Try this:

 

Measure volts right on the battery posts, with engine running and a bit of electrical load on, say dipped headlights and cabin fan on a medium setting. Write down the number.  Now move both meter probes to the alternator. The red one on the main output post (may have a plastic cover you need to remove), the black one to a scratched-clean bit of the alternator body (earth). Write down the number.

 

If the voltage at the alternator is much healthier (say 14.2V or a little more) than when measured at the battery posts, you are losing some voltage due to a bad wiring connection or corroded wire.

 

Leave the red probe on the alternator output post, and move the black one to battery positive, the reading now is how much voltage you are losing down the positive wiring. Write down the number.

Now put the red meter probe on the alternator body, the black one on battery negative, the reading is how much voltage you are losing down the earth wiring. Write down the number.

Both of these numbers should be very small, well under 0.1V, if not, there is almost certainly a bad connection that needs finding and improving.

 

If most of the voltage drop is on one or other (positive or earth) rather than equally shared, you know which one to improve first.

 

I don't think the lack of illumination of your demist button is anything to do with your charging system by the way. More likely a dodgy wire or connection in the cabin loom.

Edited 17 January, 20179 yr by Wino

I think people might be getting the wrong idea from all this? From what I can make out is your battery was suspect and now replaced it starts ok and from the readings you have given the alternator seems to be charging ok. The light you refer to is the light on the rear demist switch that is not working.

You said you replaced it twice? Did the two switches illuminate when first used or not? I would get one that has been checked to work, Devonutopia on here breaks the odd Octavia, and will have a good working order switch.

Ha, sorry for the confusion. In another thread it was suggested that someone had a similar issue on a golf and it was the alternator. That's where alternator goosed game from. I have no issue really other than the light no longer illuminates. Yes it did illuminate on each replacement. I posted the values off my multimeter which seem on the low side. As the battery hasn't cured it (bug needed changing) the next task will be to follow the advice above and check the alternator.

If it starts and gives above 13v with accessories on it's charging. A rule of thumb has always been after starting a car it will take on average 7 miles to charge back to where it was. If it's working fine then don't fiddle is the best advice.

As for the dentist switch it's just an led inside the switch when the switch is pressed in, if your capable strip it down and see if it's working with a power supply. I presume it's the yellow light your on about not the green led that illuminates the symbol when headlights are on?

Try this:

 

Couldn't of put it better myself !

 

.....and how many times the wheels turn doesn't determine how well the battery is charged!!

You need to check volts drop on earth and supply circuits when under load, there will be you-tube guide on this that will show you better than I can explain!

If I was you I'd be checking the fuses which sit on top of the battery(part of the cover?).........poor connection/high resistance here will be obvious if the plastic has started to melt/deform.

The fuses are good but the plastic next to the black lead is melted! The cable looks good. Connections are tight. I removed the black lead and cleaned the lug with scotch brite. Anything else that can be done ? Where is the main earth point ?

The plastic melts next to the black lead as this is the main supply lead from the alternator, and this is a known issue in the mk1 in so much as the lead corrodes and degrades internally creating a high resistance between the terminal and the fuse box.

 

This results in a build up of heat, eventually melting the fusebox. The terminal can also exhibit the same condition at the alternator end as well, but melting is not so common there.

 

The only sure solution is to replace the fusebox and lead before it totally melts and leaves you stranded somewhere as a worst case scenario, least case is that the dash lights up like a christmas tree.

 

Hope it helps

 

Phil.

The fuses are good but the plastic next to the black lead is melted! The cable looks good. Connections are tight. I removed the black lead and cleaned the lug with scotch brite. Anything else that can be done ? Where is the main earth point ?

Sounds like you've found your cause of volts drop...................follow the cable(s) from battery negative terminal and you'll find main earthing points. I suspect it'll have just one going to block and a separate one from block to body but you'll just have to check.

Most of the main earth points are located under the battery once you remove the tray at the body end, and as you say they go to the block at the other end.

So, replaced the main alternator feed cable as fuse box was melting ! With no load output is now 14.1v and with everything on the output is now 13.9v so I think we're good 

2 minutes ago, VrsMule said:

So, replaced the main alternator feed cable as fuse box was melting ! With no load output is now 14.1v and with everything on the output is now 13.9v so I think we're good 

I think you are owed a Mars bar lol

15 minutes ago, VrsMule said:

So, replaced the main alternator feed cable as fuse box was melting ! With no load output is now 14.1v and with everything on the output is now 13.9v so I think we're good 

Where are you measuring? Right on the battery posts?  If so, those numbers are still a little low, especially the 14.1 unloaded.  I would suggest following the earth connection(s) from battery negative and cleaning all the junctions; or at least doing the measurements I suggested a few posts ago to identify what exactly the alternator is putting out, and where any remaining voltage loss is occurring between it and the battery.

 

Much better than before though.

Edited 27 January, 20179 yr by Wino

Yeah it'll do me ! Was going to carry out further tests as suggested but when saw the fuse box was melted I thought I'd go down that route first. As everything is working and there is a marked improvement to the figures I'm going to call it a day on this particular issue ! Just have to replace fuse box when it arrives.