Can I suggest that folks read Mike Frank’s (@Michael_Frank) excellent article that explains how the 11AC alternator works, and the function of the AL signal:
https://www.coolcatcorp.com/faqs/Lucasalternators.html
One important thing that appears to have been missing from the thread so far is that the signal on the AL terminal is AC, not DC. If you measure it using the DC scale on your multimeter, you will see nothing. The signal is, of course, only present with the alternator powered and turning. Hope this helps.
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Carlo it is with the engine running at 1500 RPM or greater. With the ignition switch on (not running) AL terminal should be grounded. With engine running at 1500 RPM the voltage between the AL terminal (and ground) should be no greater than 7-7.5 DC. Any greater or less indicates a failed diode or possibly a loose or corroded connection and it will burn out the 3aw control unit.
Let me know what you find after testing.
Dick
You really should have a scope to look at AL as it AC, and worse, switched on and off in bursts by the regulator, so looking at it’s AC average with a meter is somewhat meaningless, and depends on the state of the battery charge. If its fully charged, it will be AC bursts off more than on. If low, it will be AC bursts longer on. In the middle, depending in the battery’s ability to absorb the alternator rectified current, it will be switching on and off accordingly.
Bulb tested ok when removed from gauge (LED)
Opened up 3AW and visual inspection looks OK???
AL terminal shows 7.25 volts @ 1500 rpm.
Glenn
Yes, absolutely. Thank you.
I have a similar problem on a 67 Series 1 OTS - ignition light on when engine is running (even over 2000 rpm.) Ammeter dead in the center with only a bit of tiny, occasional jitter.
Background - one of the last items in my quest to get all devices, controls, and lights working was the ignition light. It was never connected to the harness, since the 3AW only had 2 terminals left on it instead of 3. 
I purchased the CoolCat replacement unit and followed the wire tracing directions since my wires are faded beyond color recognition to the wiring diagrams. But because the light was on when running, I thought I had the wrong wires, in particular the AL “Green\Yellow” wire due to my readings.
This was because I was reading 6.08 volts DC on the AL wire with ignition on (but not running), and CoolCat says this should be zero. But when I go direct from the AL terminal to ground with ignition on, I got the same results, so I know that is in fact the wire from the AL terminal.
Could this be a bad diode in the alternator? Or is it perhaps a sign of a bad battery (I noticed my Jag is very hard to start even when the battery is fully charged, taking a long time to crank. But when I put it on a 200 amp boost start charge, it fired up very quickly.)
I haven’t yet done the engine running voltage tests
Thanks,
Dave
Dick - with the engine running @ 1500 rpm or higher, there is fluctuating ac voltage ranging from ~ 6.8 - 7 - 7.5 thereabouts. Hard to tell exactly since the meter is hopping around. Should I assume this is a proper reading?
Carlo
Sounds good to me. Youi should be safe installing a new 3aw. I would recommend you use the cool-cat solid state un it if you haven’t already.
Keep me updated if you would.
Regards,
Dick
December 1
TrickyDicky:
Let me know what you find after testing.
Dick - with the engine running @ 1500 rpm or higher, there is fluctuating ac voltage ranging from ~ 6.8 - 7 - 7.5 thereabouts. Hard to tell exactly since the meter is hopping around. Should I assume this is a proper reading?
Carlo
I did this alternator conversion to eliminate the 3AW relay. it works great.
https://www.coolcatcorp.com/faqs/Lucasalternators4.html
Just in case anyone cares about the outcome, the coolcats ss relay is in, and all is working as it should. Thank you to all who offered input and guidance.
Carlo
P.S. If anyone has the slightest doubts about the coolcats relay, rid yourself of those silly thoughts. The relay is EVERYTHING it’s represented to be. I was able to speak with Mike when I placed the order, and he took the time to explain the ins and outs of the relay’s purpose and function. Nice chap, Mike is.
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I have to agree - I have that relay, his fans, his AL radiator, the spin-on oil filter conversion and have used his Gates interchange page several times.
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Well, mine reads 10.6 Ohms - is this good or bad ? AL to E, no reading. No ignition light either.
