Forums Forums Enthusiast Forums A - Z Autowirral Machine to test Alternator?

I have a suspicion that my sisters alternator has gone on her car, where's the cheapest place to get a tester to check? I can't drive the car as its not beig taxed til Wednesday (and the batt is flat) Thanks

i have one, also have jump leads if you want to get it started to test alternator

How much willo?

Not For Sale
here is one on ebay http://www.ebay.co.uk/itm/12v-Batte...en_PowerTools_SM&hash=item3f0e6711eb

Use a multi meter,
Heres a couple or three checks you can do with a volt meter, but the real test is to have it checked by an autofactor with a proper amp tester.

Read the voltage across the battery and then start the engine and read it again with the engine running.

If both readings are more or less the same then swap the meter to read ohms, switch off the engine, and take an ohm reading between the battery negative terminal and a good earth point on the engine and also a good earth point on the car body. If there is any significant resistance at either test then check the main earth strap(s) for damage and security. If there is no significant resistance then read on:-

Change the meter back to volts and connect the positive probe to the battery positive terminal and the negative probe to the other end of the battery positive cable at the back of the alternator. If the reading is anything other than a 1, -1, or zero, then the battery positive cable needs to be changed.

If all of the above checks out and the voltage with the engine running is around 13.5 volts to 13.9 volts, or maybe a little more, then switch on the headlights, the heater blower onto full, and the heated rear window, then raise the revs to 2500 and check the volts again. If the reading remains in the 13.5 to 13.9 volt range then all is good in the charging system, otherwise the alternator is at fault.

You can also check the output of your alternator, with the car running connect the leads the same way and the reading should be between 14.1 -14.7


If all of the above checks out and the battery is not holding a charge or not taking a charge, then the battery is very highly likely at fault.

Go to B&Q, get a multi-meter, they're about 5 pounds.
Set the dial to V, put one end on a pin on the alt and the other to earth anywhere on the body (not your body, the car's). Should read 12+v at rest and then 13 - 14 with the engine running.

Thank you both very much!

You should of phoned me dude i would of tested it for you dude

Lidls leasowe rd are now selling these for £3

Its always best to take the brushes off and examine them, new brush assembly is cheap compared to needing new commutator rings as well. If one of the brushes is short, check the rings carefully all the way around - a new set of brushes don't last long if the rings are damaged.

In my experience (especially on volvos 340/740 etc), when one of the brushes is getting too short, the alternator carries on working but is burning hell out of the ring, the only sign something was wrong was the level of sparking/noise on the supply line used to upset the ecu which gets a bit erratic. Voltage wise it looked fine even under full headlamp/demister/fan load.