What the Forney did...

The Complaint: Klaus  had a dear longtime customer complain about his LGB Forney (27251). In DCC (digital) operation this Forney, the customer said, changed speed without reason. All in a sudden it went faster. All in a sudden it went slower. The Forney had a sound decoder installed and the speed change of the loco was easy to detect with the chuffs going faster or slower.

First Suspicion:Klaus' customer suspected the decoder might have a malfunction. Especially since the decoder was an older model.  -- The Forney was shipped to Klaus' workshop

Workbench Test: Klaus put the Forney on the test bench in analog mode (DC). Klaus wanted to clearly measure the Amperage that the loco was drawing. First there was nothing, then after 10 minutes of operation the Amperage began to shift and jump. With the changing Amperage the RPMs of the motor changed as well. Then Klaus switched to DCC (digital) operation and found the same shift and jumps in RPM speed.

Diagnosis: Pursuing fault finding by elimination, Klaus had to find out: was it a decoder fault or was it a mechanical fault? For that he separated the decoder from the gearbox. He then connected the gearbox to an analog power source (like a transformer with an analog outlet). The same jumps and shifts occurred. So- without the decoder being connected but the same error occurring it was clear- Klaus had a mechanical problem at hand.
Diagnosis II: Next Klaus dissembled the motor block and took out the motor. He connected the motor to an analog power source. The motor was running normal - at first. Then (here , your Famous Klaus" gut feeling comes into play ) when the motor was running at 10 Volt Klaus changed polarity ( rotating direction ). At that, the Amperage jumped to 1.2 Amps and immediately afterwards fell to the normal value of 0.2 Amps. Klaus could repeat this effect.  - The culprit was the motor.

Why did it happen? Possibly carbon brushes made of weak material had polluted the collector inside the motor. (The collector distributes the power evenly to the coils). Since the new Buehler motors feature a full shell, the carbon brushes inside could not be exchanged.

The Solution: Klaus installed a new motor, re-assembled the engine with sound decoder and the Forney is operating just fine!

Your Next Step: Call Klaus and ship your LGB engine to him to have all your LGB locos running like a charm!