Strange brake lights issue

Hi all,
I seem to have a strage problem with my brake lights / pedal,
After noticing i've been driving around with no brake or reversing lights I checked and the fuse had blown. I replaced the fuse and it hasn't gone since.

My issue is when I depress the brake pedal, the engine labours - there is a considerable change in engine noise, as if the alternator is under a heavy load. Even if I turn on all my spotlights there is hardly any engine labouring.

I've checked bulbs, connections and all the wiring looms in sight, no chafing or warm wires.

I replaced the fuse with a 3 amp and it still didn't blow, which makes me think there is a problem in the circuit before the fuse - could there be an issue with the pedal switch?

Has anyone had this problem? any help would be greatly appreciated.

1999 defender 90 td5

The brake light switch is beyond the fuse in the circuit. There is just a slight chance of a fault in the hazard switch/wiring affecting it, I'd unplug the switch to eliminate that. If the fault persists, I'd start looking at the brake vacuum area - remove the hose from the servo and test again. Are the brakes working OK? Any noise from the vacuum pump?

Thanks for your reply,
Ooh ok i'll try that, didn't think about the vacuum pump, i'll have a fiddle in the morning. Brakes are working fine and no unusual noises from engine bay.

Is the TD5 clever enough for the engine ECU to have a signal feed from the brake pedal switch? On many cars when pressing the brake a signal is fed to the ECU so that the ECU drops the rpm if you still have the throttle open (as you wouldn't want to accelerate and brake at the same time) On my Audi (and most modern VAG cars at least) if you hold the throttle at 2500 rpm in neutral and then press the brake pedal with the other foot the rpm will drop to about 13/1400. Could be that your ECU is getting contradictory signals from the accelerator and brakes?

Also have you got ABS as that will also communicate with the engine ECU?

I've checked circuit diagrams and yes, the ecu does have a feed from the brake circuit. That's interesting, and no, no ABS

Also worth mentioning the 'problem' goes away when I remove the fuse.

And the ecu has gone for a swim before

Obviously don't know your skills and ability (or even the TD5 ECU wiring) but if you can remove the pin that feeds the brake signal in to the ECU and try again then if the engine runs ok and the brakes work ok then that would eliminate all the brake light wiring and point you towards the ECU / throttle position sensor area. That said depending upon the degree of swimming the ECU has done there could be corrosion issues inside the ECU which may cause tracking between components and cause it. If it does clear the issue then I cant see any real reason why you couldn't just leave it disconnected unless it keeps throwing fault codes, especially if it delays the replacement of the ECU (IF it is corrosion unless you do something to stop it the ECU will die eventually - could be weeks, could be years).

Good idea, i'll have a go and let you know what transpires. Again, many thanks for your help

So there are two options for the ecu brake switch input, normally open and normally closed, mine has a normally open connection - pin B16. when I disconnect this, the engine goes into a constant 'labouring' sound.

I think the next step would be to read and clear any fault codes. I'm just satisfied that I know it's something to do with the ECU, not my brake circuit.

So does that mean that when you put the brakes on then +12 is fed in to the ECU? If it is then is seems odd that disconnecting the wire causes the issue unless the ECU is sensing the brake lights via the ground path through the bulbs (effectively using the brake bulbs as pull down resistors when brakes are off). Disconnecting the wire as you have done may be allowing the ECU input pin to float 'high' which is the same effective signal as applying the brakes (+12). If that is the case then grounding the ecu pin may stop it labouring and let the brakes work as usual.
This is just a bit of theory on my behalf, if you are not electronically minded or are not confident in what you are doing or don't understand the theory or the potential risks then don't go shorting out pins on the ecu just in case you pop something! Good luck

That seems to make sense, i'll try that if clearing codes doesn't solve anything

Got my hands on a snap-on gizmo, cleared the codes (nothing out of the ordinary), and the problem still exists. I think i'll just put up with it for now...