Fuel pump & gear warning failure - help

[goalstop]

Hi all!

Looking for some wisdom from you all to hopefully point my mechanic in the right direction and expedite resolution of an issue… in short, my auxiliary fuel pump and  gear warning stopped working on final approach last night. 
Details below:

+ 1 week ago the starting motor failed. Had been getting progressively slower for awhile in retrospect and finally gave up 

+ we weren’t sure if it was a battery or starter issue so tried connecting battery to a Tug and jumping it, but didn’t work

+ Left the plane stranded and drove home. Mechanic replaced the starter with a fast-turning SkyTec. Started like butter on my flight to bring her home. No fuel pump issues.

+  Descending through 3000ft, I switch fuel pump on. Start smelling an acrid/plasticy smell a few moments later. Try to figure out what it is (hadn’t connected the dots) — check breakers, look for smoke, can’t figure it out. After a minute or so the smell goes away

+ Final approach - landing checklist - fuel pump on. Wait—why is the fuel pump off? I could’ve sworn I turned it on passing 3,000ft. Switch it on. Acrid smell again. Finally connect the dots. Go to hit the switch to turn it off, and it’s already switched off.

+ Throttle back to slow to approach speed… no gear warning. Pull throttle full idle. Still no gear warning. Put gear down and come in to land

+ Back at hangar. Ground testing gets nothing out of the fuel pump. Zero operation.

Any wisdom on what the issue here is? The things that confuse me are:

+ Why both the gear horn and the fuel pump? Are the connected or coincidence?

+ Why did the pump magically start switching off? Is there a breaker integrated in the switch?

+ am I likely going to need a new fuel pump or does this sound like a wiring issue?

+ could it have anything to do with the new starter or attempts to jump the battery?

 

thank you all for your help here!!

[65MooneyPilot]

The fuel pump and gear warning are separate circuits. The fuel pump switch is also the circuit breaker. You should have a circuit breaker labeled gear warning. This circuit goes thru two switches to power the horn. The fuel pump and wiring will have to be closely inspected and trouble shot because you smelled electrical smoke. What is your planes S/N? In the service manual there are the wiring diagrams for the plane. 

[goalstop]

1 hour ago, 65MooneyPilot said:

The fuel pump and gear warning are separate circuits. The fuel pump switch is also the circuit breaker. You should have a circuit breaker labeled gear warning. This circuit goes thru two switches to power the horn. The fuel pump and wiring will have to be closely inspected and trouble shot because you smelled electrical smoke. What is your planes S/N? In the service manual there are the wiring diagrams for the plane. 

Thank you! S/N is 381, 1964 M20E

[Immelman]

Multiple electrical issues would really concern me. It doesn't sound like a coincidence. Does your airplane have a volt meter? Can you see what your alternator/generator is putting out, without risking damage to other equipment?

[65MooneyPilot]

[65MooneyPilot]

Here is your wiring diagram. If you go to the download section you will find the service manual and the legend that goes with it. It includes original equipment part numbers. This screen shot does not expand very well. Download the manual. 

[65MooneyPilot]

The landing gear system is pretty straight forward. There are two microswitches and the horn in the system. One switch in the down lock fixture in the instrument panel and the other is mounted on the throttle cable. For the horn to receive power the gear handle must be out of the up position and the throttle must be in the idle position. So when you pulled the throttle back and no horn sounded was the yellow light on? This lets you know if the first switch is working. If the yellow light is on then you can move down to the throttle switch. If that switch is working (you can check for voltage with volt meter power on or ohm meter power off) then go to the horn and check for voltage with power on. If voltage at horn with no sound then the horn needs to be fixed or replaced. When you retracted the gear did the lights go from green to yellow and back to green for landing? Note if the yellow light is burned out the horn will still sound.

The fuel pump circuit is very simple. The power from the left bus powers the fuel pump through a circuit breaker switch. If the pump froze up the switch will turn off. I am a little concerned the switch did not work correctly and allowed the wire to get to much current. Closely inspect the wiring in that circuit and a good look at all the wiring to see if you can see burnt insulation.  Good luck, hope this helps you.

