12V Hot only when engine running - Land Rover Forums : Land Rover and Range Rover Forum
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post #1 of 26 (permalink) Old 07-22-2018, 10:52 AM Thread Starter
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12V Hot only when engine running

What can I tap into to energize a relay?




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post #2 of 26 (permalink) Old 07-22-2018, 11:42 AM
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Be more specific, do you need an ignition live in the cabin or in the engine bay?

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if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #3 of 26 (permalink) Old 07-22-2018, 11:46 AM Thread Starter
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Originally Posted by sierrafery View Post
Be more specific, do you need an ignition live in the cabin or in the engine bay?


Apologies, engine bay first prize, but Iíll settle for in cabin. Must be live only when engine running.


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post #4 of 26 (permalink) Old 07-22-2018, 12:16 PM
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Apologies, engine bay first prize, but Iíll settle for in cabin. Must be live only when engine running.


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if you want one only with running engine and not with ignition on II the only option is the warning output from the alternator, the brown/yellow wire from it or you can find it in C0203/0449 pin 9 also brown/yellow wire
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2000 Discovery Td5 ES manual

Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #5 of 26 (permalink) Old 07-22-2018, 04:28 PM
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You could also use power to coils
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post #6 of 26 (permalink) Old 07-23-2018, 01:40 AM
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You could also use power to coils
Isnt there feed with ignition on even if the engine is not running ?

My friends call me Fery

2000 Discovery Td5 ES manual

Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #7 of 26 (permalink) Old 07-23-2018, 02:26 AM
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Originally Posted by sierrafery View Post
Isnt there feed with ignition on even if the engine is not running ?
There is basically no power source that only operates when the engine is running. I am not sure why he wants one. Ignition power would rarely be on when the motor is not running. The warning light wire to the alternator would be live with ignition on. The only thing that I am aware of that operates only when the engine is actually running is the trigger for the fuel pump relay. But this will still get power for a few seconds without the motor running.
Again, I am not sure why ignition power is not suitable for what he wants. Why is it so critical that the item does not have power with the ignition in the run position but the engine may not be actually running.
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post #8 of 26 (permalink) Old 07-23-2018, 02:41 AM
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The warning light wire to the alternator would be live with ignition on.
No, the alternator side is not cos the other side has ignition feed, check the diagram, the alternator's output becomes live only when it starts and that's why the warning goes out cos it gets voltage on that earth path which otherwise is a switched earth delivered by the instrument pack's internal "hardware"
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2000 Discovery Td5 ES manual

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VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #9 of 26 (permalink) Old 07-23-2018, 03:59 AM
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Originally Posted by sierrafery View Post
No, the alternator side is not cos the other side has ignition feed, check the diagram, the alternator's output becomes live only when it starts and that's why the warning goes out cos it gets voltage on that earth path which otherwise is a switched earth delivered by the instrument pack's internal "hardware"
The warning light is actually providing power to the alternator to start the charging process. So it is actually a live wire with the ignition on.
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post #10 of 26 (permalink) Old 07-23-2018, 05:05 AM
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The warning light is actually providing power to the alternator to start the charging process. So it is actually a live wire with the ignition on.
IMO that's wrong, the alternator is on the earth path.... it's triphasic with self excitation, if that brown/yellow wire was live the warning lamp would not illuminate as it has live from F27 the other side, the wire from the warning lamp to alternator is closed to earth through the brushes with alternator in rest position, when the alternator starts spinning and is delivering voltage the LED gets live both sides that's why it goes out.... i don't know how to explain that better, these kind of alternators dont need ''exciter'' wire, you can remove the whole instrument pack and the alternator will still charge normally.

let's put it this way: if you give feed to a LED one side what is on the other side of it if while it's illuminated? ... live or earth?

My friends call me Fery

2000 Discovery Td5 ES manual

Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #11 of 26 (permalink) Old 07-23-2018, 05:38 AM
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Originally Posted by sierrafery View Post
IMO that's wrong, the alternator is on the earth path.... it's triphasic with self excitation, if that brown/yellow wire was live the warning lamp would not illuminate as it has live from F27 the other side, the wire from the warning lamp to alternator is closed to earth through the brushes with alternator in rest position, when the alternator starts spinning and is delivering voltage the LED gets live both sides that's why it goes out.... i don't know how to explain that better, these kind of alternators dont need ''exciter'' wire, you can remove the whole instrument pack and the alternator will still charge normally.

let's put it this way: if you give feed to a LED one side what is on the other side of it if while it's illuminated? ... live or earth?
I am not going to debate the point. He can check whether it has voltage to it with ignition on by simply putting a volt meter on the wire.
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post #12 of 26 (permalink) Old 07-23-2018, 05:46 AM
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Yes, anyone can but what i said was already verified by me otherwise i wouldnt have insisted... also IMO the diagram is clear enough especially knowing that the warning lights are LEDs.

My friends call me Fery

2000 Discovery Td5 ES manual

Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #13 of 26 (permalink) Old 07-24-2018, 01:07 PM
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Having recently researched the operation of the alternator warning light, I agree with both sierrafery and p76rangie. I think the answer is to disconnect the brown/yellow wire from the alternator and use its terminal as the 12v, engine running-only way to get power to the relay he wants to power (for whatever reason).

'99 DII, STI wires, Champion 7070s, '04 CDL & SLABS, Garvin WILDERNESS rack, painted headliner, air springs victim, headgaskets at 109,000, plus victim of XYZ (twice) and many other ridiculous DII failures. Option B AMIGO FREE since February 2011.

Runs like a top; now if I could just afford the gas! December 2014 update - Wait, I can afford the gas! Drill baby, drill!!!
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post #14 of 26 (permalink) Old 07-24-2018, 03:40 PM
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Having recently researched the operation of the alternator warning light, I agree with both sierrafery and p76rangie. I think the answer is to disconnect the brown/yellow wire from the alternator and use its terminal as the 12v, engine running-only way to get power to the relay he wants to power (for whatever reason).
Now that's a good one.... you agree with both @p76rangie and me while me and him have a major disagreement as result of a friendly polemic i hope

and please forgive me to tell you but what you say with the wire has no logic

My friends call me Fery

2000 Discovery Td5 ES manual

Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope


if you think my english is peculiar it means you didnt hear my japanese

ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.
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post #15 of 26 (permalink) Old 07-24-2018, 04:33 PM
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Sure it makes sense. The wire indeed gets power from fuse 27 when the ignition is in Position II and the engine not running, illuminating the warning lamp. With the engine running and the alternator putting out voltage greater than the battery the warning lamp is extinguished. If you disconnect the brown/yellow at the alternator and connect a wire to the terminal and to ground you get volts, but only when the alternator is spinning and making electricity.

'99 DII, STI wires, Champion 7070s, '04 CDL & SLABS, Garvin WILDERNESS rack, painted headliner, air springs victim, headgaskets at 109,000, plus victim of XYZ (twice) and many other ridiculous DII failures. Option B AMIGO FREE since February 2011.

Runs like a top; now if I could just afford the gas! December 2014 update - Wait, I can afford the gas! Drill baby, drill!!!
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