Does the ignition light come on as the engine spins-down?

[A TR7 16V]

Because I've just stopped the car and noticed my oil pressure light does. Which I thought was odd. And thinking about it, I can't see why the oil light should come on at rpms it is normally off at, just because the ignition power is disconnected. Whereas I don't know what the dynamo and regulator do under those conditions at all.

So I'm wondering if the two lights in the speedo are crossed over. 

I'd lift the bonnet, disconnect the oil pressure switch, and see which light goes off with the ignition on and  the engine not started. But it's far too bleeding cold out there to do it now, so it'll be tomorrow evening.

Graham

[RobPearce]

Yes, the ignition light will come on as the engine spins down. It's wired between ignition supply and the field winding (this is true for both dynamos and alternators) so that it has no volts across it when the field winding is being driven (by the generator working) since both that and the ignition supply are at 12V. Before starting, the field winding is not charged so there's 12V between ignition and field. When you turn the key off, the ignition supply goes down first and there's (briefly) 12V between field and ignition. The lamp doesn't care which direction it's being driven (unlike an LED).

So yes, I suspect you've got the bulbs swapped. It's easily done.

[A TR7 16V]

Quoted from RobPearce-

So yes, I suspect you've got the bulbs swapped. It's easily done.

Oh good, cos what I though was the oil light's been comming on at very low revs and I thought the mains were heading towards being history.

But if its the dyno/regulator stating to fail, I've the parts to upgrade to an alternator already in the post.

Graham

[RobPearce]

To be honest, it's not unusual for either of them to come on at "very low revs" and doesn't necessarily indicate anything too serious. It does help to know which one it is, though 🙂

[Howard]

The oil pressure switch should be easiest to disconnect to check if it is correctly connected. On a TR7 its low down on the engine block on the offside & is a single wire plugging into a spade terminal on the pressure switch.

With the ignition turned on but without starting the engine unplug the cable & the oil pressure light should go out.

[herald948]

In my opinion, either light coming on at low revs is not cause for panic, but it certainly is worth checking out regardless!

[A TR7 16V]

Turns out it was the bulbs being swapped and it's the ignition light comming on at low revs.

That's a relief to me, as I do think the oil light coming on is a matter for concern, if hardly imediate panic. In my case, there was also a short term rattle when starting with more than minimum throttle. However, that's not been evident lately, so may have been something other than the mains being slack, and thus causing low oil pressure. 

My response was to buy a spare engine that is supposed to have a re-ground crank and new bearings in it, though done some years since and then stood - it was a bargin even after carriage. That's still being checked over, but I guess it will come in eventually,  either for staight replacement or spares  - see other thread on head rattle.

So if it turns up and the weather's good, I'll be fitting an alternator this weekend.

Anyway, thanks for the help.

Graham

[A TR7 16V]

Oh, one more question. Anybody got a suggestion for what to fit in the reguator box when it's redundant? My imediate thought is some fuses.

Graham