Technical Tips

The Welsh Poet Restarts The Charge.

A day in the life of Dai Ode

Last week I was asked to look at a club members Ruby that had a couple of problems, one of which was a lack of charge from the dynamo.

The ignition warning light came on when ignition was switched on, stayed on with the engine running and no charge showing on the ammeter. First check was a volt meter across the dynamo out put with the engine running. This showed no output at all. Removed the dynamo for inspection, and found the only visable problem was a dirty commutator. A clean cloth a drop of white spirit solved that problem.

A dynamo is a device the converts mechanical energy [engine movement] into electrical energy [charge current]. If this process is reversed, i.e. electrical energy fed to the dynamo, mechanical energy [movement] comes out. The dynamo will rotate or ”MOTOR”. This is a very simple method of testing a dynamo when removed from the engine. A lead from the car battery live terminal is connected across the two dynamo output terminals, and a lead from the earth terminal of the battery is connected to the dynamo body. A dynamo in good condition will rotate “MOTOR” when connected in this manner.

This test was applied to the Ruby dynamo which “MOTORED” successfully.

The dynamo was refitted and the output test repeated. Voltage out put was now

correct. However, no charge was showing on the ammeter on the dashboard, and the ignition warning light got brighter as engine speed increased. This brightening of the warning light is a classic indication of the cut out not working correctly. The commonest cause being a poor connection between the cut out earth connection and the car body. A lead was placed from the cut out earth terminal to the battery earth terminal and the engine run again. The result was the same, still no charge!

The cut out unit was removed [noting position of all cables], and the back removed.

Ohmmeter tests showed an open circuit in the cut out energising coil.  The unit was beyond economic repair!

I was now faced with the problem of how to proceed. I had no local access to a replacement cut out, and any others available from other sources would be second hand units of over fifty years old. Which command very high prices with no guarantee.

Now the cut out, on vehicles fitted with dynamos, is simply a one way switch which allows contact between the dynamo and battery when the dynamo is charging. When the dynamo output voltage is less than battery voltage the cut out switches out. This prevents the battery discharging into the dynamo. On vehicles fitted with alternators,

the job of the cut out is performed by an electronic device called a DIODE.

[Dia Ode welsh poet!]

I reasoned connecting a diode between dynamo and battery would solve the problem.

A call to an electronics friend produced a couple of diode bridge rectifiers, for trial.

I refitted the old cut out unit, connected a diode unit between the D+ and A+ terminals of the cut out, and restarted the engine and “Hey Presto” charging system working correctly.

The diode unit I used was a relatively old one measuring about 35mmx15mmx15mm. It was attached under the bulkhead inside the car, with leads fed to the cut out. Unless you knew what you where looking for, you cannot tell anything has been added to the cut out under the bonnet. So originality is unaffected.

One of the advantages of this modification is that it will work on any 6 volt or 12 volt vehicle that uses a three brush dynamo.

I now have one of these units under the seat as a back up in case of cut out failure.

Modern equivalents about a quarter the size of the one I used, are available. Which I can supply complete with leads and wiring instructions, at a cost of £25.00 including postage.

John Williams. South WalesAustin7 Club. 01656 786591