Funny voltage fluctuations when lights on

[adam quantrill]

The cockpit voltmeter is showing some odd dips to around 10-11V lately but only with the headlights on.

Also the dash lights go dim.

However the headlights still appear to work OK and the battery light doesn't come on so I think it's mainly an earthing or internal wiring problem.

Time to get the voltmeter out methinks!

[adam quantrill]

also it seems to do this for a few minutes, then it recovers and the voltage comes up, and the dash lights return to normal brightness. (Maybe it's haunted!)

[KKson]

Voltage stabiliser?

[RCK974X]

Couple of ideas to check.

Could be a dodgy earth somewhere - if only dash lights go dim but headlamps don't then almost certainly a faulty earth.

Could be alternator main regulator or brushes - these can start playing up before they fail. Headlamps will go dim too.

If you've got Smiths gauges, could be regulator for gauges, but lights won't go dim...

I had loads of earth issues with mine starting out, redid them all, and connector blocks too (2.8 not sure if V8 has connectors in same places) ) since then had no serious electrical issues, did get winking pods and discovered borken connector.

A Multimeter/Voltmeter is your best tool

[jeff m2]

Easiest way to find a dodgy ground is with a good one

Long piece of wire croc clips each end. One on Ground of battery then poke your ground wires with the other end. Get someone to watch your V.gauge in the car.
Should jump over 12 if you find a bad ground.
Headlights seem a good place to start.

[adam quantrill]

I found that the battery isolator is falling to bits - it's in the battery earth lead, the thread is going and it doesn't clamp very well any more. So that will be the first thing to go.

Voltage drop between the alternator output to battery + is around 0.25V when the main beam is on, so maybe the contacts there also need a clean at both ends.

[adam quantrill]

Still doing it with earth isolator eliminated so I am starting to think maybe diodes. Although I still have the alternator leads to clean up first. The sense/field lead also has a quarter volt drop so some improvement to be made there.

[ElvisWedgely]

My money is on the alternator brushes or commutator or both.

Tony. TCB.

[jimburr]

Funny that old earth. The battery gauge always reads a little low on the 350SE, until you put the lights on! Then oil pressure is better and bonus got more fuel. Wedge👍

[adam quantrill]

I first changed the regulator + brush pack but that didn't help so I have just stuck on my old noisy but working alternator, which now does the job, Nice steady voltage, drops a little with full beam to 12.3V but that is tolerable.

I think the faulty one has a diode on its way out, under mild load it was OK but heavy load warms up the diodes and then the output drops.

I'll order another diode pack for it and rebuild because otherwise it's in pretty good nick.

[adam quantrill]

Well the new (old) alternator worked for a bit then started showing the same symptoms if not worse on the way home in the dark....

just bout got home with the engine almost dying and voltmeter very low.

Under the bonnet - oil sprayed around the alternator and heat blanket just over the general area.

So how's this for a scenario:

1. Small split in oil cooler hose where jubilee clipped to the oil thermostat.
2. Sprays oil onto the fan belt. But only when the engine oil is hot.
3. So the alternator pulley slips under load.

Now it's bad enough to see what's happening I can clean it up, fix the hose and hopefully that's that. Watch this space!

Well it looks like it's finally fixed! The proof of the pudding will be tomorrows drive with the lights on but hopefully all is now OK.

The battery went down to 12.5V on idle with interior fan on and full beam, but came back up to 13.4V with 1200 ish rpm, a great improvement.

This has to be one of the more obscure problems I've fixed!

Edited by adam quantrill on Monday 16th November 21:28