Technical > Alternator - Tests
Open / download file   Battery & Volt Drop - Test Procedure

The most critical and overlooked component when fault finding Starter motors and Alternators.

The Battery should always be checked first, and then wiring for voltage drops.

If the battery is not a 100% charged and capable of maintaining its charge, the electrical system will eventually fail, leading to Alternator and Starter motor failure and possible damage.

Use a Volt meter to test the Battery.

Battery Condition:
12.6 Volts
a fully charged battery 100%
12.4 Volts
12.2 Volts
12.0 Volts
11.8 Volts



238_Battery_-_KTR.pdf  (172 KB)
Open / download file   Alternator Diagram & Tests & Common Problems - Download version in PDF

A complete guide on Alternators including below information:

  • System Checks
  • Testing Alternators - then and now
  • Alternator installation guide.
  • Alternator pre-installation check (Summary)
  • Common General Alternator Faults ( Summary)
  • Alternator trouble shooting guide.
  • External fan vs Internal fan
  • Internal regulators
  • PD vs LS connections
  • Fan Belts and Belt tension
  • Slipping fan belt
  • Pulley Fitting
  • Clutch / Freewheel Pulley
  • Coolant, Fuel, Oil contamination
  • Diesel Alternators with Vacuum pumps
  • Land Rover common faults
  • Wiring on Nissan Primera Alternator
  • Wiring differences on late Audi, Ford, VW alternators
  • Alternator Plug types:
    • COM
    • DF(M)
    • C
    • RLO
    • PCM
  • Alternator Plug descriptions - by Manufacturer
  • OE Part number location


175_KTRv3.2.pdf  (1,818 KB)
Open / download file   Alternator - Plug Descriptions - by Manufacturer
172_KTR_Plug_Descriptions.pdf  (180 KB)
Open / download file   Automotive START-STOP systems
  • Starter Motors in Start-Stop systems
  • Reversible Alternators
  • Principle of Operation
  • Diagnostic method
241_Automovtive Start-Stop Systems v1.pdf  (1,041 KB)
Open / download file   Water-cooled Alternator for B.M.W. (12V 150AMP)

Technical note:

On replacing the alternator, on these vehicles all batteries of the vehicle need to be replaced ! if not it will result in a burnt-out alternator.
Detailed description of the fault: The alternator is not designed to charge at a full rate, 100% of the time, if one of the batteries is faulty, this will lead to the alternator charging at 100% charge rate for extended periods, causing the unit to burnout. The unit will appear as if the bearing has seized, but one will be able to see that the unit is clearly burnt out.



240_Technical note - BMW Watercooled Alts.pdf  (23 KB)
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 Alternator pre-installation check:

  • 65% of all alternator failure is due to a faulty or weak battery (batteries should be replaced every 3 years with the required correct cold cranking amps). Alternators fitted to cars with a discharged / flat battery will fail within 30 minutes. The alternator is not designed to and cannot cope with the demand of handling the vehicles electrics and charging a flat battery.
  • The likely result is an overloaded alternator which will blow the rectifier, rendering the unit useless. NOT a valid claim reason.
  • 15% of all alternator failure is due to poor electrical connections, fuseable links, bad battery cables and bad grounds (Cables can be tested by verifying that the voltage drop doesn’t exceed ½ volt from end to end).
  • 10% of all alternator failure is due to belt wear or improper adjustment (check for cracks, polished wear on belts and proper tension on belts).
  • 5% of all alternator failure is due to jump starting another car improperly (failure to disconnect on alternator may cause voltage spikes).
Note: Before installing alternator you must be certain of a fully charged battery.
Note: Never remove battery cable from battery to check charging system. This will cause damage to the alternator and computer system.
Common General Alternator Faults:
No charge:
Loose or missing fan belt.
Break in wiring.
Blown / corroded fuse or fuse link.
Blown rectifier diodes, (often caused by jumpstarting a
vehicle without first disconnecting the Alternator).
Low charge:
Loose fan belt.
Poor wiring connections.
Poor battery condition.
Failed Regulator or Rotor.
Loose or damaged fan belt.
Loose pulley.
Worn Alternator bearing, (often caused by an over tightened fan belt).
No warning light:
Blown warning light bulb.
Poor Alternator earth connection or broken wire.
Relay problem.
Dim warning light:
Loose fan belt.
Loose wiring plug.
Continuous light:
Poor wiring connection (common mistake when plugging in the A127 series is the small warning light terminal gets bent over touching earth).
Loose fan belt.
Defective alternator.
Open / download file   ZEN - Tool for mounting and removal of Freewheel / Clutch Pulleys
259_Zen_Freewheel-Pulley_tool_Eng.pdf  (1,639 KB)
Open / download file   Alternator plugs
269_KTR-AlternatorPlugv4_WEB.pdf  (910 KB)

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