Trouble shooting a low voltage

[tony]

Well I don't have a J model, I have an F.  Someone did an STC done years ago which removed the generator and installed an alternator.  My Voltage regulator is on the firewall but that doesn't mean that's where yours is.  Byron has a J, I'm sure he can tell you. 

[jetdriven]

My J regultor is mounted to the inside of the firewall, on the copilot side, right behind the defroster duct. Others mount it on the inside of the fuselage under the windshield on the right side.  Anyways, check the conductor isnt cracked up and shorting to the shielding inside the field wire. The voltage then drops to zero as well, but its not a defective regulator or an open circuit.

[mooniac15u]

My J regultor is mounted to the inside of the firewall, on the copilot side, right behind the defroster duct. Others mount it on the inside of the fuselage under the windshield on the right side.  Anyways, check the conductor isnt cracked up and shorting to the shielding inside the field wire. The voltage then drops to zero as well, but its not a defective regulator or an open circuit.

 

I think that's pretty much what Tommy just said he was going to do in his last two posts.  He plans to check the wire by checking for voltage output at the regulator.  Voltage there but not at the alternator indicates a bad wire/connection.  No voltage there indicates something upstream (possibly the regulator).

[mooniac15u]

My understanding is that if you have a regulator failure (assume it's open circuit) then no field current will be supplied to the alternator magnet and therefore no output will be generated. So the voltage bus will be the battery voltage.

 

The next step is for us to locate that VR on my J (I just don't want to oepn teh glareshield if it's not the right place to look) to see if it outputs anything. If it does then there is something wrong in the wiring, if it doesn't then it's likely failed.

 

Is my reasoning correct?

 

A logical, systematic approach to troubleshooting.  Isolate components and eliminate them as possible culprits.  I like it!

 

Do you have a background in science or engineering?

[DrBill]

Beware of using a V measurement on the field wire at the regulator.  My neighbors alternator was intermittent and I first found no V on the field so we changed the regulator with a new one (one with trouble shooting LED).   The LED indicated the field was shorted and in a few mins we found a short in the coaxial wire going from regulator to the field.  (pinched at an adel clamp). Before replacing the wire, we stripped it back past the clamp and the short went away.

Somewhere along the line, the wire had made its way between the metal parts of the clamp and was shorted center to braid shield.  The ultimate fix was to replace the wire (no easy job). 

Given the MTBF of wires in the  tens of million of hours, and that of the regulator of probably 5000 hrs, one would never expect a wiring fault.

but we all know sh&* happens and if your regular, once a day.

BILL

[mooniac15u]

Beware of using a V measurement on the field wire at the regulator.  My neighbors alternator was intermittent and I first found no V on the field so we changed the regulator with a new one (one with trouble shooting LED).   The LED indicated the field was shorted and in a few mins we found a short in the coaxial wire going from regulator to the field.  (pinched at an adel clamp). Before replacing the wire, we stripped it back past the clamp and the short went away.

Somewhere along the line, the wire had made its way between the metal parts of the clamp and was shorted center to braid shield.  The ultimate fix was to replace the wire (no easy job). 

Given the MTBF of wires in the  tens of million of hours, and that of the regulator of probably 5000 hrs, one would never expect a wiring fault.

but we all know sh&* happens and if your regular, once a day.

BILL

 

Good point, but if he does see voltage there it will confirm a likely problem with the wire.  A better test is probably for continuity in the field wire from regulator to alternator and lack of continuity to ground.

[RobertGary1]

I believe that's right. It's probably mounted to the firewall in front of the copilot.

[Will W]

Any news Tommy?

[Tommy]

Ok I finally had the chance to look at the charging system again. My mechanics is still on holiday. It turns out my VR and OV are tucked right under the co-pilot side panel not on the firewall - glare shield removal was unnecessary but what the heck. (See attached). It looks pretty old with "Mooney Aircraft Inc. Voltage regulator sign on it) No LED lights to tell me if it's working.

How do I test this unit?

Presumably the crimped lines are simply grounding wires from other places. It's the blue and red lines on the right that are the field terminal to the altenator?

Thanks guys

Ps. The voltage across the battery is 12.4v

Pps. does anyone know the part number of this VR (the way it's installed I can't read or take photos beyond the words "Mooney Aircraft Inc. Voltage regulator"...

[DrBill]

Look at the color of the wire on the field terminal of the Alternator.

With the engine off and master on, you should have c 10V or more at the field terminal to gnd.

The field wire from the reg should go to the field  breaker then a wire from the field breaker to the alternator.  They SHOULD  be the same color but not necessarily.  Most I've seen are white coaxial wires.  Coax wires have a shield around the signal wire which can get pinched and short out the signal wire (found this on a Cardinal last month).

 

With the engine running (be careful of PROP !), the alternator output should be c 14V. The field terminal voltage will start around 10 or more then drop as the battery charges and load on alternator decreases.  If you turn on the landing light (assume std bulb not led), the output should not change but the field voltage will increase to maintain 14 v on the output.  

 

Hope this helps.

BILL

[Tommy]

Thanks Bill!

My understanding of the charging system of my 1981 J is that with master's on, the battery will furnish VR with power which then in turn output to the field terminal of the alternator and energizing the magnets.

So far I have established that field terminals at the alternator end is not receiving any power

My question is how do I test my VR - or more precisely where do I put my voltmeter across with Canon Plug? - to see if it receives and / or furnishes power?

Thanks