Anyone know what voltage a good alternator should throw out ?

I'm getting an ABS warning light randomly after running for a while.

Battery was flat so charged it fully and got 12.7 volts with meter. XJ sport I have sitting in the driveway was flat to so I charged that battery and got 13.5 volts. when I re-installed the battery's and used my meter to check the voltages when they were running I got 13.3 on the XJR and 14.5 on the XJ sport. Are these numbers normal - seems a bit of a difference on alternator output. Need to get to the bottom of this as failing MOT as abs lightr comes on randomly.

Cheers

 

Alternators typically put out around 13.5 to 14.5V & usually it's closer to around 14V. In cold weather the voltage will be towards the higher end & in hot weather towards the lower end. As a battery heats up it's voltage requirement falls due to increased chemical activity-the opposite is true in cold weather.

The XJR voltage of 13.3v with the engine running is too low-unless it's a hot day & you've got every electrical system switched on. In this current cold weather in the UK you should be getting at least 13.5v & preferably over 13.8v.

The XJ Sport voltage of 14.5v is about right on a cold day, although it shouldn't be any higher than that.

The correct way to check the open circuit voltage of charged car batteries is to disconnect them from the charger when they are fully charged, then leave them overnight for the 'surface charge' voltage to dissipate. If you check the voltage right after charging, you'll get corrupted results due to this surface charge.

A quick way to remove the surface charge is to apply an electrical load for a short period-although the most accurate way is still to leave the batteries overnight.

Therecould be several reasons why the XJR system voltage is too low with the engine running:

1) The alternator itself is faulty

2) There's high resistance in the earth leads

3) One of the battery cells is failing & dragging down the voltage, drawing a heavy current from the alternator. This can be spotted by leaving the battery overnight again whilst disconnected-the faulty cell would be flat the next day & the battery voltage would be closer to 10.5v. It would also get very hot with the engine running & the alternator charging into the failed cell.

 

starting to look like bad battery then. nothing was switched on in either car and it is a bit chilly here (north east Scotland. will diconnect the battery and put the meter on it in the morning.

thanks

 

If the battery is in good condition & charged, you should get at least 12.6v if you leave it overnight to settle & then check the voltage the next day. If you get closer to 12v, then all the cells may simply be old-or 1 cell has failed & is gradually discharging overnight. This can be checked by leaving it another day or 2 and seeing if the voltage falls 'over the cliff edge' to below 11v.

But certainly in NE Scotland now with the engine running & no heavy electrical load applied, I would expect to see at least 14v from a healthy charging system & somewhere nearer to 14.2v.

13.3v is definately way too low for those test circumstances.

 

The battery in the xjr is about 4 years old and the xj sport is about 18 months. so hopefully it is just the battery going and not the alternator.

 

You could always swap batteries and see if the readings follow the batteries.

 

While it is running, connect the meter between the engine and the frame. You shouldn't see more than .1 or .2 volts. Any more than that will indicate a bad grounding strap. I've had charging issues with my car caused by just that.