Brake light fuse (read all the previous posts and still stumped)

[Sallen43]

Brake light fuse blows when I start my engine?

Had been chasing this problem for a while (blown fuse). Have read all the posts I can find. Have checked the brake switch, bulb connections, reverse light switch terminals on transmission, clutch pin to switch clearance).  Couldn't work out why every time I fix it with a new fuse, test in garage all works well (only on ignition) when ever I go for a drive they've stopped working. Now I have realised that the fuse blows as soon as I satrt the engine???? Now I'm baffled. Not heard of this before. 

Any advice gratefully received before I take it to an auto electrician. What would make it blow as I satrt the engine?

[6speedmanual]

Does it do it every time? Repeatable? Are you cranking with foot on brake pedal? What if you remove fuse and put a voltmeter or test light on either side of the fuse holder and then crank? Fuse can only blow if it's carrying excessive current so must be a crossed wire somewhere or a circuit (brake light switch closed) whilst cranking. Possible compensation for poor earth somewhere by feeding through b/light cct during cranking. Continuity check to earth or the starter relay from either side of the b/light fuse holder? Do brake lights illuminate, even momentarily, when you crank before the fuse pops?

[6speedmanual]

Anything to do with HCMSL you were asking about fitting kit for on 8th May?

[Sallen43]

Hi,

thanks for your previous comments I'll try to check the things you mentioned. Foot wasn't on brake when I started it.

and not related to a chmsl. I never managed to get a loom from caterham, so never fitted one.

 

[Sallen43]

I managed to plumb in a voltmeter (on amps setting) to the brake light fuse location and every time I press the brake pedal I get 10amps (fuse is a 15) when I release the brake pedal amp reading is around -6. When I engage revers (same circuit apparently) I also get 10 amps, but when I come out of reverse I get -6 sometimes and -16 and -17amps. Could this explain it? If so any idea which wire to check next? 

Sorry to bother you, but you gave some really good advice last time. Perhaps the engine start was a complete misnomer as I'd checked reverse before starting the engine last time.  I get consistent readings with engine running and not running and also during cranking. I'm suspecting something related to coming out of reverse? But have no idea what?

[6speedmanual]

No worries!

Glad you have got a bit deeper into the investigation.  I wasn't that much help, just doing a bit of brainstorming.  Hopefully the collective of BC will get to the bottom of it!

The brake lights are 21W x 2 = 42W

Using P=IV

I=P/V

I = 42/12 = 3.5Amps  (if my o-level physiscs is serving me correctly!)

So, something is drawing too much current on the brake light circuit, something down stream of the fuse and the switch shorting to earth?  Or shorting to the reversing light circuit?

Do the brake lights come on or is there just a current draw? Also do the reversing lights come on? If there's a short circuit, normally the bulb would not light or only light dimly as the current prefers to drain to the path of least resistance.

Have you checked the earths?  (trace back the harness from the tail lights, or someone on here tell you where it earths to frame.

I've never had any problem in this area on my 7 but maybe others have?  I seem to remember reading various threads about tracing short circuits.

Peter

[SM25T]

Reverse light shares the same fuse. Maybe a connector on the reverse light switch on the side of the gearbox is touching the tunnel wall ?

[tbird]

 This thread  might help with location of reverse light switch 

 

[ECR]

M25's suggestion sounds quite feasable. The contacts on my reverse switch are very close to the tunnel wall and if the engine/box rocks during start up it could well touch and short out

[Roger Ford]

Am I reading that right?  You're getting a reverse current through the fuse when you're not in reverse, and the brake switch is not pressed?  That's pretty odd.

Try removing brake and reverse light bulbs and see if that current disappears.

[tbird]

find it difficult to see how current flow can change direction, whatever else is happening

[Jonathan Kay]

Sorted yet?

Jonathan

[Paul Deslandes]

On earlier cars the reverse light is switched to chassis so a short at the switch will make the light stay on rather than blow a fuse. The body of the switch is a terminal and scewed in to the gearbox, the other terminal going to the bulb which, in turn, has its other terminal wired to the fuse. 

Later cars (dunno when the change was made but after 95) have a two terminal switch wired into the 12V feed so a short would blow fuses.

In both cases, the brake light circuit is wired to the same fuse via an in-line switch, the bulb's other terminal going to earth/chassis/negative/ground (choose your terminology).  There may be very early cars with hydraulic switches, rather than mechanically operated from the pedal, that probably switch to chassis, the same as the reverse light, so beware.

Are we sure that the fuse blows when the car is started/cranked or is it perhaps when selecting reverse to back out of the garage?  I think I can see a mechanism for the latter which might explain why the current flows in reverse.  Basically, there's an earth wire off somewhere.

Hopefully fixed by now..........

Paul

 

[john milner]

If the bulb is blowing when the revs go up could it be alternator problem? A knackered voltage regulator could see the voltage go too high. Try checking the voltage across the battery at 2000rpm.

Also check the resistance of the bulb. My guess is that each are around 8 Ohms. All being well I=PV = 2x21W / 13.2V = 3.18A or 1.59A per bulb. R=V/I=13.2/1.59=8.3 Ohms per bulb.

Two 8.3 Ohm bulbs in parallel = 4.15 Ohms.

To get 10 Amps. V=IR=10Ax4.15=41.5 Volts. The numbers I have used are a bit of a guess but should be good enough for ballpark figures. I doubt that the alternator is pumping out anywhere near 41.5V so either the current reading is wrong or my maths are wrong or there is something else that needs to be looked at.

Another thing to consider is how is the multimeter being used. Voltages are measured across a circuit (parallel) but current is measured in series. A meter set to current has a very low internal resistance so should never be put across a circuit and is potentially dangerous.