W639 Vito Tail Light intermittent flashing

[Sherringbones]

My 2005 Vito 115 has developed a fault in the right rear unit which has me baffled and means I cant renew MOT until rectified.

When I turn the lights on the right tail light bulb flashes briefly then goes out. Then it will flash again a few seconds later and again for another 4 or 5 times with an increasing time gap between each flash - up to around 15 seconds between the last ones. After that the light stays off.
All other lights work as they should including the right brake light.

I have changed bulbs and swapped them from one side to the other to no avail. I have swapped the entire light cluster unit and the fault stays on the right side suggesting it is not the bulb holder unit.

I have checked the terminals at the connector open circuit and I get battery voltage when the lights are turned on.

I have wired a bulb directly to the relevant +ve tail light terminal (bypassing the tail light housing unit entirely) and the fault remains, however, when I trigger the brake filament from the tail light power supply it lights without flashing. When I try any other single filament 21W bulb (using the tail light 12V supply) they flash in the same manner as my tail light.

I have seen reference to electrical mods causing to the SAM unit; I installed a rear camera some months before the fault arose; it is powered and triggered from the reversing light?

I have pulled the SAM unit out and it appears clean and dry with no signs of corrosion.


I am at a loss as to what to do next;any suggestions would be greatly appreciated!

 

[Trickythemerc]

Sounds like a faulty SAM unit, had a similar fault on my JAG and that was the rear GEM/SAM module gone chips.

 

[Ted]

Are you saying that when you put the brake light filament (no 380 bulb) in the tail light circuit, it illuminates, but if you put another 21 watt bulb, it flashes as described?

It certainly sounds like a faulty SAM. The reason for the flashing is that it is applying a voltage to the circuit, and measuring the voltage drop across a resistor. If there is no voltage drop, it assumes the bulb is blown and turns the voltage off. It does this several times to be 100% sure.
Strange thing is that the circuit itself is good as the light is illuminating - so can't be the resistor as this is in series with the circuit.
Silly question, you're not using led 'bulbs' are you?

 

[Sherringbones]

Ted
No - no running LEDs

I ran wires from the ground and tail light pins on the connector, wrapped the ground wire around the outside of the 5W/21W bulb and and then touched each of the terminals in turn. The brake light filament lit up continuously but the tail light filament flashed.
When I tried a similar experiment with other bulbs they all flashed.

Thinking about it - it should be the 5W filament for the taillight and 21W for the brake shouldnt it?

 

[Ted]

Yes, it should.
Strange that the 21w filament in the side/tail doesn't flash but others do.
May be worth trying a good earth on this.

 

[wu56Shoozz]

Try changing the High Level brake light for new.. I went through this for a lengthy period blowing lights, funky flashing etc.. I put it down to the towbar wiring that the previous owner had carried out and was a mess.. got two new looms for each lamp and spliced them in/hard wired.. still playing around, Sparkydude on here said his money was on the high level brake light.. bit the bullet and bough a brand new one £56 notes lighter and ..FAULT GONE! Its been months now and no problems.. P.S Don't Buy Sh1te bulbs either they also cause problems, van doesn't like them either...Buy branded only.

 

[Sherringbones]

Problem Solved - although I think I may have treated the symptom and not the disease!

I tried putting new Bosch bulbs in, instead of the standard Halford brand ones that I have previously and the problem has gone. I had seen the suggestion to replace bulbs with good quality ones on another forum but I have swapped in new bulbs on a number of occasions previously to no avail and in each case the bulb worked on the left and flashed on the right so I had more or less excluded bulbs from the list of suspects.

I did notice that the Bosch bulb turned more in the holder. Perhaps the other bulbs werent located properly so the 21W brake light filament was connected through to the tail light circuit!!?? Weird though that all of the bulbs had the same defect?

And surely if it were simply a bulb positioning issue the fault would have changed sides when I swapped over the left and right light clusters.

So I am suspicious of the success (I had to go back out to re-check before sending this just to satisfy myself) but will get the van through its MOT before I let my curiosity get the better of me and go in search of the real reason.

Thanks for all the help