Classic cars forum & vehicle restoration.

[Rick]

Hi all, I'm fiddling with a car fitted with a Lucas 3 brush dynamo. It doesn't appear to be charging despite a new loom and recently re-furb'd dynamo.

When I look at the wiring, I see two connections on the dynamo. One (the larger) has a yellow wire connected to it that heads off into the loom (presumably back to the cut-out/junction box). The smaller connector has what looks like a recently-added wire running straight to it from the live terminal on the electric SU fuel pump. Can that possibly be correct? None of the wiring diagrams I've seen have anything like that showing.

The proper diagram shows a green wire going to the smaller connection. There is a green wire nearby poking out of the loom doing nothing, that has been cut down for some reason. Is there any reason why it'd be removed, to be replaced by a live feed from the pump?????

Thanks, RJ
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[ukdave2002]

The smaller wire from the dynamo is the field connection, is normally green with a yellow trace, it connects to F2 on the cut out box. The cut out box contains a couple of resistors that are switched in and out of the field circuit to help regulate the dynamo output. The switching of the resistors is controlled by the position of the lamp switch, when the switch is at the Headlamp position, there is no resistor selected, the field has full battery voltage, so maximum output to support the higher load.

If the field terminal on dynamo is connected to the fuel pump, it means the dynamo will be on maximum power output regardless of the load, resulting in the battery being overcharged unless the headlamps are on.

Possibly someone has done this in an attempt to get output from the dynamo?

If you have a Lucas CR2 cut out, it contains a field fuse which may have blown, the resistances (mounted on the underside of the CR2) can become disconnected. However with the current setup (field permanently connected to battery voltage) none of these faults would stop the dynamo working.

[Rick]

Thanks Dave, I'll have to do some experiments as the ammeter shows a permanent but fluctuating discharge regardless of engine revs. Maybe there's something up in the switch itself.

RJ
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[ukdave2002]

As long as you have a couple of volts at the field, the dynamo should have an output, the way it is currently wired (to the fuel pump) any problem with the switch will be circumvented. I'd do the following:

1)Measure the voltage at the field, as long as its over a couple of volts that's fine for testing the dynamo.

2) If there is a field voltage measure the field current, if its very low or zero, then the 3rd brush isn't earthing the other side of the field winding, or the field winding is kaput; investigate further

3) If the field is between 1-4 amps, its in the right ball park, now measure the Armature winding resistance, it should be around 0.4 ohms, if all good, then the dynamo should be working ok, measure the output voltage of the dynamo to check.

A quick dynamo check is to stick a headlamp bulb across the output and observe how the brightness varies with revs. At least if you are confident the dynamo is working , you can focus on sorting out the field control.

[Rick]

Cheers, forgive my ignorance but when you say "As long as you have a couple of volts at the field" do you mean at the field (smaller) terminal on the dynamo itself?

thanks
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[Rick]

I fired it up (I'm not convinced about the accuracy of the ammeter). At the smaller connection on the dynamo (still connected to the fuel pump live) I'm seeing 12v as expected, on the larger connection I'm getting 14v+ at a brisk idle.

With the ignition on, before starting the engine, the ign light is bright as you'd expect. Once running, at idle the ignition light is glowing slightly, and brightens up as you rev it.

RJ
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[ukdave2002]

[Rick]

Thanks, I don't think I'll ever quite get my head around it.

So in addition to the ign light glowing when idling, and going brighter when revved, I made a few notes of dashboard ammeter readings with the light/charge switch in different positions at fast idle.

HEAD(lights on) shows -8A
SIDE(lights on) shows -4.5A
and in both of the two other unlabelled positions, I also see -4A (I assume one position is WINTER, the other SUMMER) so really in the last three switch positions there's little difference reported on the ammeter.

A voltmeter over the battery when running shows a smidge over 12v regardless of revs, I have just charged it up with a mains charger though.

RJ
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[alastairq]

Does the fuel pump still run if you disconnect the wire from the dynamo?
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces.

[Rick]

Just as an update, I had another poke around and gave the points in the cut-out a closer look. They were opening and closing ok depending on whether it was at idle or revving, but the V at the battery wasn't altering and the ignition light was on all the time, getting brighter when revved.

A gentle clean of the points in the cut-out appears to have improved things, it now shows around 13v at the battery, at c2k revs, and the ign light now goes out as it should. The ammeter (the needle of which sticks unless you thump the gauge) now shows around +4A at around 2k revs which doesn't seem too far out to me. Certainly a big improvement over before.

Is the 12v feed direct from the fuel pump to the dynamo likely to cause any issues going forwards?

Thanks for your help,

RJ

PS I think I'll have a look in a few boxes and see if I have a better ammeter.
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[alastairq]

Ford USA had real issues with their ammeters during the late 60's & '70's... The idea was good, the reliability was abysmal.

They junked their old idea of major current being fed to a gauge on the dashboard...because of the fire risk should a break or short occur.

They reverted tot eh type that had a thinner cable, forming a 'loop' inside the gauge, which 'read' the urges without direct contact.

These gauges ceased to work just as the guarantee ran out.
Today there are US businesses who will rebuild those meters, or produce better working items, for vast amounts of rent money...
_________________
Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces.

[ukdave2002]