Dynamo repairs

Based in Cookstown in the heart of Northern Ireland, Mid-Ulster Rotating Electric's specialise in the repair of Starter Motors for both modern & classic vehicles as well as HGV, Commercial, Plant Machinery & Marine.

All completed dynamo repairs are tested and ready to go straight back into your vehicle. Most common makes of dynamos can be repaired.

Repairs are completed professionally to a high standard, with 98% of repairs completed same day.

As well as repairs we also stock a wide range of new dynamos, to find out if we can help with your starter problems or repairs feel free to give us a call on +44(0)28 867 65354 or using the online enquiry form.

How To Test Dynamo For Faults

Make these checks with a voltmeter if possible. If not, use a circuit tester or test lamp.

The instructions are for a car with a negative (-) earth system. For a positive (+) earth system, read negative for positive, and positive for negative.

  • Connect a voltmeter across the battery terminals while the engine is running. Have a helper rev the engine up from idling speed. The battery voltage should rise, or the tester lamp (or headlamps) should brighten.
  • If it does not, and if checks on connections and the drive belt have been satisfactory, switch off the engine and disconnect the two cables from the end plate of the dynamo.
  • The terminals are usually marked D and F. They are of different sizes, but label them if necessary, to avoid confusion.
  • Use a short length of fairly heavy cable to clip the D and F terminals of the dynamo together. Start the engine and let it idle at not more than 1,000 rpm.
  • Connect the positive lead of the voltmeter to the D terminal and the negative lead to earth. The meter should read about 14 volts (or the 12 volt bulb should shine brightly). If so, the dynamo is working.

Testing the cables

  • Reconnect the dynamo cables, leaving the short bridging cable in place. Disconnect the cables at the control-box end, where they are also labelled D and F.
  • Start the engine and allow it to idle at not more than 1,000 rpm. Connect the positive lead from the voltmeter to the cable disconnected from the D terminal at the control box to see if it is sound.
  • Then do the same with the cable from the F terminal at the control box.
  • If the cables are sound, and if the dynamo is charging as previously checked, the meter should read about 14 volts and any fault must be in the control box


A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later electric-power conversion devices were based, including the electric motor, the alternating-current alternator, and the rotary converter. Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and cost reasons.