Electrical problem

[Hawklord]

I am experiencing strange electrical problems with my 300TDI which I assume are all related.

For over a year now the 15A fuse for the brake and reversing lights periodically blows but it may be months, weeks or days before it blows again after replacing.

This week when I switched off the headlights via the stalk on the steering wheel I noticed that the headlights stay on and they will not go off until I turn off the ignition. When I turn the ignition back on the headlights remain off until I switch them on again and then they won't turn off again. This doesn't happen every time but probably 9 out of 10 times.

Yesterday the headlights and gauge lights came on when selecting reverse gear and go off again when coming out of reverse.

I have a boomslang wiring harness so the headlight switch opens a relay and the headlights are wired directly to the battery so my initial thoughts were that the low beam relay is sticking on but this doesn't seem to have any link to the reversing light issue.

When the headlight switch is pushed into the second on position I can hear a relay sounding click from the right side of the gauge panel and can also hear this when selecting reverse gear.

This had totally stumped me and can anyone suggest where to start looking for the cause(s) of my problem.

 

 

 

[Peaklander]

The reverse light is fed from the switch at the gearbox , which itself has an ignition controlled +12v (green).

If weird things happen elsewhere when reverse is selected I think it must be a fault at the back corner where the reverse lamp wire runs near the others. Maybe an intermittent short there. Not sure really. 

[Hawklord]

The electrics have been fine since my original post until yesterday when the reverse and brake lights stopped working together with headlights coming on when selecting reverse gear. So today checked the wiring in the back corners and all seems to be okay with no sign of any shorting out or corrosion anywhere. I then took apart the reversing light unit and again no signs of anything wrong there, but I then removed the reversing light bulb and the problem when away, brake lights worked and no headlights when selecting reverse gear. I can live without a reversing light but any ideas how removing the bulb fixes the problem I had. The other odd thing is that the 15 amp fuse hadn't blown this time.

[Peaklander]

Removing the bulb breaks the circuit and so any faulty bits are now 'out of it'.

I would disconnect the wire at to the reverse lamp (green/brown), at the switch at the gearbox. Then I would apply +12V to that disconnected wire, with the bulb in place. You could pop an in-line fuse in the temporary cable.

Or you could test the resistance between that disconnected cable and metalwork, without the bulb. You should see a very high resistance. If you don't then you need to follow the wire to the back end and disconnect it from the first thing you reach and measure again.

[GregK]

On 2/11/2024 at 3:38 PM, Hawklord said:

The electrics have been fine since my original post until yesterday when the reverse and brake lights stopped working together with headlights coming on when selecting reverse gear. So today checked the wiring in the back corners and all seems to be okay with no sign of any shorting out or corrosion anywhere. I then took apart the reversing light unit and again no signs of anything wrong there, but I then removed the reversing light bulb and the problem when away, brake lights worked and no headlights when selecting reverse gear. I can live without a reversing light but any ideas how removing the bulb fixes the problem I had. The other odd thing is that the 15 amp fuse hadn't blown this time.

Long shot. Can you remove rear light lenses and check if you have by any chance a wrong bulb fitted [single filament instead of twin] and it back feeds power  to the sidelights while braking.  [ I have came across a bulb that has twin connectors at bottom of bulb but only 1 filament.]

Have you got twin electrics to the tow bar? If so maybe a short inside one of the sockets especially when they get wet. 

I would also check the wires as they go thru back body in case there is no rubber gromets and insulation has  chaffed  and shorting out.

[Hawklord]

@Peaklander

Surely when selecting reverse gear there is a 12v and earth source going to the reversing light unit whether the bulb is fitted or not, so I would have expected the symptoms I am experiencing to be the same in either case. 

@GregK

The rear lights and front lights are all sealed led units so the incorrect bulb can be ruled out.

I do have a tow bar electrical socket and is one area that I haven't really looked at, but the wiring diagram doesn't show a pin in the socket for a reversing light which could be shorting. As stated in an earlier post, what happens is when the reversing light bulb is fitted, occasionally when selecting reverse gear the reverse light doesn't come on but the headlights, rear lights and dash gauge lights do and sometimes the gauge lights are only about half brightness (headlights full brightness). And when this happens the brake lights do not come on when the brake pedal is pushed down, but by removing the reverse light bulb everything works correctly. Also when the reverse and brake lights don't work the headlights cannot be turned off by the stalk on the steering column, only by switching off the ignition.

 

[Peaklander]

55 minutes ago, Hawklord said:

Surely when selecting reverse gear there is a 12v and earth source going to the reversing light unit whether the bulb is fitted or not, so I would have expected the symptoms I am experiencing to be the same in either case

Yes but if you have a bad earth, it comes into play when reverse is selected because current is flowing to the reverse lamp. Removing the bulb prevents that current from passing through the bulb from its +12V side to the 0V metal connection. The metal should be at 0V but may not be because there is a poor connection presenting a resistance on that side. Now you have a voltage sitting there and current trying to go back along other black wires to things that aren't supposed to be ON. 

[GregK]

55 minutes ago, Hawklord said:

@Peaklander

Surely when selecting reverse gear there is a 12v and earth source going to the reversing light unit whether the bulb is fitted or not, so I would have expected the symptoms I am experiencing to be the same in either case. 

@GregK

The rear lights and front lights are all sealed led units so the incorrect bulb can be ruled out.

I do have a tow bar electrical socket and is one area that I haven't really looked at, but the wiring diagram doesn't show a pin in the socket for a reversing light which could be shorting. As stated in an earlier post, what happens is when the reversing light bulb is fitted, occasionally when selecting reverse gear the reverse light doesn't come on but the headlights, rear lights and dash gauge lights do and sometimes the gauge lights are only about half brightness (headlights full brightness). And when this happens the brake lights do not come on when the brake pedal is pushed down, but by removing the reverse light bulb everything works correctly. Also when the reverse and brake lights don't work the headlights cannot be turned off by the stalk on the steering column, only by switching off the ignition.

 

As @Peaklander says check the earths as incandescent bulb will need better earth for it to work than LED as it draws more current/power.

If you have a white tow socket [12s] this will have reverse pin connection. But there is nothing to say that someone did not use one of the side light pins on your standard 12N tow socket for reverse. [I have seen it done] but this would have to have trailer and the vehicle connected the same way for it to work.