'Morning all.
I recently bought a nice '89 760 turbo where the previous owner had swapped out the old lego block style tail lights (pictured below) for the much more stylish newer design.
Everything worked but unfortunately the bulb failure warning light refused to go out. I have been working on it for some time, changing all the parking light bulbs and even pulling out the bulb failure relay and re-soldering a bunch of pins. All to no avail.
Yesterday, I opened up where old mate had spliced in the new lights and harness and found a huge knot of cheap crimp-on bullet connectors, bad grounds and miles of insulating tape. I pulled it all apart and re-did it all using solder and heat shrink. It all looks much better but the bulb failure warning light was still on.
I just resolved the issue and thought I might share my findings.
Firstly, to install the newer style lights you will have to cut out the vertical sections of the mounting holes. That had already been done on my car, after a fashion. As you an see above, It looks like an axe was used, but rather than fill the boot with grindings, I left it as it was.
On my car, there are four wires that drive the rear parking lights. 2 white / red wires, and 2 white / yellow wires. But on the new harness there are only two wires. The red wire goes to the left hand parking lights (two bulbs, inner and outer) and the Brown/White wire runs the RH two bulbs.
How to hook them up?
If you look carefully at the new bulb holder plate you will notice that the polarity of the inner and outer bulbs is reversed. There is some discussion on-line about this but I do not think it matters in the slightest. At least not with simple incandescent bulbs. It probably just made manufacture and production easier.
My car had been wired with the two White/Red wires running the parking lights, left and right, and the two White/Yellow wires hanging loose and doing nothing. That is where the problem was.
All four of those wires need to be carrying about same current, or load, for the system to be balanced and the bulb failure warning light to be off. Originally I guess, each wire ran its own bulb. ( I do not have the original lights and harness to check.)
All I did was solder one red/white and one yellow/white together and connect it to the left parking lights. Then I did the same with the other pair and connected that to the right hand parking lights. Now all four wires see the same load and the warning light is OUT !! If I pull out one bulb, the warning light does not come on, but If I pull both bulbs out of one side, the warning does come on. Good enough.
I'm not sure which wire is supposed to run which bulb, but I don't think it matters. As long as they all run something. You could probably just solder them all into one big lump and everything would work too. (except the bulb failure warning . )
As a side note, on my 760, tucked into the left hand corner of the boot, is a little box that drives the tail lights. I suspect that this little box is really the brains of the bulb failure warning system . The relay under the dash is really very simple and it did not look clever enough to do the entire job. This might only be on 760s though, I am not sure. My old 740 does not have it.
