MK 5 Battery warning light

[vaya82 0]

Recently took my MK5 for its MOT, and on the way back noticed the dash battery warning light was on - only dimly but pretty much all the time, fluctuating with the engine revs.

 

As the car doesn't get much use, I assumed it was the battery. Put a new one in, but still the same problem.

 

Checked the voltage across the terminals with the engine running, and it was barely above 12v. Belt tension okay, so checked output from alternator by popping the back cover off the multiplug (with it still plugged in) and testing the two large terminals with the engine running. These both not much more than 12v, so came to the conclusion in was the alternator.

 

I've picked up a spare A115 from somewhere, so swopped it in. Voltage across the battery now around 13.6v, with the output from the alternator a little higher than this.

 

BUT, the dash battery light still stays lit (if anything a little brighter than before) and seems to brighten and dim in line with engine revs and/or indicators & hazards when used.

 

I've re-used the old belt (which seems fine) and the external interference suppressor off the old alternator.

 

Anyone any ideas what the issue could be - I'm concerned the original alternator packing in was symptom rather than cause.

[Incarsolutions 0]

Check for corrosion on the fuses. Had the same thing on mine a couple of months back. Twisted the fuses at the box and it stopped. Cleaned them all up and its been fine since.

[GhiaMk4/5 207]

As well as the fuses the block connector that goes to the fuse box from underneath, these corrode, and can give the glowing light syndrome.

[sheffieldcortinacentre 751]

Also check dash earth & same for eng.

[vaya82 0]

Thanks for the replies - I'm working part of this weekend but hopefully I'll get a chance to have a look at some point over the next couple of days.

[MK5 V8 1]

A glowing charge lamp usually indicates a faulty rectifer, a diode blown which is also why you are only getting 13.6 volts. regulators are set to 14.2 and this is what you should be getting. you say you swapped with an unknown unit but this could also be faulty

[vaya82 0]

Finally (!) had some time to look into this over the past few days.

 

I've checked and cleaned the fusebox and its block connectors (all looked fine) and replaced all the fuses on principle.

 

Then gone through all the under-bonnet and engine earths and cleaned and re-tightened them (although I wasn't sure where the dash earths to?) but again they all seemed fine.

 

Sadly the problem still persists - warning light on faintly at idle, but glows more brightly when revved, or any drain/load placed on the electrics (lights, radio, washers, rear screen etc.)

 

Given what MK5 V8 has said, and the unknown quantity of the spare alternator I've swopped in, I think my next move is to get a rebuild kit for the original alternator and then re-fit that.

 

I'm not convinced the spare that's in is 100% okay - voltage across the battery is about 14.1v, but not a constant 14.2v. The car's never had any electric problems in the past (in the family since new) and I could have just changed one faulty alt for another.

Edited November 2, 2018 by vaya82

[dgo 80]

You do have a real bulb in the ignition warning lamp not an LED?

[vaya82 0]

It's a real bulb - to the best of my knowledge it's never been replaced, so it's probably the original as well.

[btboy 50]

When you re-used the old belt you did put enough tension on it and/or tighten up the bolts? A slack fan belt can cause this problem as can an old shiny belt.

[vaya82 0]

Yep - checked tension before and after per what it says in the workshop manual, and double-checked all the bolts we/are tight.

 

I've got a new belt, so when I've (hopefully) rebuilt the original alternator, I'll fit that when it goes back in.