MK1 Golf VR6 Alternator / Battery Light Wiring issue

Hi all,

I'm hoping someone on here can help with the issue I'm having. I have a 1990 Mk1 Golf GTI Cabriolet and I've recently completed a VR6 engine swap on. I had a custom loom made which was spliced into the existing Mk1 Loom. Car starts and drives fine but the battery isn't charging, Well, it is but at around 12-12.2volts. I have checked all earth points and even ran a separate new earth from the engine to the battery to see if this would increase the voltage and resolve the issue but it didn't. I've also checked the voltage with the car running at the battery and straight off the alternator and I'm getting the same voltage. Looking at the loom I then found a black single spade plug which runs to the alternator plug. Checked continuity on this cable and all is well but there's nothing on the rest of the loom for this to plug into.

When I put the ignition on, I don't get a battery light at all. so I'm guessing this black cable from the alternator plug needs to get to the original wiring loom for the alternator/ battery light. I'm hoping this all makes some sort of sense. Please see below the photo of the plug if it helps.

Can anyone advised of which cable this may need to attach to on the original mk1 loom / into the fusebox?

I've uploaded a photo to this post so hopefully this will show as I haven't used forums much before.
Thank you in advance, Matt

The battery isn't being charged at all 12.2V indicates that you are using battery power only.

So I have to ask as I never assume (yeah right this is why I am typing it after my 37 page post) you have had your alternator tested, as the Voltage Regulator circuit can go bad and you see the same indications that you have. It also verifies that your alternator can produce voltage and isn't having an issue to the regulator. So before proceeding to tear your hairs and car apart, Take it off and have it tested this validates in your mind that it is good, and you can proceed to the ripping your car apart and hairs out trying to find the bad part of the equation.

As I don't know which type of alternator that you are using I will be roughly generic.

On the alternators there is a large red wire that feeds back to the battery.

On the alternator there is a itty-bitty 14-18 gauge wire that is either a ring connector to bolt on thing, or part of the plug, it is usually blue but I have seen black used.

This little wire is what tells the alternator to charge.  It is called the exciter wire, as it is fed off the battery to the clusters battery light, through a diode back to the alternator.

When the Ignition is switched on, current goes from the battery to the lamp through the diode and to the alternator.  This starts the coils in the alternator to build a field to allow charging or the alternator to produce current expressed as voltage…Some alternators and Voltage regulators have a speed sense, and you have to blip the throttle above 900-1100rpm to make the alternator charge.

When you start the car and the engine is turning over, then your alternator starts producing voltage as the 13.75-14 volts of the alternator reverse biases the diode in the dash turning off the Battery light and your voltage gauge will show 13.75-14 volts typically.

So if we look at your issue you stated that the battery light never comes on, so that would in effect be either a 12V loss of power to the alternator, or a Broken Exciter circuit.

The Exciter circuit is fed from main power into the cluster (clocks) on the black wire, through a resistor to the top of the diode which should be 12-12.2V (battery power.  To power the light ground comes up through pin 2 (brown) on the cluster, through the light to reach the 12v at the top of the diode. (diodes are one-way switches).  Since the Diode is correctly biased, it allows the current or voltage which ever you choose to call it to pass and the light for the battery comes on when you turn the key, this voltage is also applied to the alternator to start building the field to allow a charge.

When the Alternator is producing voltage or current, the voltage it produces is more than the bias point of the diode, and it switches off the current path to the lamp in the cluster (through reverse biasing) and the lamp extinguishes.

So back to basics.

You may have a defective alternator or exciter circuit, and or clocks (a burned out battery light can cause the same issues).

With the Key on, then you should have 12 volts to ground on every wire on the alternator. This tests the
12V to battery wire, and the exciter wire sense from the cluster.  If you don't have 12v on the exciter wire, and the battery light is out or not turned on a switch on, then we have to make some more tests.

If you have 12v on the black wire of the cluster, then you need to validate that the lamp is good.  At the same time if you don't have 12V on the Exciter wire leaving the  cluster (blue wire) then your clocks circuit isn't good, and I would lean towards a burned out light or a bad diode, typically rare, but the bulbs do burn out.


You can fake it a bit by finding the exciter wire on your alternator and when the car is running attach a 12v wire from the battery through a 12V lamp and see if your voltage goes from 12.2-13.75-14V.  if that happens then you are looking at an exciter issue possibly the lamp being burned out.

You can google Hot-Rod alternator exciter circuits to see how to make one if you don't relish taking your dash apart to verify that your exciter is working.

If you go that way, make sure that you make it watertight if externally done to the cabin.

 

Thank you very much for that, That will help a lot and you seem to know what you're talking about!! you don't  live local to Newcastle Upon Tyne do you haha?

I think I will try and get the alternator tested to start with then go from there. I know the LED in the dash has always worked as I've owned the car for over ten years now and never had any issues with it until I swapped the engine out not to say that the LED may have stopped working…

The black wire from the VR6 alternator that I had photographed, this is the exciter wire? This is a single black wire coming from a plug on the alternator to that single spade plug. So this needs to make its way ( somehow ) to the back of the clocks as you mentioned above, would this go "To power the light ground comes up through pin 2 (brown) on the cluster"

OR

"The Exciter circuit is fed from main power into the cluster (clocks) on the black wire, through a resistor to the top of the diode which should be 12-12.2V"

I'm sorry If I sound like I'm repeating myself I'm just trying to get my head around it all.

Thank you again

Also, I have found the below:

If this has anything to do with it?


On the rear of the fuseboard:

Blue multiplug ( 28 pin one) On pin 16 ( Blue Cable) this is for Dash plug T7a/5 alternator warning light.

AND

Yellow multiplug ( 22 pin one) On pin 2 ( blue cable) this is for Alternator Pin D+.

They seem to be the only two wires or pins I can find with anything to do with the standard Alternator/ Clocks warning light.

They would probably be the right ones to tie into.


The exciter circuit starts at the cluster from the blue wire.  It leaves the cabin and makes it's way over to the alternator.

The exciter circuit gets the power it needs 12.v from the black wire in to the cluster Battery 12V+in at the top of the clocks on the diagram at the junction of the resistor and the diode on the clocks.

yes connect the square plug pictured to the original mk1 blue wire. its found in a spade plug near the battery live wire from the fusebox

So I managed to get the issue resolved thanks to the help from you guys ! Found that pesky blue wire and tested it only to find it must be broken at some point in the loom so replaced it with a new cable. Now I have the battery light coming on with the ignition and going off when the car starts ! Getting around 14.2-14.5 Volts when the car is idling so it's charging properly now !! Also fixed a bad earth on the clocks that was causing the rev counter to die and dash lights to dim, took her for a drive out and she died on me randomly, then wouldn't start, turned out the starter motor has failed. Carried out a few tests on it and it's not fixable so another one is on order. Oh, and when reversing her into the garage I noticed my rear side lights no longer work … But they did when I took her out 20 minutes before hand
… I've had enough for the night so it's put away and ready to be looked at again when I have the time and patience !! Thanks again for the help guys I really do appreciate it!!

side lights bad connections if you tap them while they are out if they come on yep bad connections that need to be cleaned.