What voltage should green circuit be at fuse box?

Batteries, Alternator, Wiring, Lighting, etc. (Please discuss ignition problems in the Engine category)
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Llessur
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What voltage should green circuit be at fuse box?

#1 Post by Llessur » Thu Dec 20, 2018 6:15 am

I'm trying to trace an issue with my indicators being dim and am slowly working my way around the circuit and another question has cropped up. I wanted to put this as a separate post in case it leads on to a separate problem/discussion.

What voltage should the green circuit be at the fuse box with the engine off? The purple circuit shows full 12v+ battery voltage, whereas the green circuit is showing <11v. It is both sides of the fuse (i.e. the white wire from the igntion switch - I think - so it doesn;t seem to be a fusebox related issue).

Is 11v correct?

At present the car doesn't run so I can't test with the engine running.

I'm not the greatest electrical mastermind so be gentle please :)
Adelaide, South Australia (ex-Brighton, UK)
1977 2500S - White

My 2500 blog: https://triumph2500blog.wordpress.com/

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Llessur
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Re: What voltage should green circuit be at fuse box?

#2 Post by Llessur » Thu Dec 20, 2018 10:46 am

Here's an update on this one...

The green circuit is fed from the white circuit at the ignition switch. This circuit is live when the ignition switch is on (i.e. in the Run position).

I have tried two different ignition switches - both show a voltage drop from the positive feed to the white (live when ignition on) circuit without the engine running. Voltages at the various terminals of the ignition switch are as follows:
  • Switch off - 12.6v at the brown wire from the battery +ve feed. 0v at other terminals.
  • Radio on position - 12.6v at the brown wire from the battery +ve feed. 12.6v at the white & purple feed to the radio. 0v at other terminals.
  • Ignition on position - 11.3v at the brown wire from the battery +ve feed. 11v at the white & purple feed to the radio. 10.9v at white wired to ignition-switched circuit.
Is this voltage drop of 1.7v acceptable between the battery +ve feed and the white ignition-switched circuit when the engine isn't running?

Am I perhaps just chasing a non-issue that will resolve itself when the engine is running and the battery is showing 13+ volts?

I plan to get the exhaust fitted this weekend and start her up so I'll find out sooner or later I guess...
Adelaide, South Australia (ex-Brighton, UK)
1977 2500S - White

My 2500 blog: https://triumph2500blog.wordpress.com/

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Alec
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Re: What voltage should green circuit be at fuse box?

#3 Post by Alec » Thu Dec 20, 2018 4:56 pm

Hello llessur,

I wouldn't worry too much at those, from your last post it's only 0.4 volts. I would say your battery is not fully charged as it should be over 13 volts.

Alec
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Re: What voltage should green circuit be at fuse box?

#4 Post by Mike Stevens » Sun Dec 23, 2018 12:32 pm

I agree, I wouldn't worry too much until you've tried it with the engine running. I have found that there is often quite a volt-drop across the ignition switch, due to, I suspect, the relatively high current drawn. Remember that with the ignition on but the engine not running, the coil will be drawing more current that it will with the engine running. I have fitted an ignition-controlled relay to overcome this problem.

Try disconnecting the feed to the coil when you do your tests. I would expect to see a higher voltage on the ignition side of the ignition switch. Also check for volt-drops across the fuses. Sometime the fuse box contacts get dirty.

When the engine is running, the voltage will be much higher - at least 13.8 so the flasher bulbs will be brighter.

Cheers,
Mike.
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