VR6 - Lighting dims on idle

[P3rks 11]

Hi everyone!

 

I've noticed when driving at night that sometimes (not always) the lighting on my car dims when stationary and on idle. If I rev the car the lighting picks up to normal again, but then when it settles to idle the lighting dims again.

 

I thought it could have been a tired alternator so I swapped it for a fully working 120amp alternator and I changed the wiring which goes from the alternator to the starter. It's not made a difference.

 

The battery is about a year old but tested and fully working.

 

I've looked at the volts at the battery end and on first start up and fast idle it's 14.4v, on normal idle with no electrics on it's about 14.2v, but as soon as I turn every electric on in the car the volts drop to 12.3v.

 

Can anyone point me in the right direction on what to look at next and any tests I can perform? :scratch:

[Kevin Bacon 5]

Assuming the earths are OK, I'm going with a knackered battery.

 

A healthy battery shouldn't pull any more than 14V from the alternator after a cold start and quickly settle to 13.5-13.8V.

 

As yours is hovering around 14.2V and dropping to ~ 12V with load, I'd say the battery has seen better days. The voltage shouldn't drop below 13 with load.

 

Stick a 74AH Varta Silver dynamic in it (cheap from battery2u.co.uk) and be done with it :D

[P3rks 11]

Thanks for the fast update.

 

Is there a quick way I can check the earths before I fork out on a new battery?

[Kevin Bacon 5]

Hmmm, not that easily tbh. It's using the meter to check for voltage drops between the live feed of the ancillery component and it's earth point. You'd need the complete ELSA wiring diagram for the car really.

 

TBH, a quick check for corrosion is enough. Maybe when you get time, disconnect all the earths on the body point behind the battery, clean up the spades, retighten them and then cover them with vaseline or copper grease.

 

I find non-branded batteries don't last for toffee. Even a VAG branded battery is past it's best half way into it's 3rd year. And it's always winter that shows up any weaknesses!

[P3rks 11]

Nice one!

 

I'll clean up the earths and re-test.

 

If not it could be a new battery time. Strange though as it doesn't lose drain and starts first time every time, even when the cars been sat for a few weeks. Although, once or twice I have had to bump the car if I do any work in it with electrics on but the engine off, which i thought was a quick drain.. hmmmm..

[P3rks 11]

Right, I've removed all the engine earths and cleaned the contact points.

 

From cold start-up on fast idle with no electrics on the battery is at 14.38v .

From cold start-up on fast idle with all electrics on the battery is at 13.8v +/- .2

From warm on normal idle with no electrics on 14.3v

From warm on normal idle with all electrics on 12v +/- .2

 

I've tested the circuits from the alternator, starter and engine earths and there is a maximum difference of 0.1v which suggests all the engine bay wiring is spot on.

 

Now, the tests with all the electrics on just needed a ever so slight rise in RPM (the tiniest feather of the throttle) to get the volts back up to 14.4v.

 

Does this still sound like the battery or could it indeed be the alternator? I don't want to fork out £100ish for the wrong one.