Pitot Heat Failure troubleshooting

[ronr]

2003 M20R DX 29-0289 (28V system)  with factory installed TKS FIKI system.

Yesterday I turned on Pitot heat and the current draw, which usually increases from about -2A to -9A did not budge at all.

Troubleshooting today, we found 11.9V at the input to the Pitot Heat Breaker Switch (unexpected in a 28V system) which dropped to 70-80 mv when the switch was turned on.

In the line going to the hot side of the Pitot Heat Breaker Switch, we found a "box" which I could not locate on any of the diagrams I have.

This appears to be a rectifier according to the part number and appearance. The voltages read about 23.9 V in and 11.7 V out, so it appears to be faulty.

But I can't locate this item in the various maintenance manuals I have, nor does it appear on the electrical diagrams I have access to.

I'm guessing it is there to block spikes when turning off the pitot heat switch. Is it a standard Mooney part costing hundreds of dollars?  Or something I can buy on line (I've seen them for under $5)?

Thanks.

    Ron

[N201MKTurbo]

https://www.newark.com/solid-state/kbpc5006/bridge-rectifier-single-phase/dp/10P1435
 

Hard to imagine why it is there. Or, why it failed. 

 

[ronr]

Thanks, Rich.

Yes, I know what it is. And your link is arguably the least expensive. 

But my question is really if the diagrams I have are just lacking that item?  Or if it was put in for some other reason. I don't have my logbooks handy so can't refer to them.

[GeeBee]

The pitot heat on a Fiki airplane is different. I would talk to CAV and get the scoop as when they install the system it is a different pitot tube.

[ronr]

I will do that Monday. But I've got the Install manual and diagrams for the TKS system from CAV, and, although it mentions replacing the pitot heat breaker switch with a different one, there is nothing on that diagram indicating that the power to that switch is anything other than a line from the 28 V bus. No sign of any rectifier, diode or anything else in that line.

[rgpilot]

In a FIKI Mooney the full heat requires the plane to be in flight. Simuulate flight by putting a hose on pitot tube and squeeze it slightly until your airspeed indicator moves up to

about 70 knots.  Then your stall vain and pitot should get very hot.

[ronr]

That does not explain the absence of increased current draw when testing this on the ground (which has been present for the five years I have owned this a/c on every pre-flight). There is also no mention of this being a requirement in the AFM or in the TKS supplement of this being a requirement for the routine pre-flight testing of the pitot heat.

And where is that pre-flight testing procedure documented?

[LANCECASPER]

@StevenL757 knows these systems and heated stall warnings.

[StevenL757]

Lance, thanks for the callout.  I know a little, but not as much as Ron is probably looking for.  He sent me a PM, so I'll have a look and follow up.  I don't want to convolute everything by sharing only what I "think" I know.  :-)

[ronr]

In a PM from @StevenL757 to me, he mentioned that the item is used as blocking diode for the standby alternator system. It prevents the non-essential bus from being fed by the standby alternator. The schematic for this installation was not available to me in the documentation I had from Mooney, but I was eventually able to locate a schematic for my Ovation serial number. It seems that when running the standby alternator, this diode will block power to adjacent switches to the left of the Pitot Heat breaker switch, none of which are on the essential bus.

But under battery power, there should not be any drop in voltage across this diode. Since we measured a significant drop, my assumption is that this diode has failed. A replacement is on order.

Thanks to all.

Ron

[PeteMc]

On 4/27/2024 at 9:48 AM, ronr said:

This appears to be a rectifier according to the part number and appearance. The voltages read about 23.9 V in and 11.7 V out, so it appears to be faulty.

So you have a box in your 28v system that drops the voltage to 12v going into the Pitot switch. And why do you assume it is faulty???? 

What is the input voltage to the pitot heater?  Is it possible it is 12v and that's why they dropped the voltage?  Guess it is also possible that a prior owner couldn't find a 28v heater and they added the box so they could put in a readily available 12v replacement.   

Maybe a call to a Mooney shop or someone here can confirm if it is supposed to be 12v in an R (or your year R). 

 

[ronr]

That was something we actually considered for a short time. Once we reviewed all the schematics (especially those for the stand-by alternator install), the log books and the specs for the KBPC5006, that clearly could not be the case.