One for the electronic gurus

[snowbird30ds]

I'm looking at ditching the volvo warning lights and using separate leds and trying to fathom the circuitry in the standard vp panel but the red circled bit in the diagram makes no sense to me, two diodes facing each other surely may as well be a wire break and diodes leading to another diode may as well be one diode, can anyone tell me what I'm missing please?
All the rest is fairly straight forward to recreate and I know I will need a shunt across the alternator light to kick the alternator into life (33 ohm I believe), I will likely do away with the alarm test function as it gets checked each time it's started in effect.

 

[Cariadco]

At a guess, maybe an ESD (electrostatic) protection for the Transistor Emitter or maybe whatever the + on Component No 9?

 

[snowbird30ds]

9 is just the alarm buzzer.

 

[LynnC]

The diode pointing upwards is a zener diode. It is not correctly shown in the diagram.
A zener diode is a device which prevents current flow (in this case downwards) below a set voltage. Above the set voltage, current is alowed to flow.
In this case the voltage is set at alternator voltage (say 13 volts)
The idea is that when the engine is first switched on, the engine is not running. Therefore the voltage on that diode is battery voltage (say 12 volts) and current cannot flow.
With the engine not running the oil pressure light will be on. But the voltage on the diode is at 12 volts and current cannot flow, meaning the alarm will not sound. The idea is to avoid an annoying alarm when first starting the engine.
When the engine is running the voltage (from the alternator) will be 13 volts. Should there be an oil pressure failure, the current can now flow. This will sound the alarm.
The down side is when the engine is running and the V belt falls off, the alternator stops running and the votage to the diode falls to battery voltage (12 volts). The V belt often runs the coolant pump. Should the V belt fall off the engine will overheat. The overheat warning light will come on but the alarm will not sound.

There was a work around for this but I cannot recall what it was. Since you are making your own circuit I guess you can work something out.

This circuit was for the older alarm unit which had a black body and was replaced 20+ years ago
The newer alarm unit has a grey body and is a much more complex unit and overcomes the problems of the old (black) unit. I don't know the circuit of the newer unit. But I belive it still works with no bulbs in it.
If you have the grey unit I suggest you check that the alternator will start without the bulb in place. If it does, just relace the existing bulbs with LED ones.

 

[snowbird30ds]

Thanks for that, I have the old black alarm, I'm tempted to ditch the buzzer altogether and just rely on the lights as I get the irritating alarm on startup anyway, I have TMD41A'S.

 

[LynnC]

Perhaps I should have mentioned this before. Engines have been destroyed because the warning light came on but went unnoticed (in bright sunlight) because the alarm did not sound. Not all because the black unit's characteristics, I might add.
I urge you to have a working alarm (buzzer).