27-04-2020, 02:56 PM
Hi again David H
There is clearly something amiss somewhere.
I wondered if you would find it helpful to know how the charging is supposed to work on an A7 using the Lucas SM5 switch panel. Deep breath taken. The dynamo has an internal "summer charge" resistor from its main output (D or +) to its field winding (F or SHunt). If you start the engine and rev up, the dynamo will start producing a voltage. The dynamo main output is wired to the +D terminal of the SM5 and also daisy chained to the +D terminal of the cutout. Inside the cutout it goes through a fuse and is applied to the "voltage" coil, the other end of which is grounded. As the revs rise the dynamo voltage rises to the point where the (small) current through the voltage coil makes enough magnetic field to pull the cutout contacts closed. The cutout contacts then short from +D of the cutout (after the fuse) to A of the cutout. Current now flows to A of the switchboard, through its Ammeter and out of terminal B to the battery, and the battery is charged. A refinement inside the SM5 is an internal switch operated by cams from both the Left hand (Charge) switch and the Right hand (Lighting) switch. If either Full charge or Headlights are selected this internal swich shorts between D and SH on the SM5 with SH on the SM5 connected to SH or F on the dynamo. This effectively shorts out the dynamo's summer charge resistor and allows it to develop full whack.
As we have already discussed, the Ignition warning lamp is wired between +D and IGN on the SM5. Once the ignition is turned on, IGN is connected to A, so the lamp is effectively between +D and A. This places it across the cutout contacts. It will glow when they are open, and go out when they close.
All your descriptions seem to indicate that the cutout contacts are not closing. This could be because the cutout fuse is blown, the cutout voltage coil has gone open circuit, the earth connection is faulty or the points are very dirty. You should see some movement as the engine is revved up.
I don't know why you are seeing a rise in battery voltage when the ammeter is registering a discharge. Don't believe anything you read on a digital multimeter as they cannot cope with the fearsome electrical noise of a A7. Try and get an old fashioned analogue (needle) type voltmeter, even a cheapo one will do. Failing that wire a spare sidelight bulb directly across the battery and see if it gets brighter ( = more volts) when the engine runs.
The only way I can see that the battery could genuinely be charged without the ammeter registering would be if there was an incorrect connection somehow from the generator to the +B terminal of the switchboard. If you follow the car's wiring from the live side of the battery, there should be a connection to the starter motor and a connection to +B on the SM5. Nothing else.
Hope this might point you in the right direction
Cheers, John.
There is clearly something amiss somewhere.
I wondered if you would find it helpful to know how the charging is supposed to work on an A7 using the Lucas SM5 switch panel. Deep breath taken. The dynamo has an internal "summer charge" resistor from its main output (D or +) to its field winding (F or SHunt). If you start the engine and rev up, the dynamo will start producing a voltage. The dynamo main output is wired to the +D terminal of the SM5 and also daisy chained to the +D terminal of the cutout. Inside the cutout it goes through a fuse and is applied to the "voltage" coil, the other end of which is grounded. As the revs rise the dynamo voltage rises to the point where the (small) current through the voltage coil makes enough magnetic field to pull the cutout contacts closed. The cutout contacts then short from +D of the cutout (after the fuse) to A of the cutout. Current now flows to A of the switchboard, through its Ammeter and out of terminal B to the battery, and the battery is charged. A refinement inside the SM5 is an internal switch operated by cams from both the Left hand (Charge) switch and the Right hand (Lighting) switch. If either Full charge or Headlights are selected this internal swich shorts between D and SH on the SM5 with SH on the SM5 connected to SH or F on the dynamo. This effectively shorts out the dynamo's summer charge resistor and allows it to develop full whack.
As we have already discussed, the Ignition warning lamp is wired between +D and IGN on the SM5. Once the ignition is turned on, IGN is connected to A, so the lamp is effectively between +D and A. This places it across the cutout contacts. It will glow when they are open, and go out when they close.
All your descriptions seem to indicate that the cutout contacts are not closing. This could be because the cutout fuse is blown, the cutout voltage coil has gone open circuit, the earth connection is faulty or the points are very dirty. You should see some movement as the engine is revved up.
I don't know why you are seeing a rise in battery voltage when the ammeter is registering a discharge. Don't believe anything you read on a digital multimeter as they cannot cope with the fearsome electrical noise of a A7. Try and get an old fashioned analogue (needle) type voltmeter, even a cheapo one will do. Failing that wire a spare sidelight bulb directly across the battery and see if it gets brighter ( = more volts) when the engine runs.
The only way I can see that the battery could genuinely be charged without the ammeter registering would be if there was an incorrect connection somehow from the generator to the +B terminal of the switchboard. If you follow the car's wiring from the live side of the battery, there should be a connection to the starter motor and a connection to +B on the SM5. Nothing else.
Hope this might point you in the right direction
Cheers, John.