Power circuit management
How terminals S, 15, 50 and 75 are switched: an ignition lock with four switches or a start button, the “two out of three” rule for the terminal 15 relay, and starter operation — based on self-study program SSP 611.
How power supply circuit management works: which unit generates the terminals, why the ignition comes on even if one switch fails, and how the starter is engaged.
Who controls the terminals
Which control unit manages the power supply circuits — the terminals — depends on the equipment: without keyless access it is the steering column electronics control unit, with it — the steering column lock control unit. (without keyless access — J527 · with it — J764)
Without keyless access, the ignition lock has four switches: contact S, two terminal fifteen requests and a terminal fifty request. (ignition lock — 4 switches: S, terminal 15 requests 1 and 2, terminal 50 request)
From them, the steering column control unit determines the status of terminals fifteen, fifty and seventy-five and contact S, and continuously transmits it over the CAN bus. (J527: status of terminals 15, 50, 75 and S — via CAN convenience)
Terminal fifteen: two of three
Contact S closes when the key is inserted; the unit reports this over the bus and via a separate wire to the onboard power supply control unit. (contact S: key inserted → CAN and separate wire in J519)
The terminal fifteen relay is switched by the onboard power supply control unit: it reads three wires — the two terminal fifteen requests and contact S. (J519 reads 3 wires: request 1, request 2, contact S)
If at least two of the three wires carry voltage, the relay switches on — so the ignition comes on even if one switch fails. (voltage on 2 of 3 → relay J329 on)
Terminal fifty
The engine control unit receives the start request via a separate wire and over the CAN bus. (start request to J623 — by wire and via CAN)
The control unit switches both starter relays to ground, the starter receives terminal fifty voltage, and the unit checks via a diagnostic wire that the terminal is actually switched on. (J623 → relay J906, J907 → terminal 50 on the starter · diagnostic wire)
With keyless access
With keyless access, the ignition lock is replaced by a start button with two microswitches — they are read by the steering column lock control unit. (button E408: 2 microswitches → J764)
If one microswitch fails, neither the ignition nor the start can be switched on — the engine can only be switched off with the button once. (one fails — no ignition, no start · switching off — once)
There is no longer a separate terminal seventy-five relay: the control units receive its status over the CAN bus. (no terminal 75 relay — its status via CAN)
Summary
- Which control unit manages the power supply circuits — the terminals — depends on the equipment: without keyless access it is the steering column electronics control unit, with it — the steering column lock control unit.
- The terminal fifteen relay is switched by the onboard power supply control unit: it reads three wires — the two terminal fifteen requests and contact S.
- If at least two of the three wires carry voltage, the relay switches on — so the ignition comes on even if one switch fails.
- The control unit switches both starter relays to ground, the starter receives terminal fifty voltage, and the unit checks via a diagnostic wire that the terminal is actually switched on.
- If one microswitch fails, neither the ignition nor the start can be switched on — the engine can only be switched off with the button once.
Next episode: interior lighting.