7 replies to this topic

[victorludorum]

I'm having problems with the battery in my 2000 Cooper. I did a few tests this morning and have the following issue. When The car is running, my voltmeter is giving me a readout of 13.5 ish, but when I turn on all of the electrical goodies in the car (lights, spots, radio etc etc) it drops to a little over 12.5. Do you think this indicates a duff battery or a duff alternator?

[sonikk4]

More likely to be the alternator than the battery. There will be a drop in power produced because of the loading but if you up the revs does the output increase??

[RedRuby]

When you turn everything on are you at idle or with some revs on? I believe that with everything turned on you should check the voltage output running at about 3000 rpm. A better qualified auto electrician will no doubt advise shortly, but I think there are a number of posts on here detailing the exact way to test battery/alternator outputs.

Edited by RedRuby, 20 April 2014 - 10:25 AM.

[ACDodd]

At the same time that the drop is happening, what is the voltage at the output terminal of the alternator? Do this test at 3krpm. Measure at the alt output terminal and then at the battery terminals.
With this test you are checking the connections between the alt and battery. If alt voltage is fine ie 14volts and 12.5 volts at the battery your problem is not the alt but your wiring.
Ac

Edited by ACDodd, 20 April 2014 - 10:48 AM.

[victorludorum]

At 3000 rpm I'm getting a reading of about 12.7 at the battery. I will check the alternator terminals later...

[victorludorum]

AC,

 

Thanks for the advice How do I test the output of the alternator, please? I see 2 connections on mine, one which says B+ and one which says D+. It's a 2000 Cooper.

[ACDodd]

Negative of the voltmeter connected to alt case. Positive to the B+ terminal! the one with the large cable connected to it.

Ac

[victorludorum]

I just did the alternator output test and I'm getting 14+, even at lower revs, so this indicates a wiring problem. Am I right in thinking that the thing to do is to do the test while eliminating certain electrical connections like lights etc? Do them one by one to see which has a big effect?

 

Is it possible that the engine earthing strap is part of the problem?