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Alt (ignition) light behaviour - solved thanks.

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bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
So. Before my alternator upgrade, I would have to rev my motor just after starting to get the alt light to go out, and the charge to register on the ammeter. A quick blip to around 1500 revs would do it. After the upgrade, I have to rev it to about 3500-4000 rpm for the gauge to read charge and the light to go out.

Is this something that can be adjusted somehow? It's a bit of a pain really. Ok if I drive off straight away, but if I want to let it warm up a bit (like I usually do) I'm not real keen to rev a cold motor like that.

cheers



Edited 1 time(s). Last edit at 2019-02-02 12:27 AM by bikeboy.

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LFMTR4 Avatar
LFMTR4 Lou Mijares
Scottsboro, AL, USA   USA
What did you replace it with and do you have the current production curve for it?

Is the belt tight and connections clean and tight?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
My old Ford Motorcraft alternator did the same thing (although it was a conversion from a generator on a TR3A). The cure in my case was to add a power resistor in parallel with the dash lamp, so the alternator would get more field current when the light was on. Might be worth a try if your new alternator doesn't have a direct connection to the switched (ignition) circuit.

Unfortunately, that was a long time ago; I don't recall the value of resistor I used. Maybe something like 10 ohms.



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 # 1593170 by LFMTR4 What did you replace it with and do you have the current production curve for it?

It was a direct swap, 65amp unit. Didn't have any docs with it, but I had an auto-elec supply and fit it for me

Quote: Is the belt tight and connections clean and tight?

yep. all good.


Attachments:
alt.JPG    37.9 KB
alt.JPG

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
In reply to # 1593197 by TR3driver Might be worth a try if your new alternator doesn't have a direct connection to the switched (ignition) circuit.

The wiring wasn't altered, so I'm guessing it does?

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1593279 by bikeboy
In reply to # 1593197 by TR3driver Might be worth a try if your new alternator doesn't have a direct connection to the switched (ignition) circuit.

The wiring wasn't altered, so I'm guessing it does?
Nope, the original Lucas did not.

I couldn't find much about the part number in your photo, except it appears to be a Jaylec part number. I found them for sale in lots of places, but nothing about output curve, minimum speed or anything like that.

If it was me, I'd give the resistor a try. I can't say that it will work for you, but it worked for me, for many years.



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

Uberxy Avatar
Uberxy Steve Fox
Va, Charlottesville, USA   USA
Try a resistor or changing the light bulb.

I replaced my bulb with a LED, and it has such low resistance that I have to blip the engine to about 2500 to get the light to go out and consequently put the system in a charge state. Gonna go back to incandescent.



SR
73 TR6
86 930

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
In reply to # 1593285 by TR3driver
Nope, the original Lucas did not.

I gotta admit wiring isn't my strong point, so the way I interpret this diagram leads me to believe the warning light gets power from the ignition switch? Or is it that power to the ignition goes through the bulb? That would seem a bit dodgy?



Edited 1 time(s). Last edit at 2019-01-30 07:52 PM by bikeboy.


Attachments:
warning light.jpg    24.8 KB
warning light.jpg

LFMTR4 Avatar
LFMTR4 Lou Mijares
Scottsboro, AL, USA   USA
Do you have the original type filament bulb in the alt light or have you swithched to LED? FWIW, a 65A alternator is about 30A more than the OEM. It would be a good idea to run another 8 gauge wire in parallel from the alternator to the battery to handle the current that baby can put out.

bikeboy Avatar
bikeboy Silver Member Ian Fox
Lara, Victoria, Australia   AUS
Bulb is filament type, only LEDs for instrument lighting. I did read that about the addition Brown back to the battery, and will get on to that as well.

thanks

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Right. The light gets power from the ignition switch. Initially, the other side is grounded through the alternator, which for some designs , also provides the initial current through the rotor.

That initial rotor current has to develop enough voltage in the stator to first over come the forward drop of the diodes, then power the regulator circuit enough for it to apply power from the battery. Somewhere around 2 to 2.5 volts. Not reaching that initial threshold is why the light stays on until you rev up the engine.

A corollary of this is that the alternator may not work at all if the light burns out. I actually copied the resistor idea from a GM wiring diagram.



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 # 1593197 by TR3driver The cure in my case was to add a power resistor in parallel with the dash lamp

Is this the sort of thing I need Randall? What power rating should I get?

https://www.ebay.com.au/itm/10-25-50W-0-1-470-Ohm-Watt-Shell-Power-Aluminum-Housed-Case-Wirewound-Resistor/273659355071?hash=item3fb75e5fbf:m:m4IhUiYGSszC5b82ZnlvZaw:rk:7:pf:0

cheers

Jeff Clunn Avatar
Mount Dora, FL, USA   USA
Would a different size pulley cause this? not spinning fast enough initially at idle

LFMTR4 Avatar
LFMTR4 Lou Mijares
Scottsboro, AL, USA   USA
I would say 5W but 10W will have a higher satety margin.

In reply to # 1593355 by bikeboy
In reply to # 1593197 by TR3driver The cure in my case was to add a power resistor in parallel with the dash lamp

Is this the sort of thing I need Randall? What power rating should I get?

https://www.ebay.com.au/itm/10-25-50W-0-1-470-Ohm-Watt-Shell-Power-Aluminum-Housed-Case-Wirewound-Resistor/273659355071?hash=item3fb75e5fbf:m:m4IhUiYGSszC5b82ZnlvZaw:rk:7:pf:0

cheers

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
In reply to # 1593355 by bikeboy
In reply to # 1593197 by TR3driver The cure in my case was to add a power resistor in parallel with the dash lamp

Is this the sort of thing I need Randall? What power rating should I get?

https://www.ebay.com.au/itm/10-25-50W-0-1-470-Ohm-Watt-Shell-Power-Aluminum-Housed-Case-Wirewound-Resistor/273659355071?hash=item3fb75e5fbf:m:m4IhUiYGSszC5b82ZnlvZaw:rk:7:pf:0

cheers
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



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

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