Odd voltage on brake light circuit

[Bat21]

I am currently in the process of wiring up the new lights on the camper build and have noticed an odd occurrence.
When turning on the ignition there is a 2v feed down the brake light live wire. Is this normal? Seems very odd to me.

 

[StarCruiser]

Do you have a tow bar and electrics?

 

[Bat21]

I did have but these have been removed.
The 2v reading is taken from the loom which has been trimmed back to just behind the passenger seat.

 

[Grimbo]

Try pulling the brake light switch and continuity test it ,
seems to me it is partially sticking ,..... 2 volts won't light a bulb so wouldn't show up in use .
Willing to bet you've got continuity through the switch even when not activated , test with a decent multi meter that can read resistance and don't just rely on a "beep " test .
No reason why the vehicle would be putting 2v down the brake light live ......80's are not clever and the brake light circuit is a simple circuit so no ECU driven voltage about to produce strange readings.

 

[Bat21]

Thanks Grimbo, I'll have a look at that and see if that's where the problem lies.
The brake lights work with the ignition switched off but this 2v feed only appears when the ignition is turned on, so I suspect the switch won't be the culprit here, but will certainly have a look.

 

[uHu]

That voltage would be typical for missing ground for one tail lamp. If one side rear lights cluster is missing ground, the 5 watt tail lamp filament would get it's ground through the brake light filament, and through the cabling over to the other side's brake light and to ground through it.
You say you have cut back the loom... If that means that the tails are disconnected, then that doesn't make sense. One possibility then could be corrosion in a plug on the loom.
Or, if its an auto, the brake wire would be connected to the ecu, for checking that the brakes are applied for allowing it to be taken out of Park position.

 

[Bat21]

The 2v reading is taken with no rear lights fitted.
I discovered this weird anomaly when wiring up a new cluster of rear LED lights. On the left cluster with everything wired up the rear side light comes on slightly when reverse is selected, when the wire to the brake light is disconnected this doesn't happen.
With both left and right clusters wired up this doesn't happen.
Yes it is an auto but an early one with, as far as I know, no ECU.

 

[StarCruiser]

I did have but these have been removed.
The 2v reading is taken from the loom which has been trimmed back to just behind the passenger seat.

Could be water or corrosion on the brake light switch. Try disconnecting at the switch and see if that changes anything. You can also try pulling fuses until it goes. Pull each one and leave it out then when it disappears leave the last one out and start refitting the fuses checking as you go. If another fuse brings it back, leave that out and continue. That will help narrow it down as to where it’s coming from.

 

[Bat21]

An update.
When turning on the ignition there is a 2v feed down the brake light circuit.
When selecting D, 3, 2, or 1 there is a 10v feed down the same wire.
Pulling the fuses, it seems the one marked ECU-IG (15a) stops this happening.
Anyone know what that fuse does? A quick Google doesn't bring up anything obvious.

 

[uHu]

Measuring voltages with a high resistance multimeter on an open circuit can be deceiving. Looks like its the ECU sensing if the brakes are applied. The circuit will not work right until you have the brake light bulbs in, giving the ECU zero volts through the bulbs, until the brake pedal is applied.

 

[Bat21]

Measuring voltages with a high resistance multimeter on an open circuit can be deceiving. Looks like its the ECU sensing if the brakes are applied. The circuit will not work right until you have the brake light bulbs in, giving the ECU zero volts through the bulbs, until the brake pedal is applied.

Okay thanks that makes sense, just
I will wire in both sets of new lights and take a reading then. They are LED units so not sure if that will make a difference.

 

[StarCruiser]

Uhu may have hit the nail on the head here. If your truck has cruise control, the cc ecu looks at the brake light circuit for a load. You may need to fit ballast resistors to your LED rear lights to give a suitable load and stop them from glowing on the 2v sensing.