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Alternator Repair & Servicing

Repair Guide Alternator Repair & Servicing

If you have a charge light on or starts behaving oddly, or a battery that goes flat, then the chances are your alternator is toast. If it starts squeeking, siezes or rumbles, it also needs attention, so this guide will show you how.

At minimum you will need:
  • Standard spanners
  • 5 & 5.5mm sockets on a 1/4" drive
  • 25Watt or more soldering iron
  • Flux core solder wire
  • Digital meter with diode test function

First of all, test the alternator with the battery fully charged, and the engine running. Check the voltage at the battery. This should be getting 13+ volts if all is well, maybe around the 14 volt mark.

If you are getting more than this, then you will only need a new regulator for the unit, explained below.

Presuming you are getting nothing, turn off the engine, (check for bad connectors) and remove the alternator completely.

First, clean the outside, and remove the black cover with the 5.5mm socket.

Inside should be something like this


You may have more or less wires in there, but don't worry at this stage.

Tip over the sink, and using a paintbrush with water and detergent, clean off any muck you can see, and as much as you can clean the thing.

Leave to dry.

To identify the main components:

The Square block is the regulator,
The Finned unit is the rectifier
The large round components with one central connector are either a capacitor, varistors or in the picture - both.

Rectifier unit:

This is a group of large diodes forming a bank of 9 in total around a central pillar. The central lead passes through other plates to keep the design compact and help cooling. Make sure there are no bridges between the plates causing a short.

Regulator unit:

These can be desribed as machine sensing or battery sensing, depending on how they are wired, and may have 2, 3 or 4 leads from them. The main thing to remember when replacing them is, replace with the same type. If your unit is overcharging - just replace this part.

Remove it and note where the connectors go. Testing on the bench really isn't an option, so if everything else checks out ok, then replace it. Over 60% of alternator failures are due to a defective regulator, so it is a good bet to replace it.

The Regulator has the job of adjusting the feed to the rotor, thus making the magnet within it stronger or weaker. This in turn affects how much electricity is generated from the outer coils.

Alternators are "self exiting", that is, the power they produce is rectified and fed back (via the regulator) to the field coils again. The problem is, if the load is removed (i.e. removing the battery) there is a sudden spike of energy - that goes into the rectifier - and usually blows it up. With this spike of unrectified power, it usually blows up the regulator too (sometimes the diodes survive, but the regulator doesn't)

With this out of the way,


Remove the brush housing (plastic centre section). The brushes should protrude (not fall out) by about 5mm+. If they are shorter then replace them. Check continuity too.

The slip rings should be firmy attatched, not too pitted or degraded.

With a meter, check the ohm reading between the inner and outer copper ring. It should be around 4 ohm. Put a probe inside to contact the rotor body, and check insulation resistance between body and rings. Should be up in the megohm range.

Checking the Diodes:

Desolder the 3 stator wires from the diodes. Note where they go! With a diode check function, check between one of the plates where the diode is fitted, to it's leg on the centre ring, first one way then the other. You should get .466V drop one way with 0v the other if all is ok. This is high because you are testing 3 at once.

Check Stator coils:

With ohms scale, check between each of the 3 coils in turn. Should be very low, again maybe 5 ohm. Check insulation resistance again between body and coil, should be in megohm range again.

Check spin of rotor:

Should be quiet with no roughness or clicking. If it is rough remove the outer clamp bolts, mark line on outside to help assembly. Take apart, and replace both sets of bearings. Use vice to crack the bearings if a puller is not small enough.

Kit of parts available:

You can buy bits separately or as a kit


Presuming everything else checks out ok, re-assemble with the new parts, at least a new regulator, and try fitting it to test it running. This is really the only way to test it fully at home.
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