CDT-Gauge dead ...

[NicoN]

I have a 1980 M20k with the -GB engine.

Several years ago a JPI830 was installed with tons of sensors - but not CDT!

Now, the original dual-pointer gauge Shows a TIT, but the CDT pointer indicates alway Zero.

According to my co-owners they never have seen it working.

As I do not like non-working-Features in General, I started searching for Information about CDT.

Fo the aircraft itself, I did not find much about this sensor. The maintenance Manual  says only something about cables, but does not tell me anything about where that sensor is located and how to test it.

Next question is, how important this sensor is  and if it is worth the Money to hook it to the EDM830.

 

[Mjknick@gmail.com]

Sorry CDT not sure what your talking about.


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[NicoN]

That should mean "Compressor discharge Temperature". It is one of the pointers in the Gauge located on the right side Panel. The other pointer is "TIT".

It should Show the temperature BEHIND the turbo-charger, AFAIK this temperature can be pretty high.

[N231BN]

I had to replace the probe when I got my 231. I forget who makes the replacement, the part from Mooney was $$$$$. I'll check when I get to the office.

[MIm20c]

3 hours ago, NicoN said:

That should mean "Compressor discharge Temperature". It is one of the pointers in the Gauge located on the right side Panel. The other pointer is "TIT".

It should Show the temperature BEHIND the turbo-charger, AFAIK this temperature can be pretty high.

There are two sides to the turbo. The exhaust side that has a TIT probe before the turbo and the air intake side that compresses the induction air going to the engine. On my aircraft (not a 231) I have an upper limit for induction air temperature because I don’t have an intercooler.  I’m not sure what would be a normal temperature range for your CDT but my WAG is in the 150-200 range if you are lacking an intercooler. 

 

[CaptRJM]

KS Avionics supplies the TIT probe to Mooney. They also may supply the CDT probe. I’d call them to find out if they can help you. KS price for the TIT probe is about 1/3rd the price from Mooney. 

If you find out that it is the gauge, and you decide to replace it,  first see how many wires are used.  Mooney used 3 different gauges ... two versions have 6 wires and there is 1 version with 5 wires.  It will save you some headaches if you are looking for a used, serviceable gauge.

 

[N231BN]

I looked back through my notes and I bought my probe from LASAR.

https://lasar.com/probes/cdt-probe-a002-cdt

Edited August 24, 2018 by N231BN

[carusoam]

Nico,

How are you setting power for your engine?  MP and rpm, cross referenced to a chart to give %bhp? Great for N.A. engines... but could be misleading for your TC’d engine....

Logic behind knowing the temps...

TIT.... Turbine inlet temp... this is for the protection of the turbine drive side... too hot... the blades turn to nubs... kind of hot... the blades stretch and scrape the housing, turning them to nubs....

CDT... Compressor discharge temp... this is for the protection of your engine... too hot, your engine is risking pre-ignition.... PI melts Pistons in a short amount of time in the climb....

Having an intercooler adds another important piece of information to the mix.... what do they call the intercooler exit temperature? Or do they give a DeltaT for temp dropped across the intercooler...

The engine’s power comes from MP being delivered... the temp of the air is a quality measurement of the MP....

If your operations guide says use this chart or that chart make sure your instruments give the right data to support using those charts.

If you got an aftermarket turbo, or intercooler or complete engine set-up... don’t expect to operate the engine using an old POH that no longer applies... or borrowing a power chart from a plane like a rocket or 252... and using it on a 231....

When you have 50amu costs associated with the engine... spending an amu to keep it running healthy makes a lot of sense...

So many things affect these temperatures, it is hard to make judgements without all the individual thermocouples... including OAT...

if there is instruments to determine the DeltaT across the intercooler, and the DeltaT is close to zero... this indicates the intercooler isn’t working as expected... an FOD blockage has probably occurred...?

All these instruments keep you from...

• overheating and damaging the turbo...
• over heating the intake air and damaging the pistons...
• On cold days, Possibly over pressuring the engine intake and cylinders in return... manually controlled waste Gates...

When it comes to price... the TIT thermocouple lives in the worst environment, firy, acidic, and oxidizing.... at the highest temperatures possible... expect to change it out every few years...

The CDT lives in nicely filtered air, warmer than OAT, but not very hazardous to the thermocouple... if it fails replace it... but don’t expect to do this very often....

Now, go review those power charts to see if they apply to your current set-up...

PP thoughts only, not a turbo expert...

Best regards,

-a-

[NicoN]

We do not have an intercooler or something like that.

When powering MASTER Sw on, I can see a small pointer movement downwards  on the TIT scale even with the engine switched off. Of course TIT is excessively low. After cranking the engine this pointer shows correct values.

But the CDT pointer does not move a bit.

I learnt a lot about the CDT, thank you.

The next step would be to learn if there is a certain method how to test  the gauge and the thermocoupler.

Where is that thermocoupler located?

 

[N231BN]

We do not have an intercooler or something like that.
When powering MASTER Sw on, I can see a small pointer movement downwards  on the TIT scale even with the engine switched off. Of course TIT is excessively low. After cranking the engine this pointer shows correct values.
But the CDT pointer does not move a bit.
I learnt a lot about the CDT, thank you.
The next step would be to learn if there is a certain method how to test  the gauge and the thermocoupler.
Where is that thermocoupler located?
 

The CDT actually uses a thermistor I believe, it provides higher accuracy at low temps. The probe is located just above the turbo in the induction tube.

[N231BN]

Some quick troubleshooting tips for resistive temperature gauges, this also applies to the factory Oil Temp and CHT gauges. If the needle pegs hi, the circuit is open. If it pegs low, there is a short. Since yours doesn't move, the CDT side is either dead or someone replaced the probe with an incorrect unit.