That would suggest that your nichrome wire is broken - but I think that normally results in the light not going out.
So… I suppose you could have a failed 3AW and a burned out bulb or other failure.
If you connect the WL wire (brown/black I think) to ground the light should be on ((ignition on too). If not then something other than (or in addition to) the 3AW is faulty. Check the bulb and if okay, check if you have 12V on the white wire to the bulb.
Since you have no continuity from AL to E you might want to open up the 3AW and inspect. You’ll probably find the nichrome wire is broken (that is the spiral heating element). Mine was broken:
I don’t think that wire is easily repairable so you may be looking at a new 3AW or the electronic version Mike sells:
Note - order the correct polarity.
I wired mine through the old can and hid the black box behind it:
If the nichrome wire is not broken then even a dead 3AW may be brought back to life. I cleaned the points in the unit on my other E and increased the tension a bit to get better contact.
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Many thanks, Awhanee. Yes, I checked the bulb by earthing it, works fine. Alternator works well too, 14V + on the terminal and good charge on the ammeter.
I also just removed the 3AW from its casing and again, all looks fine. The coily wire is intact, likewise the straight length that actuates the contacts, and all seems to move ok. I’ll have a go with it tomorrow and see if there’s any movement.
So we have the circa 8V from the alternator AL terminal going to the 3AW and activating the contacts via earth, then transferring the power to the WL terminal which takes it out to the bulb. What I cannot grasp is how the 8V somehow becomes 14V, which must be needed to equalize the voltage from the white 12V wire to the bulb.
Anyway, I’ll test the continuity of the AL & WL wires, otherwise must conclude that the 3AW is dead even though it looks fine. I have yet to address the oil pressure warning wiring, which cannot be working either, but could that in any way be interfering with the ignition light ?
The current from the ALternator is merely operating a switch (by heating and bending a contact). The switch then provides a ground to the bulb which always has power when the ignition is switch on.
Not at all like the generator bulbs that have to have power to both sides (battery & dynamo) to go out.
In which case you should be able to measure continuity with some resistance. You said before ‘AL to E, no reading.’ but try again. You may need to clean up the lugs to get an accurate reading, that thing has been in the engine bays for decades and the current used by an ohmmeter is puny.
This isn’t a trio alternator. The lamp has 12v coming in on one leg from the ignition switch. The other leg is the WL wire. When this is grounded through the 3AW to the “E” terminal, the light is on. The 3AW gets about 8V AC from one winding of the alternator when it’s running. This causes the thermal element to heat, which opens the circuit so the light goes out. Unlike a trio alternator, the potential across the bulb doesn’t balance.
The ignition light has nothing to do with the oil pressure sensor when a 3AW is present. Nor does it tell you much of anything about the alternator. At best you know that one winding is generating AC, which leaves two windings and the diode pack untests. The real reason you have this light is to let you know that the key is on, but the engine not running. Otherwise, it’s pretty much useless. The oltmeter is a better indicator of charging system condition.
Make sure you have continuity between WL and E when the unit is unpowered. If not, the 3AW is bad. You can try to clean the contacts, but take care, it’s delicate.
Thanks, Geo and Michael. I’ve had E Types on and off since the '70s, and only now do I fully understand the 3AW ! Although until now I’ve never needed to understand it. Once you realise it’s just a switch, it’s simple.
Well, I did the checks you suggested and tested the continuity of wires etc., eventually it became clear that the light being on all the time was due to the points in the 3AW being in permanent contact and not separating when the AL voltage rose. So then it just became a matter of fiddling with the points until they were just touching but would separate when the voltage increased.
So there we are, result - hopefully sorted although the coiled wire does seem to get a bit hot. Might eventually need a new 3AW, but at least we’re on the right track now.
One function of the light, though, is that it would indicate a broken fanbelt, which may not be immediately apparent from the ammeter with a fully charged battery.
Thanks again, guys,
Anthony
I believe that is exactly how it is supposed to be.
Ok - if it had been in the housing I would have been blissfully unaware !
It’s a thermal relay. The coil gets hot and causes the bimetallic contact to bend until it opens.