What Is a Relay 109 Issue


Any technician specialist at Volkswagen AG (Audi – Volkswagen – Seat – Skoda…) knows this relay well and its assignment: let’s discover what this mysterious relay 109 is used for.

Assignment of the relay 109

The relay 109 is only valid for the Volkswagen group: easily recognizable; it is a classic relay with, on its cover, the name in big and white numbers, the number 109.

This relay controls the power supply to the engine management computer.

Relay 109: description

Often, its pinning is specific, but its operation is that of a standard relay.


A relay is composed of 2 circuits:

  • Primary circuit

The primary circuit or power circuit is the circuit that supplies, in the case of the relay 109, the engine management computer. It consists of an input marked by the number 30 (power supply), a switch, and an output of 87 (ECU side).

Good to know: the schematic diagram with the terminal numbers is often reproduced on the relay cover.

  • Control unit

The control circuit consists of an electromagnetic coil. The output of the loop (86) is connected to the ground. A prohibition can condition the ground supply: for example, the central computer delivers the ground when the automatic gearbox is disengaged.

Operation of relay 109

Relay 109 Issue

When the coil of the control circuit is powered by terminal 85 (+ after contact) and grounded at terminal 86, it creates an electromagnetic field that closes the switch of the power circuit: the engine management ECU is then powered.

Malfunctions and maintenance of the relay 109

Preliminary checks

When the relay is energized, it initially emits an audible click that can be heard and heard when a finger is placed on it: in this case, the control circuit is operating.

Several failures can affect the relay 109; this post delivers you the various checks to be carried out in the form of a table and the conclusions drawn from it.

Checking the relay



Values found

Manufacturer’s values


Control circuit

Ohmmeter terminal 85 and 86 (relay disconnected)

0 Ω or ∞ (infinite)

~85 Ω

Coil in short circuit or cut

Voltmeter terminal 85 and vehicle ground

0 V

12 V

No power supply, see fuse and line.

Voltmeter terminal 86 and vehicle ground

12 V

0 V

No ground, see ground line or prohibition of the calculator

Power circuit

Voltmeter terminal 30 and vehicle ground

12 V

0 V

No + power supply, see fuse and line.

(Relay supplied in + terminal 85 and – terminal 86)

voltmeter terminal 87 and vehicle ground

0 V or <12 V

12 V

cut or abnormal resistance of the relay contact

Note: the above checks require mastery of the multimeter and basic knowledge of electricity.

Replacement of the relay 109

You can easily find a 109 relay in the Volkswagen network, car parts distributors, or on the internet.

The prices vary from $10 to $35, including shipping.

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