i've been trying to source a problem with my right rear park/tailight lamp on my 90 780 ...sometimes it works sometimes it doesn't. the rest of the cluster works fine as do the rest of the lights, i have tried replacing the bulb but is to no avail. what is the next step, ground? relay? any help would be appreciated

most of the time the bulb is out, but on my drive home last night the indicator on the dash turned off and low and behold the tailight was back on, a mile later the indicator came back on and the tail light hasn't worked since, pls. help this is my daily driver

i haven't the time/weather to check the relay or sensor yet, but i have noticed that the light on the street side of the housing is shared between the tailight function and brake light function in other words, it lights when the parking/talight lamps are on, and when the brake light is triggered. I have noticed that it lights for the brakelight but not for the talight/parking function does this help narrow down the possible suspects????

thanks,

Quite possibly it could be the bulb out relay. My 89 740 had cracks in the solder traces a number of years back so yours is the right vintage for problems. The current actually flows through the solder traces on the relay. If there is a crack in one trace, you may get only one light and a bulb out indication.

You can reflow the solder traces by heating it up with a soldering iron and letting it reform. A replacement relay from FCPgroton is pretty reasonable if you're not handy with a soldering iron.
--
Paul NW Indiana '89 740 Turbo 140,000

thanx to all that have posted , yes i do have a bulb-out indicator on when the tailight is out, i wil post back when i get this thing figured out thanx again for all the help

The Bulb out sensor has segments in series with brake and head lights
It is famous for going open and causing failures.
Pop the cover off it and resolder the bad solder joints.
You can get plenty of info via the FAQ's and search engine

Good luck, Bill

Does the "bulb out light" come on the dash?

It's possible that the bulb out idiot light can be open/dead,
in which case you could have a crapped out bulb out sensor solder joint.
see the FAQs for all the info about that.
The bulb out sensor is in series with the brake/tail light legs,
so if it goes open your brake light goes out.

I don't have specific info for a 780

But you can clean corroded lanp sockets and bulbs
Check/clean the local ground connections to the body.
Use dielectric silicone on the sockets ,bulb bases, and ground connections.

If it uses twist-in bulb holders make sure the spring contacts are tight to
the printed circuit board. (use more dielectric silicone)

If that all fails- get your meter back there and see if you are losing the
Plus 12 volts or the ground leg to help you find the fault.

Bill

Is it only the tail lamps that are giving trouble, or is the stop or turn affected too? There are 3 bulbs (2 tail + 1 side) per side. Do they all go out together? Check for bad connections at the 3-pin Molex connector.

Check the circuit board within the Assembly itself, i found that turning the blulb holder slightly back alittle kept the connection on the my 700;s. i once used tin foil between the bulb holder and connection points to keep it solid.

There is no circuit board on a 780. These lights are quite different than 740 and 760.