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Pesky Tr8 Ignition Light

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I'd driving me nuts.

 

TR8 (original). Ignition light does not go off when running.

Alternator? Tried that - had it checked and all's good.

Checked a wiring diagram (TR7) and the circuit seems simple - the small wire from the alternator goes direct to the ignition light bulb and either provides an earth or doesn't.

Earth when stopped, something near 12v when running. As the bulb has 12v on the other end of it it is either on or off.

I do electrics, but am no expert on alternators. The wiring between alternator and lamp is good. If I take the wires off the alternator and put an earth on the smaller one, the lamp goes on and off.

 

To me, it HAS to be a fault with the alternator, but it's been off to be checked twice and last time it had its diode pack changed.

 

Any ideas?

 

Thx

 

Warning - also posted on CT!

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I thiink the way it works is that when the ignition is on but the engine not running, the lamp gets 12v from the ignition switch and earths through the alternator, causing it to light. It is also the "exciter wire" that provides the current to energise the rotor winding. Once running, the alternator no longer earths the wire, but actually produces 12v, meaning that the bulb has 12v on each terminal, resulting in no current flow and no light.

 

Does the alternator actually charge? Should get approx 13.8 - 14.5v across the battery terminals with the engine running vs. 12.6v battery voltage.

 

Does it glow equally brightly with the engine running? If you have an ammeter or multimeter that does current (should be very small current) you could connect it in series with the bulb and see if the current flows the same way at all times or whether it reverses when the engine is running.

 

Nick

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Thanks for applying yourself Nick!

 

I don't think it is quite as above. The other side of the bulb (not connected to t'alternator) has a 12v feed switched from the ignition, so when the ignition is on, t'alternator MUST provide an earth for the bulb to light.

 

Once t'alternator is generating though, I assume the voltage on the 'exciter' connection should raise and therefore the lamp should go out - gradually sometimes, depending on revs and efficiency of t'alternator.

 

But it's not doing that. I do have a MM, and the voltage on the exciter connection raises a bit on running 2.5v ish - therefore lamp stays lit.

 

My conclusion - fuc!ed alternator. But big expert man he say no.

 

Confusion isn't help 'cos this car is in the shop for a lot of work, and is an ex-pace car with some race history. Plus, the light seemed to operate normally until the ignition barrel needed changing. So I suspected a loom fault, with a permanent earth being added to the ignition lamp circuit somewhere between t'alternator and t'lamp - but no, the loom tests ok.

 

Hmmppph.

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Hi Fred.

 

Yes it is. My understanding of alternators is a bit vague - but I assume a diode is used in the exciter circuit as well as a bridge rectifier arrangement to rectify the AC coming from the coils.

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So it was the diode pack in the alternator. A simple swap out at little cost.

 

Also - the alternator resists starting to generate (and turn off the ignition light) before it gets to 3000rpm ish - and that's a lot of bluster from a Rover V8 30cm from the side of your head :)

 

Thanks for the thoughts - sorted.

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