Brake Light Weirdness

Found out earlier today that my brake lights aren't working, and have just been trying to find out what's wrong. Here's what I've done so far...

Checked fuse - okay.

Check tail lights - okay (so earth at lamps okay).

Split connections at brake light switch. Ignition on, the feed shows 12V. Reconnect feed and press pedal - other side of switch shows 12V. Release pedal, get 0V. So, everything up to switch okay.

Measure for resistance from switch to lamps down green/purple wire. Get about 2 ohms. Remove one lamp, get about 2.5 ohms. Remove both lamps, get open circuit.

Reconnect switch wiring and split bullet junction at rear offiside. Ignition on, pedal down, measure feed wire (green/purple) for volts - nothing. Measure for resistance through lamps - same as before.

Run a separate wire from the switch to the bullets. Ignition on, pedal down, get 12V at far end of new wire. Connect wire to lamps - nothing.

Measure resistance between earth tab on lamp holder and a plated bolt in the back - less than 1 ohm. I variously used the bolt and the lamp holder's earth when measuring volts, depending on what was nearer/easier.

To summarise the anomalies...

Green/purple wire from switch to rear is okay when measuring resistance through the lamps, but I can't measure volts on it when it's connected to the switch and disconnected from lamps.

When I supply volts to the lamps with a separate wire, they don't light up - not even a faint glow.


I must be missing something, surely?

Nomad Z said:

When I supply volts to the lamps with a separate wire, they don't light up - not even a faint glow.

Click to expand...

So in conclusion your bulbs are not working?

My suggestions would have been to supply 12V directly to the bulbs to check to see if they're working, then stick a multi-meter into the sockets to see whether that's working.

If the bulbs are working but not being activated when you push the pedal have you checked the switch to see if it switches on when you push the pedal?

I didn't explicitly verify the lamps by getting them to light up, but the filaments on both looked okay, and the change in resistance from two lamps, to one lamp, to no lamps, worked both ways round (they were in and out multiple times, both lamps tried in both holders). I'll try verifying them tomorrow in any case.

Did put a meter into the socket - I got about 0.1V with the pedal down, and 0V with it up. This was with the original green/purple wire making the connection from the switch.

The brake light switch is okay - as I said above, with ignition on and pedal up, there was no voltage on it, and pressing the pedal gave 12V.

After an hour or so in nut-chilling weather, and with the light fading, I decided enough was enough for one day - better to try again with a fresh mind and with the benefit of knowing where the bits are (and already having the covers off the back corners).

have you checked the bullet connector going to both lamps and the condition of the bullets themselves for bad corrosion dry solder ,checked lamp holder connections for corrosion and that the bulbs are correct pin config and they contact the lamp holder contacts when inserted

The bullets were quite tight, but a squirt of WD40 and a bit of wiggling back and forth got them out. They all looked decent - not brand new, but everything was reasonably clean. No corrosion to speak of. The contacts in the holders were actually fairly shiny. As I said, removing and inserting lamps gave the expected changes in resistance, which suggests that the contacts and lamps were okay. At one point, I measured between the sidelight and brake light terminals in one holder (lamp out, probes in the holder), while the other holder still had the lamp in, and got a low resistance from the two filaments in series. Took the lamp out, and the reading changed to open circuit.

I will fix it - I just decided to stop when I did due to the cold and lack of light, and because I was getting a little frustrated. When you're standing there, saying things like, "this is bollocks - that's ****in' impossible!", it's time for a tactical retreat and a rethink.

Progress, of sorts...

Went out, tried a press on the pedal before going anywhere - no brake lights. Drove round the corner to my lockup, pulled out a lamp to measure resistance directly on the lamp before messing about with a wire to check that the lamp lights up. Got resistance okay (both filaments). Decided to do a quick check on the conenctions in the lamp holder, just in case - ignition on, pedal down, come round to the back to measure, and the other brake light is lit. Put removed lamp into its holder, and it lights as well.

Then I put things back together, drove back round the corner, and had another look at the brake lights - not working again. Back to lockup, quick check before unscrewing stuff, working again. Drove home again, lamps still working.

So, intermittent fault. Methinks the next step is new bullets on all the wires, plus new female bullet-thingies.

link incoming feed to out going green/purple to eliminate brake switch .if still no joy connect feed from battery to green purple to eliminate a loose fuse connection or corroded or bad fuse or holder or connections your volt meter will sense voltage but could be breaking down when load applied to circuit

Tracked it down - dodgy brake light switch. It seemed to have two things wrong with it. Inside, the contact on one side had been arcing - black crappy stuff on both parts. The other thing was the little U-shaped bit that moves up and down was ever so slightly loose between the two contacts (which would have led to the arcing).

Can't get a new one til later in the week, so I scraped some of the crap off and splayed the U-shaped bit to ensure it applies pressure to both contacts. Now works great on my analogue multimeter, and will stick it back in presently.