I am not sure about a 79 SD, but my 86 SDL has a manual engine shut off valve inside the engine compartment. Look for a lever that says "Stop" or something to that effect.

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86 300 SDL - Vesuvius

96 SL 500

Assumption is the mother of all screw ups.

Do not go gentle into that good night. Rage, rage against the dying of the light. - Dylan Thomas

All that is necessary for the triumph of evil is that good men do nothing - Edmund Burke