At 09:29 AM 3/11/2007 -0600, Greg Bleakley wrote:
"I apparently have a trickle drain somewhere in the system, as the battery goes dead in a couple of days. Any suggestions on where to start looking, or any likely culprits?"

Yes. This is fun, but a little tricky. A good battery may have about 40-ampere hours storage capacity. Going dead in 40 hours would imply an electrical drain of about one amp, which is substantially more than a "trickle". One amp is enough to light up three dash illumination lamps to full intensity, or almost enough to power up the field coil in the generator.

First check to be sure you have a good battery and it is not going dead on its own in 2 days. Fully charge the battery, and disconnect the charger. Switch on parking lights and headlights on high beam. Come back periodically to check on headlight output intensity. All lights combined may draw up to 14 amps from the battery. The battery should be able to power the lights for about two hours before they go dim. If it lasts for less than one hour you need a new battery.

If the battery is good, charge it up again and bring out a voltmeter. Electrical circuits in the MGA are all fairly simple. Think battery, wires, switch, load, and return to ground at the battery in a complete circle. If you disconnect any wire it breaks the circuit, and you have no current flow, and no discharge. When lights and ignition are switched off everything should be open circuit.

If you have any current flow at all there is a closed circuit somewhere. Current flow of about one amp indicates a load with about 12 ohms resistance (against the 12 volt supply). A voltmeter has very high internal resistance and will conduct only a very tiny current at these voltages. You can disconnect a wire anywhere in the closed circuit, and connect the voltmeter in series with the circuit between that wire and the terminal it was connected to. If it is open circuit the meter well show 0 volts. If it is a closed circuit you may see something close to 12 volts on the meter, even if the circuit has fairly high resistance and draws only a trickle current. In this manner you can detect a closed circuit.

Start with the basic circuits diagram. Note that when parked the starter switch, lighting switch, and ignition switch should all be open circuit, and no current will flow there. But you should check these circuits anyway to assure that all is well, because you know something is wrong and you do not know yet where the fault lies.

First disconnect the fat brown wire at the A terminal on the control box, leaving the smaller brown wire connected. Connect a voltmeter between the disconnected wire and the A terminal. Normally this should show 0 volts indicating all open circuits and no current draw. If so, you're barking up the wrong tree, and there is no current drain, so go back to suspecting a bad battery. If you do find voltage here, then you do have a closed circuit and current draw somewhere downstream.

Reconnect the fat brown wire and disconnect the smaller brown wire at A1 on the control box. Connect the voltmeter between the wire and terminal. If this shows any voltage, you have a trickle current to ground through the horn(s), because the horn switch is open circuit. See the horn circuit diagram. Assuming this is 0 volts you can reconnect that wire and go on to the next test.

Getting back to the charging circuit diagram, disconnect the wire from A1 on the control box. Connect the voltmeter between this wire and A1 terminal. If it shows any voltage here, you have something strange going on downstream, like maybe a radio drawing current from the battery side of the ignition switch. If this shows 0 volts, there is no drain downstream through the ignition or lighting circuits. Then you can reconnect that wire and go on to the next test.

Having passed the prior tests, this leaves only the control box and generator circuits to be suspect. See the diagram for the charging circuit. Disconnect the Yellow/Green wire from terminal F, and connect the voltmeter here. If you find any voltage here it indicates a power drain through the field coil in the generator. Question then is where does the power come from. Looking at the internal diagram for the control box, and starting with the battery connection at terminal A, the circuit runs through two coils to the cutout relay contact, which should be an open contact when parked. If you have power on the D terminal, the cutout contact must be improperly closed, and you have a faulty control box.

Furthermore, if the cutout contact is closed when parked (generator not running), it makes a direct connection from battery to dynamo through the low resistance coils in the control box. This allows current to drain through the armature of the generator, which is also very low resistance. If this happens it can draw very high current, enough to burn the fat yellow wire between control box D and generator D terminals, and may also cause internal meltdown of the generator. I sincerely hope this is not your problem.

If you have installed an alternator, and some internal diode is bad, it is possible to have a small current drain through the alternator when parked.

If you have installed a radio or any other aftermarket electrical device that connects to the Brown circuit before the ignition switch, that device(s) would be suspect of causing the small current drain. For any such device, find the wire connected to the Brown or Brown/Blue circuit that supplies power to that device. Disconnect this wire, and connect the voltmeter in series there. Any voltage showing there would indicate a power drain through that device.

By the time you get this far you should have found voltage somewhere to identify the circuit with the current drain.