Dynamo Testing

[noel shelleyParticipant @noelshelley55608]

It doesn’t matter what charging method you use, if the battery is to small a capacity or on it’s last legs it will not hold the power required to start the engine. The voltage drop at start up will also prevent the coil giving a good spark – to some extent this was improved by using a 9V coil and dropping resistor that was took out of circuit for starting on some vehicles, though not MGBs. That the MG magazine recommended a 44Ah battery to replace an 80Ah bank I find strange. Classic cars seem to get through batteries in no small part due to standing – this is the best way to ruin a good battery. In one case a 90Ah battery was fitted and left – not used & 12 months later it was sulphated up and scrapped. EDTA can sometimes work. A modern maintenance charger will help to save the battery and at about £25 well worth it !  Must go.  Noel.

[Howard LewisParticipant @howardlewis46836]

We assume (dangeroius!) that the brushes on the dynamo are in good condition.

The cut out should not conect the dynamo to the battery until it is producing about 14 volts (A fully charged 12 volt battery would have a terminal, open circuiut voltage of 13.2 (2.2 x 6 cells).

So until the dynamo is delivering more than 13.2 volts, it will not chege the battery.

Since the current draw of the starer motor will be in the region of 300 Amps, ALL the battery and starter connections need to be secure and CLEAN. And the main battery / starter leads need to be in good condition.

Otherwise a small resistance will result in a voltage drop between battery and starter when drawing high currents. At 200 Amp current draw it needs VEY little resistance to drop two or three volts before the starter.

AND with the battery heavily loaded, the on load voltage will be will below the open circuit voltage, reducing the power of the spark.

This is why, Ford and Vauxhall used 9 volt coils,, normally fed through a ballast resistor (To reduce the voltage seen by the coil, in normal running, to 12 volts). When cranking, the ballast is shorted out feeding full voltage to the 9 volt coil for a good strong spark.

Often, it pays to depress the clutch while cranking,to disconnect the engine from the viscous drag of the oil in the transmission. (I once came across an engine that was a poor starter. It had a carbon thrust race on the clutch, rather than a ball bearing one). A few drops of oil on the thrust race improved matters markedly.

As ever, the devil is in the detail. Clean connections on terminals and leads (Of adequate capacity and low resistance) and a battery properly filled (Plates covered) and in good conditions.

AND a cold battery will not deliver the same power as a warm one.

Howard

 

[Robert Atkinson 2Participant @robertatkinson2]

Don’t jump to conclusions or follow “advice” without data. You do NOT need a bigger battery when changing from 6 V to 12 V. 12V equipment is much more efficent. Resistive losses are a 1/4 of a 6 V. A physically similar 12V battery of 1/2 the Ah rating of the 6V has the same energy capacity.
First check is, as Grindstone Cowboy said, to check the terminal voltage of the battery while cranking. Put the voltmeter probes directly on the battery terminal posts not the clamps. If battery voltage DOES NOT fall below 10-11 V The fault is elsewhere. Either connections, starter or contactor if seperate from starter. If it’s below that the battery is suspect.
Check ground return first. Connect meter from battery negative post to body of starter (or bellhousing if you can’t access the starter). Measure voltage drop while cranking. If it is more than 1-2 V there is a bad connection or broken / corroded cable. This could be battery to chassis or engine /gearbox to chassis. You can confirm this by connecting a jumper lead from battery negative to body of starter or bell housing and seeing if it improves.
Check Positive next. Connect meter from battery positive post to terminal on starter . Measure voltage drop wihile cranking. If it is more than 1-2 V there is a bad connection or broken / corroded cable.
If both voltage drops are low it’s likely the starter that is at fault.

If you re buying a new battery DON’T buy a “improved” modern type such as Calcium or stop-start. These are generally less tolerant of the older charging systems.

Robert.

[noel shelleyParticipant @noelshelley55608]

We have not been told that it has been changed from it’s original Positive earth to negative and it still has a dynamo, so testing needs to take into account polarity. The vehicle IS 12v it is Not being changed from 6V and had 2 x 6v 80Ah in series = 12v 80Ah asking a 12v 44Ah to replace it may not work for too long. It is why I used a 12v 90Ah on my MGB.

The starter is not pre engage and the solenoid is separate from the starter motor.  Noel.

[Robert Atkinson 2Participant @robertatkinson2]

OK, I misread that. I’ve not worked on any early B. Istill don’t think a bigger 12V battery is required though. They will have used 80 Ah batteries because that was the size used in the 6V cars. You only need capacity to run electrical loads with the engine off. 44 Ah is enough for a modern car with lots of electrical load. A bigger battery also take longer to charge up when it has been discharged. The cranking capacity of the battery currently fitted is 420 A which is probably more than the orginal 6 V batteries. A modern 80Ah 6 V is higher at 600 A but you really don’t need that much for a A series engine.

Robert.

[noel shelleyParticipant @noelshelley55608]

Forgive me Robert but it’s a B series of 1798cc. Noel.

[Howard LewisParticipant @howardlewis46836]

A car of the MGB vintage will have left Cowley with a 12 volt system.

Even in the 1950s, cars had ceased using 6V systems, even Ford!

(Probably the last 6V fitted cars were the last of the E93 Fords; the 100E certainly used 12V. My 1955 one did, as did my father’s 1955 803cc Morris Minor.  At that time, the MGA would have been the BMC sports car; the MGB came later, and the engine gradually increasing in swept volume upto 1798 cc with the narrow bead for the cylinder head gasket, which then tended towards head gasket leakages).

On the MGB, two 6V batteries would have been wired in series to give 12 volts, to fit into the space available.

Howard

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