The PO of my 231 had replaced the probe with a Type K thermocouple. Needless to say it didn't work.

[NicoN]

Okay. So, the CDT sensor seems to be a simple one.

Let's see ...

[jlunseth]

On 8/23/2018 at 3:44 AM, NicoN said:

I have a 1980 M20k with the -GB engine.

Several years ago a JPI830 was installed with tons of sensors - but not CDT!

Now, the original dual-pointer gauge Shows a TIT, but the CDT pointer indicates alway Zero.

According to my co-owners they never have seen it working.

As I do not like non-working-Features in General, I started searching for Information about CDT.

Fo the aircraft itself, I did not find much about this sensor. The maintenance Manual  says only something about cables, but does not tell me anything about where that sensor is located and how to test it.

Next question is, how important this sensor is  and if it is worth the Money to hook it to the EDM830.

 

I hate to be the bearer of bad news, but the CDT is very important especially to the GB engine if there is no intercooler, and in fact its not a matter of irritation with non-working items,  the problem is that the aircraft is not airworthy without a working CDT.

Compressor Discharge Temperature is the temperature of the induction air when it comes out of the turbo.  The air is heated quite a bit in the turbo, partly because of the fact that it gets compressed, and partly because the turbo itself is very hot and imparts some of that heat to the induction air.  There is a redline for CDT in your engine, and it is 280 dF.  The redline is not there to protect the turbo, Turbine Inlet Temperature protects the turbo.  The max on that is 1650 continuous and 1725 for a very short period to set the mixture. 

The purpose of the CDT redline is to prevent the engine from going into detonation as a result of the fact that the induction air is too hot.  If you were to look in the Type Certificate for the aircraft, you would find the gauge listed, and that specific gauge, or a gauge that is STCd to act as the primary gauge, must be present and working.  Simply adding CDT to the 830 will not do it unless the 830 is STCd to act as primary for CDT, and I don’t believe it is.  The 930 is, but the 930 has some extra items required by the FAA to permit it to act as the primary set of gauges for the aircraft (mainly a secondary display called the RAD).  JPI may have gotten an STC for the 830 to act as primary, I don’t know, but you would definitely need to check.  As I said, simply getting a CDT readout on the 830 will not return the aircraft to airworthy condition unless the 830 is STCd as primary.  

I think your best course is to get the factory gauge fixed, and then figure out what you can do about the 830.  I am surprised this has gotten through annual.  I hate to tell you this, but in its present condition the aircraft cannot be flown because it is in unairworthy condition.  The gauge needs to be repaired in place, or you must get a ferry permit to move it to a place where it can get repaired.

Look in your POH in the Limitations section you will see the CDT redline limit there.  

[Aerodon]

The EDM can be setup to measure CDT on pins 5,6 on P1.  Its called IAT (Induction Air Temp) and uses a type K thermocouple.

Now is the time you should seriously consider an EDM900.  Stop throwing large amounts of money at old instrumentation.  I'll ask some of my previous EDM900 customers if they have any spare hardware.

 AeroDon

[jlunseth]

If the 830 is not STCd as primary, it can be hooked up to show CDT but the aircraft is still not airworthy.  There must be a Type Certificated or STCd primary display of CDT.  Sorry, but that is how it is.

[vance.k]

I had the same problem and found the wrong probe and a miswire. The replacement part was 880006-501 and I got it from Lasar, as an earlier poster mentioned. 

I’m also attaching a copy of the gauge spec sheet / drawing that helped me sort out the wiring and troubleshoot the readings. 

Good luck!

880000.pdf

[Awful_Charlie]

15 hours ago, Aerodon said:

The EDM can be setup to measure CDT on pins 5,6 on P1.  Its called IAT (Induction Air Temp) and uses a type K thermocouple.

 AeroDon

CDT is only the same as IAT when there is no intercooler - otherwise CDT is what comes out of the turbo (and goes into the intercooler), and IAT is what comes out of the intercooler (and into the engine). My elderly EDM800 has inputs for both and I have them both installed. FWIW, you can use these to track the max MP adjustment on the Bravo, where IAT is the variable that defines the max allowable MP

[NicoN]

@vance.k

[NicoN]

Thank you for your detailled information.

I think, the first approach will be checking the wiring.

 

BTW: Does the EDM900/830 CDT-probe use the same principle or is it a thermo-couple?

[N231BN]

Thank you for your detailled information.
I think, the first approach will be checking the wiring.
 
BTW: Does the EDM900/830 CDT-probe use the same principle or is it a thermo-couple?

JPI uses a thermocouple.

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[N231BN]

Ref. page 5 of the 900/930 primary installation manual, CDT is listed as a primary function. IAT is listed as non-primary.

[ArtVandelay]

Ref. page 5 of the 900/930 primary installation manual, CDT is listed as a primary function. IAT is listed as non-primary.

I believe that doesn’t mean it can’t replace factory IAT, it just means there is no factory limits on IAT.

[N231BN]

I believe that doesn’t mean it can’t replace factory IAT, it just means there is no factory limits on IAT.

That is correct.

[jlunseth]

I went on JPI’s website.  The 830 is not STCd primary.

[Miguel Fisac]

I bought my M20K 231 without both TIT an CDT with cutted cables and only a JPI 700 to bad track both parameters. Just in the firs hot day my engine went into detonation, and I can tell you won't like to experience this situation since it was sounding that the engine was going to get all apart.

After that I decided not to fly before the restitution of the old indications.

Now I have TIT indication working perfectly after installing new cables that insane where cutted at time of installation of the JPI engine monitor.

Still I have to restitute the CDT indication, but since my 231 have a Turbo-Plus intercooler. Can anyone can tell me how reliable would be this CDT indication to avoid future detonation in my GB engine?.