Power issue

[-TheVixen-]

My battery light come up the other say and after driving 5 minutes my dash light up like a Christmas tree. Then it happened again, the second time it happened I was bringing my car to a stop and dash lights up again and car died. Was a fuel issue so replaced fuel pump and fuse and all seemed fine.

The next day the battery light is still coming up from time to time and loses power and can hear my subwoofer cut out. I have since replaced all fuses and relays to be sure. Battery and terminals are all in near new condition. I decided to do a multimeter test, the battery from postive to negative reading was within normal range not running, running, and didn't drop under load either.

I then tested the negative to alternator was around 0.22-0.25 and the positive to alternator was 0.05. After looking around on YouTube I discovered it was meant to be the other way around. The negative to chasis ground was within normal range. Wondering what could cause this?

 

[accentstencil]

Do you have figures for what you consider to be normal range, especially to one with the engine running?

 

[-TheVixen-]

I have resolved the negative to the alternator. The reading from positive to the back of the alternator with the car running is about 0.05 and it's meant to be about 0.25 I think. Is that correct? And if so what could cause this? I'm thinking there is a short somewhere

 

[accentstencil]

I have resolved the negative to the alternator. The reading from positive to the back of the alternator with the car running is about 0.05 and it's meant to be about 0.25 I think. Is that correct? And if so what could cause this? I'm thinking there is a short somewhere

I'm not sure about that, maybe a bad earth. How many volts at the battery with the engine running?

 

[Skylarking]

You can’t determine battery condition by measuring volts alone. You need to do a load test which checks the batteries ability to provide current and voltage while staying within specs. If the battery fails a load test, replace it.

You also need to have clean battery post and battery cable terminals. Sometimes they look clean but a film coats them and increase resistance. Even if they look clean, take them off and scrub both the post and the terminal until they are nice and shiny...

Also check the terminals at the alternator as loose or corroded terminals can cause issue. Best to disconnect at the battery first, then disconnect wires at alternator and clean the terminals and cable lugs and reattach the cables.

All wiring has an internal resistance but it’s trivial in a vehicle setting. What’s important is wiring insulation is doing it’s job which requires mechanically checking the wiring for fraying or insulation rub through. On that front, what are you actually measuring, resistance (ohms) or voltage (volts)? And how?

If battery, terminals at either end, wiring itself are all ok, look at the alternator voltage regulator as being faulty (I think VX uses old school dumb alternators and not BCM... it’s a good thing). Even the alternator internal brushes (part of the regulator) can wear out and cause issues.

So with a low battery voltage, when the car is running, the alternator should pump out 14.5 (ot there about s) volts which will force current into the battery and charge it up. If it doesn’t pump out 14.5v when the car battery is say 11.5V, then there is something wrong, usually the regulator or brushes...