1997 S40 dashboard lights

[ElVentu]

Hello to everyone, I'm Davide from Italy.
I own a 1997 Volvo S40, 1.8 116hp, for 10 years now.
What a car, never had major issues.

But I have an issue now.

Just some context.
In last days here it was freezing cold.

When starting the car on the coldest morning, during cranking the battery had some issues.
The car started, but when the engine revved to the "cold idle", for a moment it seemed that the alternator had issues to keep the voltage to the right level.
All the lights went up a little bit too much, then the situation returned to normal.

In that moment, the 20A fuse for the internal lights popped.
All the rest is working, but not the dashboard lights, both the instrument ones , the lighter and the AC ones went black.

I thought it was only an issue related to the voltage peak, I found that the fuse blew and I replaced it.
Nothing to do, it blew in a second.

So I tried to find the issue, I disconnected the instrument panel and checked, but it's fine.
I tried to feed 12V to it and the lights worked fine.
Also the dashboard works, no issues.

I also checked the light dimmer (I hope it's called that way), and it's ok.

I found that the issue happens only when the car lights are on.

With them switched off, everything works fine.

It seems that there must be something broken which is connected to that circuit when the car lights are switched on.

But what?

I don't have any wiring diagram for the car, did someone can help me?

Thanks a lot!

Davide

[matthew1]

Welcome to MVS, ElVentu. Dimmer is the correct term, yes.

I'd look for wires chafing/rubbing against other wires, near the headlights. Sounds like a short circuit.

[ElVentu]

Welcome to MVS, ElVentu. Dimmer is the correct term, yes.

I'd look for wires chafing/rubbing against other wires, near the headlights. Sounds like a short circuit.

Thank you for the reply!

But the fuse that popped isn't related to the headlights, and they work fine.
It's the internal lights that doesn't work.

There must be something on the internal lights circuit that is turned on by the headlights switch.

[matthew1]

I found that the issue happens only when the car lights are on.

Ohhh the internal lights, not headlights. I mis-read and thought the fuse popped when you turned on the headlights.

Well hmm. I'm not good with electrics, maybe someone else will have a thought on this problem.

[ElVentu]

It seems that the issue is related to the plate lights/lateral lights (the orange ones).
I checked and the plate lights are ok, but the circuit is shorted between the wires.

But the lateral ones aren't simple to remove, their connector is locked in the bodywork.