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Diagnosing the Rear Wing Control Module on a Carrera 4

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A malfunction in the control box located under the passenger seat may interfere with the operation of the rear aileron. The blue, slotted component visible on the circuit board is a protective varistor; its replacement should only be considered after checking the power supplies, ground connections, limit switches, and the aileron motor.

Figure 7.1 — Closed rear aileron control box, identified by its part number.
Figure 1 — Rear aileron control box (closed), identified by its part number.

Overview

A malfunction in the control box located under the passenger seat may interfere with the operation of the rear aileron. The blue, slotted component visible on the circuit board is a protective varistor; its replacement should only be considered after checking the power supplies, ground connections, limit switches, and the aileron motor.

Figure 7.2 — Circuit board diagram showing the damaged varistor circled.
Figure 2 — Circuit board diagram showing the damaged varistor circled.

Varistor Inspection

The removed circuit board reveals a cracked varistor. Before replacing it, carefully note its part number and check for any other overheated components or damaged traces.

Figure 7.3 — MC14538B and LA6324 integrated circuits marked on the board.
Figure 3 — MC14538B and LA6324 integrated circuits marked on the board.

Explanation

When a transient pulse occurs, the varistor’s impedance drops sharply, protecting the downstream circuit by short-circuiting it. Under these conditions, it limits the voltage to its rated value. Here, it protects the small electric motor controlled by two micro-relays — see photo — one for opening the flap and the other for closing it.

The timing circuit is built around an MC14538B reference integrated circuit — see Photo No. 2 — which is a monostable multivibrator, meaning it has only one stable state.

Figure 7.4 — Overview of the circuit board and its two control microrelays.
Figure 4 — Overview of the board and its two control microrelays.

Control Board Architecture

The general view shows the two micro-relays that control the raising and lowering of the flap, as well as the delay stage.

Figure 7.5 — 50-V split varistor, the probable cause of the observed malfunction.
Figure 5 — 50-V split varistor, the probable cause of the observed malfunction.

Explanation

The LA6324 integrated circuit is a quad operational amplifier. It processes speed-related signals before controlling the delay stage and the relays. Any repair to the circuit board must be followed by a complete test of the flap’s retraction, extension, and manual control.

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