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Ross
Ross, Diagnostic and repair technician
Category: Car
Satisfied Customers: 1180
Experience:  Licensed diagnostic and repair technician
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Rear brake lights not lighting , bulbs are new..fuses are fine

Resolved Question:

Rear brake lights not lighting , bulbs are new..fuses are fine . Please advise
Submitted: 2 years ago.
Category: Car
Expert:  Ross replied 2 years ago.
Ross :

Hello and thanks for trusting me to help you today. I am a licensed diagnostic and repair technician, I look forward to working with you.

Customer:

Okay thanks! Well the overhead brake light works above the tail gate but the 2 rear lights don't.

Customer:

Oh! And Ross it's a manual gearbox, Glyn

Ross :

Ok, the fact the center brake light works is a good start as this means the electronics, relays and switches are all OK.

Firstly this is an obvious question, but are the bulbs correct? There are a few bulbs which look identical but have very slightly different contacts at the bottom which breaks the connection when they are twisted into place.

Otherwise, either the power feed is missing or it has a bad earth.

Ross :

Bare with me while i take a look at the wiring diagrams to see where to start checking.

Customer:

Yes the bulbs are correct, as they have been working fine till yesterday evening...One thing I noticed was when examining the contact strips for all the lights (brake, tail and indicators) on one side it was wet , however there was no indication of shorting as would have been expected

Customer:

The power feed block to the tail lights and indicators , which also supply the brake lights is fine as these 2 functions work properly, it's just the brake lights not operating. ..so your idea of a faulty earth seems very plausible

Ross :

Yes a bad earth is probably one of the most common causes. I think ideally your going to need a volt meter or multimeter to get a better idea of what is going on. My wiring diagrams are not very helpful for this model but its a simple circuit. From memory the power is fed to one side then crosses over to the other side, so if both have stopped and you don't have a multimeter you could try to put an earth to the bulb and ground it on the chassis somewhere to see if it works.

Ross :

Also, anything that got wet could have corroded slightly on the connector contacts so be sure to check these are all clean and dry.

Customer:

Yes I did dry all the area in question before trying again

Customer:

I do have a tester here but need advise as to what setting I should use please

Ross :

Sure. Firstly, check to see if there is 12v (14v if engine running) at the bulb holder. wedge the brake pedal down and with the meter set to Volts DC (20) Use the positive probe on the center pin of the bulb holder and touch the negative probe to the outer metal part of the bulb holder you should get voltage if the bulb is working but expect to see nothing. Then find a clean bit of metal in the car to use as an earth and touch the negative probe onto this. If there is voltage now showing on the meter... it is a bad earth.

Ross :

Set the meter to Resistance or continuity (beep test) You will need to pull some trims off to trace the earth but it will be bolted somewhere near by. Check resistance between the outer part of the bulb holder and this earth point. you should either get a Beep if there is continuity or 0.0 ohms Resistance. it may not be exactly 0 ohms but if there is high resistance or the meter reads blank, there is a bad connection or break.

Customer:

Okay can we step back a little...I have a Gunson's pocket meter with digital readout to the left of off is ohms (resistance) to the right is milli volts so, is 20 v setting okay ??

Ross :

20v yes.

Customer:

OK will try will take me 10 ms Hi okay Im getting 12volts lamp holder to ground

Ross :

Is that the lamp holder ground or to a different ground?

Customer:

That's the general ground to all plus positive in the lampholder giving me 12 volts when brake depressed

Ross :

Ok, probe between the ground you used and probe the negative part of the bulb holder on the ohms setting to see if this is a break or high resistance

Customer:

OK there is no reading

Customer:

When done as you described the meter showed no resistance reading

Customer:

Ross are we talking bad earth here do you think? .

Ross :

Yes it will be a bad earth between the bulb holder and your ground point. You can keep bringing the probe closer to the holder to isolate the problem

Customer:

And if it is the case how does one rectify it ?

Ross :

It depends exactly where the bad earth is. keep the probe on the holder negative and probe somewhere on the common ground where you had no resistance, then keep moving it in closer to the point it stops reading, this will isolate the problem.

Ross :

Alternatively you can add a bit of wire in from the bulb holder pin to to a good ground. If that doesn't work... the bulb holder will be the problem.

Customer:

Thanks Ross it's clear there is a bad earth so I guess it's a job for a auto electrician

Ross and other Car Specialists are ready to help you
Expert:  Ross replied 2 years ago.
Our chat has ended, but you can still continue to ask me questions here until you are satisfied with your answer. Come back to this page to view our conversation and any other new information.

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Expert:  Ross replied 2 years ago.
No problem. If you do need further help with this please feel free to ask, Im always here to help. Best of luck getting the problem sorted! thanks. Ross.

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