[N201MKTurbo]

Did you hook it to a 28 volt tug? You might have fried it while trying to start it.

[65MooneyPilot]

I do not recommend ever jumping your plane like a car. My neighbor recently tried to jump his plane and fried the alternator. Another friend of mine had his plane upgraded at the avionics shop and when he went to pick it up every piece of equipment that had the power on was fried including the brand new GPS they just installed. We are sure it was a power supply problem. 

[goalstop]

20 hours ago, 65MooneyPilot said:

 

 

20 hours ago, 65MooneyPilot said:

The landing gear system is pretty straight forward. There are two microswitches and the horn in the system. One switch in the down lock fixture in the instrument panel and the other is mounted on the throttle cable. For the horn to receive power the gear handle must be out of the up position and the throttle must be in the idle position. So when you pulled the throttle back and no horn sounded was the yellow light on? This lets you know if the first switch is working. If the yellow light is on then you can move down to the throttle switch. If that switch is working (you can check for voltage with volt meter power on or ohm meter power off) then go to the horn and check for voltage with power on. If voltage at horn with no sound then the horn needs to be fixed or replaced. When you retracted the gear did the lights go from green to yellow and back to green for landing? Note if the yellow light is burned out the horn will still sound.

The fuel pump circuit is very simple. The power from the left bus powers the fuel pump through a circuit breaker switch. If the pump froze up the switch will turn off. I am a little concerned the switch did not work correctly and allowed the wire to get to much current. Closely inspect the wiring in that circuit and a good look at all the wiring to see if you can see burnt insulation.  Good luck, hope this helps you.

Super helpful. I did notice that when I went to push the throttle in for priming, I didn’t hear the typical ’click’ of the gear microswitch… would that be a mechanical (eg fell off the throttle cable) or an electrical issue with the switch?

the green light did go on, so that switch is fine

Edited December 8, 2024 by goalstop

[goalstop]

17 hours ago, 65MooneyPilot said:

I do not recommend ever jumping your plane like a car. My neighbor recently tried to jump his plane and fried the alternator. Another friend of mine had his plane upgraded at the avionics shop and when he went to pick it up every piece of equipment that had the power on was fried including the brand new GPS they just installed. We are sure it was a power supply problem. 

Wow note taken, thank you

[65MooneyPilot]

Yes, the micro switch has a mechanical finger that actuates the switch. If it is broken it will not work. It is fairly easy to see looking up at the throttle cable on the back side of the instrument panel. Take a flashlight and inspection mirror to view it. 

[goalstop]

Offering an update here:

+ Fuel pump was just bad. Had started leaking a little bit, as evidenced by a few drips of fuel out the front tube after each flight. Finally went fully bad and stopped working

+ Gear issue entirely unrelated. Mechanical finger had slipped off the right positioning and wasn’t actuating

otherwise good to go. Glad it happened at home!

[65MooneyPilot]

Thanks for the update. Older planes can have multiple problems. 

[Bartman]

Are you sending your fuel pump to Aeromotors for rebuild ?  Mine has the Dukes pump, and now the rebuilt pump should outlast the rest of the plane.

[MikeOH]

On 12/18/2024 at 3:47 PM, goalstop said:

Offering an update here:

+ Fuel pump was just bad. Had started leaking a little bit, as evidenced by a few drips of fuel out the front tube after each flight. Finally went fully bad and stopped working

+ Gear issue entirely unrelated. Mechanical finger had slipped off the right positioning and wasn’t actuating

otherwise good to go. Glad it happened at home!

Good to hear.

But you did smell something electrical burning...that should NOT have happened.  Have you tried to locate what 'melted'?

[Will.iam]

On 12/8/2024 at 9:59 AM, goalstop said:

Wow note taken, thank you

Also when jump starting and airplane usually means because the battery is dead. When you turn on the alternator it is now getting a workout because of the huge amp draw the battery is pulling to charge back up. This could potentially damage the alternator if it over heats or the alternator coupling to the engine especially if it’s old could break under the new high torque the alternator is pulling trying to charge the battery. In the world of airplanes where everything is so much more expensive, charging up the battery using a battery charger will be more kind to the battery and other components than a rapid charge by the alternator.