box trailer to take the Max, then off to its new home at the airfield next week. Yaay!

 

Bruce

 

 

[spacesailor]

Many thanks for the reply. Bruce.

 

I would have thought the extra induced volts would make the gauge read higher than normal.

 

spacesailor

 

 

[Soleair]

One CHT needle did. The others were maybe opposite polarity? Just guessing here - electricity is mostly white man's magic to me.

 

Bruce

 

 

[spacesailor]

I wonder if you "twisted" the egt/cht wires it could stop the induced voltage?.

 

spacesailor

 

 

[Bruce Tuncks]

Yep spacesailor, they said to do this with detonator wire at an explosives school I once went to. The induced voltage from magnetic fields can actually set off detonation with separated wires.

 

The thermpocouple wire I bought recently has a wire braid which should also stop interference.

 

 

[kgwilson]

FWIW I had a similar problem with my tacho reading erratically. I thought it was the sensor not being correctly set to read the 2 pulses (1 tag being set at a different distance to the other), then the instrument which was removed & tested. I rang & spoke to jabiru technical who were very helpful but nothing they said worked. Eventually by temporarily running the wires across the top of the deck & into the cockpit away from the other wiring... Voilla! Perfect. Re-routed the tacho wires & problem solved. Induced voltage from all the wiring from the engine bay running through the same loom & the tacho wires probably being alongside one of the others with a decent voltage running through it.

 

 

[Nobody]

I have found this reference good for understanding aircraft wiring systems. Page 14-11 is the start of the section on thermocouples.

 

http://www.aeroelectric.com/Book/AEC_R12A.pdf

 

 

[spacesailor]

Hi Bruce,

 

The braid has to be earthed at both ends to disperse any residue volt-spikes IMH opinion (very humble ) LoL

 

spacesailor

 

 

[Soleair]

Thanks for your input, Spacesailor.

 

The braid from my EGT lines aren't earthed at the panel end. But it stops before the instruments, and converts to an unbraided 'adaptor' length of 4 core cable, which connects to the instrument. Same as the CHT wiring. These 'adaptor/extension ' wires were supplied with the sensors and their main wires.

 

I know nothing of these things. But both instruments do now seem to be giving credible readings, although the CHT's are down approx 70F relative to my infrared thermometer.

 

Cheers

 

Bruce

 

 

[spacesailor]

If it works, wonders. LoL

 

spacesailor