![]() The voltage should rise (possibly to around 8 or 9 volts.) Raise the engine speed slowly and the voltage should rise – take care not to exceed 20 volts or there is a risk of damage to the dynamo’s insulation. Link the field and power terminals with a piece of wire, taking care not to short either terminal to earth.Measure the voltage between the power terminal and earth – this is typically 2 or 3 volts but may initially be lower.Start the tractor and run at low speed (about 600 RPM).Disconnect both field (the thinner wire with smaller spade or ring connector) and power connections (thicker wire with larger ring or spade connector) ensuring that they don’t short to surrounding metalwork in case there is also a regulator fault.Too slack a belt leads to slip, belt wear and reduced dynamo output. Too tight a belt leads to belt wear and overloads bearings. Check that the dynamo belt is serviceable and correctly adjusted – ½” to 1” movement when moved up and down… and that the fixing bolts are secure.Operation of all dynamos and their regulators is essentially the same so these notes apply equally to other makes. The dynamos fitted to UK built tractors in the 50’s and 60’s were often supplied by Lucas and were very similar or identical to those on cars in the same period. A cold engine just started has discharged the battery and the dynamo at this point is asked to provide its maximum output, often with a damp belt and pulleys. When a belt squeals it is slipping because it cannot transmit this power. At 90% efficiency this would require 420/.9 or 467 watts from the engine… a little over 0.6 hp. A dynamo outputting 14 volts and 30 amps is producing 420 watts. They are relatively efficient, possibly between 80 and 90%. Tractor dynamos convert external power via a belt to electrical power. WARNING – When testing a dynamo take great care with the drive belt, pulley and fan to ensure that body parts or clothing are not caught or dragged in. Currently restoring IH tractors ’61 B275, ’67 B434 - Andrew has contributed to the International B Series Tractors Group and a number of instructional files to the International Harvester Club of Great Britain. ![]() ![]() With a degree in engineering Andrew, now retired, has time to pursue his interest in tractor restoration. Andrew grew up on a dairy farm and completed his first tractor engine rebuild around 50 years ago. ![]()
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