Brake lights fusing - not while parked, only when running

Hi folks,

It's been a while!

My 300tdi is blowing brake light fuses.
With the engine running and stationary, it takes 40 seconds of constant brake
use to blow a 5amp fuse. With the 15amp in it takes around 2 mins constant use. If the engine isn't running ( so just have ignition on) I can't get fuses to blow. This is the only fuse that's blowing too. Have checked for cable rubbing in and out the chassis, have checked brake lights,, towing connections, haven't changed any wiring, getting to the point where I think I have to run a new cable from the break switch ( which I've changed too...). The only clue I have is that I get interference on the radio when I brake - until the fuse goes and it stops. Presume that means it's shorting...

Any suggestions appreciated!
Cheers
Ken

do head lights get brighter when you rev it ,have you checked the lamps and connectors

Hi - checked all the connectors and earths - checked alternator, lights don't increase / decrease with revving..

This is a bit unusual. Are the fuel and water gauges working properly? Don't know what age your car is, but the diagrams that I have handy show that the gauges are powered from the same fuse, so if one is breaking down, the load added by the brake lights could be the cause?

Hmm - it's an early model 94 - one of the first 300tdi's - the only thing the brake lights share is the engine management - on early tdi's I don't think there was any!

I've only got Haynes here, but it shows all '91-96 as having those gauges and the hazards all running from that fuse. As you say, not a lot of engine management going on.

That's the confusing thing, it's fuse 17 on the cover panel ( which shows brake and engine management icons and reversing light) but in the Haynes vague I've got, (86-94) none of the colours match up, yet the diagram says its for 300tdi...

My gauges etc are on fuse 2 - the only thing that pops the fuse is the brakes... Very odd.

Since it's an older car. it may be best to assume that it isn't exactly as the book says. Take the fuse out and make sure that everything else still works, just to eliminate the possibilities.

If it really is just the brakes, then I think that your suggestion of running a new wire from the switch back might be simplest, but you may need to replace the one between the back lights and also any trailer socket connection one at a time.

trailer socket is indeed a good place to look

In a basil fawlty moment of madness/ temper loss this morning I gave the towing electrics a talking to then cut them off. I've isolated the ends so I know it can't be that. I've also checked the splice for the towing too, and it's not that either.

Taking the fuse out doesn't help - the only other thing on it is the reversing light, and I've taken that off / disconnected it - there is no voltage going through it as I've metered it. Only other thing is the mysterious engine management symbol and the Landy runs fine... Everything else works.

Kennymac said:

In a basil fawlty moment of madness/ temper loss this morning I gave the towing electrics a talking to then cut them off. I've isolated the ends so I know it can't be that. I've also checked the splice for the towing too, and it's not that either.

Taking the fuse out doesn't help - the only other thing on it is the reversing light, and I've taken that off / disconnected it - there is no voltage going through it as I've metered it. Only other thing is the mysterious engine management symbol and the Landy runs fine... Everything else works.

Click to expand...

engine management is for egr controller under center seat if fitted

No egr under middle seat - it's blanked off on manifold so likely butchered by previous owner. No wiring etc either. I've had the Landy 8 years now, even after all this time it's a mystery!

Let's assume that the fuse box schematic is right - and that there was an EGR valve and wiring. It seems to me that it must be something like a few strands of wire that are shorting and when you add the load of the lamps it becomes enough to blow a fuse. That short could well be in the wire that somebody disconnected from the egr system. Problem now is that I can't find a different wiring diagram that shows how the egr sys gets it's power, whether it's to the underseat unit or to the valve itself - or even what wire colour to suggest that you chase.

Thanks for your efforts on this - it's appreciated!

There doesn't appear to be an egr valve fitted. Just the blanking plate on the manifold. As this was an early 300 it's possible it didn't have the egr system fitted. When I removed the middle seat there was no wiring to be found.

I'll try the brake light test the morrow ( take a line from brake switch to each light individually .) and see how we get on. I'll report back.
Cheers
Ken

Kennymac said:

Thanks for your efforts on this - it's appreciated!

There doesn't appear to be an egr valve fitted. Just the blanking plate on the manifold. As this was an early 300 it's possible it didn't have the egr system fitted. When I removed the middle seat there was no wiring to be found.

I'll try the brake light test the morrow ( take a line from brake switch to each light individually .) and see how we get on. I'll report back.
Cheers
Ken

Click to expand...

certainly an odd problem

Another thought for you tomorrow. Check the earth wires from the lamp units. A short in those could cause the slow blowing of the fuse.

Thanks for that - I'll have a bash at those too. Cheers k

Is the reverse lights not on the same fuse ?
Had any gearbox work done recently ?
Did you check the cable runs with a tester while wiggling the wires ? or just give them a glance over ...
It must be the fuse box side of pedal switch, if the blowing fuse with ignition on and pedal is not pressed ... check green wire or run replacement ...

Or use lamp or Led and start disconnecting stuff- when it goes out you find short.

Low watt Lamp or Led will limit current and indicate if fault still pressent
Or an ohm setting on Dvm

If you have a multimeter, set it to read current, and measure the current across the fuse with the engine running and don't apply the brakes. It should read 0. Anything else means you are draining current to another circuit. If it does read 0 , then the brake circuit is probably earthing somewhere. Disconnect the wiring at the rear of the tub where the loom comes in, try again with the current reading and the brakes on, if its more than 0, the fault is from the loom forwards. Next step is to measure the resistance between the brake wiring and earth, with the fuse out and disconnected at the rear. Anything less than a very high reading is a fault. You may be able to disconnect the rear loom where it plugs into the main loom in the engine bay (in mine at least) to help isolate where the contact is.