Charging problem

[bristolracer]

Hi all

My S type Jag (2004 ford based variety) keeps getting a flat battery.

Earlier this year I replaced the battery, the car will run when jumped, but i did notice the dashboard and headlights flickering while the engine was running.

I have had a dig about in the manual and have checked the voltage across the battery terminals when the car is running and am getting about 12.3 to 13 volts
The manual says 13.6 to 15.3 is the required voltage.

Im thinking alternator is at fault, but is there anything else I should check before I get another?

[GreenV8S]

bristolracer said:

Im thinking alternator is at fault, but is there anything else I should check before I get another?

The low voltage suggests it isn't charging, but I suggest you get it diagnosed by a competent electrician before you replace the alternator. The alternator may not be at fault, and if it is at fault it may be possible to repair it without replacing the whole thing.

[SlimJim16v]

Try measuring again, but revving the engine a bit higher.

[tapkaJohnD]

No argument with the above, but how often do you use it?
Lead acid batteris are designed to be used, discharged and then recharged - they deteriorate if left idle.
If occasional use - think about a "smart" battery charger, that you can leave connected while the car's not used.

And is anything discharging the battery while it sits there? Could be a faulty bootlight switch or similar, or even a clock. Out with the multimeter and check the current flowing when everything should be switched off.

JOhn

[chammyman]

Take the drivers wheel off the the wheel arch liner. Theres a junction terminal in there.

Its where I jump start them if I cant get access into the cars.

Be prepared for it to be a corroded mess though.

[bristolracer]

GreenV8S said:

The low voltage suggests it isn't charging, but I suggest you get it diagnosed by a competent electrician before you replace the alternator. The alternator may not be at fault, and if it is at fault it may be possible to repair it without replacing the whole thing.

Is there a replaceable regulator?

SlimJim16v said:

Try measuring again, but revving the engine a bit higher.

Tried that and the voltage does no alter, still 12/13 volt

tapkaJohnD said:

And is anything discharging the battery while it sits there? Could be a faulty bootlight switch or similar, or even a clock. Out with the multimeter and check the current flowing when everything should be switched off.

JOhn

Where would be the best place to measure this? The Jag is Negative switched if this makes any difference?

[GreenV8S]

bristolracer said:

Is there a replaceable regulator?

I expect there will be a replaceable electronic module. If it has failed, it could cause the symptoms you're getting. But it is not the only possible cause so you should get a confirmed diagnosis before you start trying to fix it.

bristolracer said:

Tried that and the voltage does no alter, still 12/13 volt

Evidently it isn't charging. This would explain why the battery is going flat, although it may not be the only fault.

bristolracer said:

Where would be the best place to measure this?

Residual current draw can be measured at the fuse box with an accurate volt meter and a little knowledge.