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Alt (ignition) light behaviour - solved thanks.

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1593370 by Jeff Clunn Would a different size pulley cause this? not spinning fast enough initially at idle
It could. But it would be in proportion to pulley size. So if, for example, a 2" pulley lets it "pull in" at 800 engine rpm; an 8" pulley wouldn't pull in until 3200 rpm.



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

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bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
In reply to # 1593386 by TR3driver I dug out the one I used. The markings aren't visible any more (it was probably surplus even in 1984), but my ohmmeter shows it to be roughly 33 ohms. At that resistance, 5 watts will be big enough (and that's about what it looks like). Similar to
https://www.ebay.com/itm/Ceramic-Power-Resistor-Cement-Resistor-Axial-Guide-White/312159993705?epid=15020362858&hash=item48ae2f7f69:m:m8zynjhf6DPaAnkz7F7GfnA:rk:8:pf:0

You're a champ. Thanks Randall.

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
I think I've got it sorted Randall. Does this look like it will work? I've used the 5W 33ohm resistor you suggested. My wiring loom has wedge-type bulb holders, but the only insulated spare I had was a screw-type holder, but I doubt that is critical. It's more important to me not to chop the harness up, so I came up with this 'adapter'.

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Attachments:
powerResistor.jpg    74.9 KB
powerResistor.jpg

LFMTR4 Avatar
LFMTR4 Lou Mijares
Scottsboro, AL, USA   USA
Looks good, did you try it?

Wonder what the resistance difference between the bulb you are using and original screw in bulb?



Edited 1 time(s). Last edit at 2019-02-01 09:41 PM by LFMTR4.

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Looks like it should work.

Resistance of an incandescent bulb is a bit tricky, because it changes sharply depending on whether the filament is lit (white hot) or not. On a TR3, that bulb is rated 2.2 watts, probably at 14 volts, making it's operating resistance around 89 ohms.

I don't know offhand what bulb a TR6 takes, but I'm guessing it's similar.



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
In reply to # 1593671 by LFMTR4 Looks good, did you try it?

Wonder what the resistance difference between the bulb you are using and original screw in bulb?


Well. It's official. I'm an idiot!

I got as far as removing the indicator bulb, only to discover its an LED!!!!! Way back in post #7 I reassured Steve that it was incandescent, based solely in my 'feeling' the bulb in it's holder. My only excuse is there's bugger-all room up under there, and I couldn't get a clear look at it. So I went to all this trouble, including involving the good people of this forum, for nothing! I switched the globe out for a 'proper' one, and it went out the instant the car fired!

I apologise profusely for wasting everyone's time. If I'd been a bit more thorough, this would have been solved half-way through the first page.

All good information from you all, just a lack of application on my part.

Sorry guys, lesson learnt. I now know why they call them Idiot Lights ;-)

Ian



Edited 1 time(s). Last edit at 2019-02-02 03:03 PM by bikeboy.

Toot Avatar
Toot John T
Northbrook, IL, USA   USA
Is this a good reason to not switch instrument bulbs to LED?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
The resistor is a simple solution, and a worthwhile mod even without the LEDs, IMO,.

But I would worry about what happens when you turn the key off. It may be necessary to add a diode as well, especially with some electronic ignitions. At shutdown, the alternator is still supplying 12v to the lamp, so the LED will see reverse voltage. Some assemblies will be protected, but bare LEDs usually can't stand more than just a couple of volts reverse.

The resistor will also be supplying current back into the ignition circuit, which may confuse some ignitions (like the MSD 6 I used to run).



Randall
56 TR3 TS13571L Once and future daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Toot Avatar
Toot John T
Northbrook, IL, USA   USA
Good thought... The Pertronix modules are very sensitive. I would also be concerned that after some time, this module might fail.

LFMTR4 Avatar
LFMTR4 Lou Mijares
Scottsboro, AL, USA   USA
Good point Randall, I wonder if Ian just put the incandescent bulb back in. That alone may remedy the alternator problem eliminating the need for the resistor all together.

In reply to # 1594267 by TR3driver The resistor is a simple solution, and a worthwhile mod even without the LEDs, IMO,.

But I would worry about what happens when you turn the key off. It may be necessary to add a diode as well, especially with some electronic ignitions. At shutdown, the alternator is still supplying 12v to the lamp, so the LED will see reverse voltage. Some assemblies will be protected, but bare LEDs usually can't stand more than just a couple of volts reverse.

The resistor will also be supplying current back into the ignition circuit, which may confuse some ignitions (like the MSD 6 I used to run).

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
In reply to # 1594296 by LFMTR4 Good point Randall, I wonder if Ian just put the incandescent bulb back in. That alone may remedy the alternator problem eliminating the need for the resistor all together.

Yes, that is what I ended up doing. The original bulb seemed to cure the situation, and following the KISS principle, that's where I left it.

Thanks again for all the support.

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