Faulty Rotax 912 Alternator Wiring

Pieces of wiring from Rotax 912 alternator to voltage regulator/rectifier

During repair of the motor mount, a major project, we discovered that the wiring of the integral alternator supplying power to the ignition module and the voltage regulator/rectifier had badly corroded.

The above photo shows fragments of the wires going from the stator of the alternator to the voltage regulator/rectifier.

Next is a photo of the wiring going from the stator to the ignition module. Here, too, the insulation has broken off and the wires are badly corroded.

Defective wires supplying power to the ignition module

Surely the damage must have been building up during several years. How could we have missed it during the annual inspections?

The Answer

There are two places where the wires are visible: At the voltage regulator/rectifier, and at the ignition module. These wires looked okay until a recent major repair of the motor mount, where everything firewall-forward had to be removed.

At that time, the wiring had to be disconnected from the voltage regulator/rectifier. Right then, the insulation turned out to be brittle, broke off, and revealed oxidized copper wires.

We then traced the wiring back toward the alternator by stripping off the braided metal shield, and discovered that the entire wire was ruined.

Turning to the wire supplying power to the ignition module, we saw just a small and hardly visible crack of the insulation. But when we disconnected the wires from the ignition module, the insulation broke off, and the copper wires turned out to be badly oxidized.

The Hazard

Now it is one thing to lose the connection of the alternator to the voltage regulator/rectifier. There is still the battery for powering essential equipment, and flight to the nearest airport is assured.

However, if the power supply of the alternator to the ignition module is interrupted, the engine stops immediately, a possibly terrifying situation depending on when and where this is happening.

Wiring Repair Versus Stator Replacement

For replacement of the wiring from the stator to the voltage regulator/rectifier and ignition module, you must remove the flywheel at the rear of the engine and then remove the stator. At that point, there are two options.

First, you may have a repair shop for electrical equipment such as electric motors attach new wires to the stator. 

Second, you buy a new stator, which has new wires appropriately attached.

Replacement stator with wires

We recommend the second, more expensive, choice as a matter of safety and reliability. As outlined above, the supply of power to the ignition module is crucial for engine operation, and stator replacement seems the best way to assure this.

Stator Replacement

The replacement process is described in the Rotax 912 Heavy Maintenance manual, Section 74-00-00, page 48. It requires three tools: The locking pin for locking the crankshaft, the flywheel puller, and the special washer for protecting the crankshaft from the bolt of the puller.

The entire process is straightforward, except that you may have difficulty removing the big bolt securing the flywheel. 

The bolt has a 24 mm hex head. If you do not have a metric socket of that size, you may also use a 15/16 in. socket, since 15/16 in. is 23.8 mm. 

It takes a considerable force to turn the bolt. We used a 2ft breaker bar with an iron pipe extension to do this. Just increase the torque gradually until the bolt moves. 

You may also consider using an impact wrench. In our opinion, this is not a good idea since it introduces considerable shaking into the crankshaft. But that is just an opinion, of course.

Preliminary Test

Before you install the new stator, do a preliminary test of the wiring, as follows.

Measure the resistance between the two wires going to the voltage regulator/rectifier. It should be 0.1 - 0.8 Ohm.

Measure the resistance from one of the two wires to the body of the stator. It should be infinite; that is, there is an open circuit.

For additional details about these and other tests, see the post Testing Rotax 912/914 Generator and Voltage Regulator/Rectifier.

Performance of New Stator/Wires

The new stator produces 13.7 V at much lower rpm than before, and the running of the engine at idle rpm as well slightly above that value has become much smoother. 

We guess that, at low rpm, the deteriorating wires interfered with proper charging of the battery and provided insufficient voltage to the ignition module. 

Getting Help

If you have questions about the stator, the wiring, or the replacement process, contact the Lockwood Aviation Tech Line at 863-655-6229 for help. In particular, Lockwood Aviation is the go-to place if your engine has the old style stator, which is a difficult-to-find replacement part.

The Lesson

During each annual inspection, examine the wiring going from the alternator to the voltage regulator/rectifier and to the ignition module with a magnifying glass, and scrape the insulation lightly to check if it is still in good condition. 

If there is any doubt, disconnect the wires and bend them a bit, to make sure that the insulation and copper wire are still in good condition.


Have any questions or feedback about faulty alternator wiring? Please share your thoughts in the comments.