Putting my 70 roadsters dash back together and have a few basic question?
Interior light is for the dome light behind the seats
Panel lights are for the instruments
Can’t answer the alternator question
Fuel warning light …… take out the sender unit from the tank and the arm will drop …. Making the light come on…… or if there is no fuel in tank will show.
Map light doubles as the courtesy light…. When doors close the circuit is open and the light goes off …the switch creates a closed circuit turning light on…… much more convenient than holding the door open at 60mph to read the map 
You’ve done that…too?
Or you can detach and earth the white/green wire at the top of the sender.
I think that’s correct. I’ve never fitted a single wire alternator but this is what I think happens.
The light normally connects to the alternator AL terminal when the ignition switch is on via a brown/yellow wire. There is a 3AW relay in line in this wire that cuts out at about 8v.
So with the ignition on but engine off there is a small amount of voltage in this wire from the field coils (say 5v) which lights the globe.
Once the car starts the alternator is putting out about 14.5v which triggers the relay and switches off the light.
So unless you have some other way of connecting the brown/yellow wire to the alternator the light won’t work.
I’m looking at the wiring diagram now. It looks like the light gets power from a white (switched/unfused) ignition wire and the 3AW circuit acts like some sort of earth maybe, through the alternator.
Tracing electrons has never been my forte. I spent some time yesterday trying to wire up the reversing light on the Mk2. The best I’ve done so far is make the left rear indicator come on when I put the box into reverse.
That, my friend, takes sheer genius!!
I don’t think there is a dome light on a OTS? got to have a dome first?
The light that is on the rear wall of the cockpit
With regard to the alternator, if you have a true one-wire alternator, you won’t be able to have a useful dash indicator. You could rig up an oil pressure sensor or low voltage trigger to operate the light, but neither would give you any information about the alternator.
I’m not a fan of one-wires. People mistakenly put these in thinking they’re getting reliability. In fact, no OE manufacturer makes a one-wire voltage regulator. So every one wire alternator is built up from aftermarket parts of varying quality. A one wire needs to get to high RPM’s before it bootstraps, because it depends on residual magnetism in the core for excitation. That can be disturbing. Some of them don’t charge well at low revs. Sometimes they’re chosen for high peak output. Most cases, it’s output you don’t need. And a high amp alternator puts a great strain on the belt system. Then there’s the bling of powder coating, which makes grounding difficult. Above 100A, you should consider a ribbed belt and overrun pulley. Finally, you lose the light.
Hi Andrew,
The 3AW is a normally closed switch between the WL terminal, to which the Warning Light is connected, and the Warning Light’s source to Ground, the E Terminal of the 3AW. When the original alternator is running, its generating circa 7.5 volts AC seen at the alternator’s AL terminal. When this is connected to the AL Terminal of the 3AW it heats up a Bi-Metal coil, inside the unit, that acts like a winch to break contact between the WL and E Terminals, thus breaking the Warning Light’s source to Ground and extinguishing it.
Brent
And if you’re wondering why AC? Why a thermal relay? The answer is that the 11AC system was designed to work with either positive or negative ground. All that needs to be done is switch the alternator diodes from side to side. All the other components of the charging system are either floating ground, polarity agnostic, or AC operated.
Brilliant!
Michael and Brent thanks for explaining how it works; I think I finally understand it.
Is it different with the generator in the 3.8s?
And it does double duty as in addition to lighting the interior it also illuminates the boot.
Granted, it does a piss-poor job of both… useless for the interior if the hood is down and useless for the boot if you have a lot of stuff in there.
Hi Andrew,
They used an Oil Pressure switch. The logic must have been:
if there is oil pressure, the engine must be running
if the engine is running, the generator must be charging
Whether a generator was actually working must have been irrelevant.
Brent
Must’ve been different in Australia: my 3.8had a generator, and it used some circuit in the regulator to operate that light.
Hi Paul,
I suspect that it was added to your car. Even the early S1 4.2 cars used an Oil Pressure Switch.
The following picture is an extract from a wiring schematic for an S1 4.2. The wire shown in Red and all that inside the Red Ellipse is not present when the system under the heading:
IGN. WARNING LIGHT PRESSURE SW
AND WARNING LIGHT
is used.
Brent
Hmmm: here’s what I know!
Tweety had a mechanical OP gauge, installed in the mid-60s. Up till he went off the road, in 1983, he had the stock generator, which worked fine, and there was no ancillary OP switch to be found on the engine. The light went off when the engine was on, and when I converted to a Delco alternator, Ray L. gave me a simple hack to make the light work with the Delco. It was by hooking up two wires, right off the regulator, and the light operated the same as it had with the generator.
I’m not disputing your info, btw: I’m just now puzzled.
Hi Paul,
A previous owner may have added the alternate arrangement; did you own the car from new? I can only state what I’ve seen and what’s backed up by wiring schematics I’ve worked with.
Brent
