1967 109 oil pressure light in the speedo

[clearcut]

my light has never come on. how does one properly test that it working? thanks

[siii8873]

this light should come on when you turn the key to run position without starting the truck. If it does not come on when running that is a good thing.

[siiirhd88]

There is a white wire (unfused power with the ignition switch 'on') at the fusebox (originally the A3 terminal) that runs to one side of the warning lamp. The other wire on the warning lamp is green with a yellow tracer and it runs to the low oil pressure switch mounted at the oil filter housing.

The low oil pressure switch is the path to ground for the warning lamp circuit. Power is supplied thru the warning lamp any time the ignition is 'on'. If there is no or low oil pressure, the switch contacts close, and the power passing thru the warning lamp bulb goes to ground completing the the circuit. The bulb should light..... Since your low oil pressure lamp doesn't light when the ignition is on and the engine is not running, you need to check the circuit. The problem can only be a few things:
1) Bad bulb
2) Lamp not connected to power (unplugged at fusebox or broken or disconnected wire)
3) Lamp not connected to ground (unplugged at switch or same wire problem)
4) Bad switch

You can check for power at the wire where it connects to the switch. If you don't have power there when the ignition is 'on', you need to check the wiring and the bulb. There are typically two of the plug in wiring Lucar connectors between the lamp and the switch that can become disconnected or corroded. If you have power at the switch wire, the lamp should light if you ground the wire by touching it to a good clean surface. Running a jumper to the ground on the block or battery works well.

If the lamp lights, the switch is likely bad.

Bob
'02 D2 SE Kalahari
'02 D2 SD
'97 RR 4.6 HSE Vitesse
'80 SIII 109
'75 SIII 88 V8
'68 SIIA 109 V8