First off, let me say that there is almost never a problem with the flasher unit, column switch, or the wiring itself. So don't make any rash assumptions, the fix is usually simpler than you think!
When the yellow dash indicator does not work or works intermittently that is a sign of either burnt-out or wrong wattage bulbs and/or bad electrical contacts which leads to low current draw. If there is not enough current being drawn by the lamps, the indicator is not activated - this is a design feature so that you will be warned of burned out or dim turn signals. Pretty clever, eh?
Before anything else, check the fuses. The ceramic fuses used in early German cars have a tendency to corrode and essentially "wear out" over time. Inspect and clean or replace the fuses and clean the contacts in the fuse box. Sometimes you have to carefully bend the contacts in the box to tighten things up and ensure good contact.
Next, check that the bulbs are good (obvious, but often neglected). If so, make sure they are well seated in their fixtures, the bulb bases and fixture contacts are clean, and that the grounds to the fixtures are good. When buying replacement bulbs, try to get quality bulbs with nickel plated bases. Cheaper plain brass base bulbs are more prone to corrosion and will cause more trouble. Osram is high quality and OE for BMW. If the bulbs are old and darkened but still work, replace them anyway.
The front turn signal fixtures are a notoriously bad design and are usually the problem. Typically, the area of the fender where they are mounted is rusty, leading to bad grounding. They ground through the screws that hold the fixture to the fender. Very dim front signals and no running lights are a sign of a bad ground (the turn signal filament gets grounded through the running light filament). The signals themselves are also prone to filling with water and corroding internally. If the socket is dead, you can usually wire in a generic one in place of the original... somehow.
It may ultimately be necessary to add a ground wire from the inner frame of the fixture to a clean, solid ground point on the inner fender. Note that if you are testing the system when you have the turn signal assembly unscrewed from the car, nothing will work unless you run a jumper lead to ground (duhhh, been there).
Sometimes, the springy contacts at the bottom of the bulb socket get loose or corroded or bent. Very commonly, the wires to the fixture get reversed so that the low power filament of the dual filament bulb (21W/5W) is used for the turn signal rather than the running lights. If you have very bright running lights and dim turn signals, there's your problem.
In the rear, make sure the indicator bulb is a 21 watt unit and not a 5 watt one. Again, check all the connections. Look carefully at the grounding points in the trunk - you will have to remove the trunk floor panel. On German cars, ground wires are always brown.
If, in the end you determine that all is well and it really is the turn signal flasher relay, then you can use a standard, generic, $5, three-terminal round relay from the local Schucks. I can give more detail if needed but you can carefully free the spade connectors from the plastic connector block of the original flasher, leaving the brown ground one in place (it is not needed for the generic flasher). The stock metal connectors will fit right onto the generic flasher unit - you do not have to cut or splice a single wire; this is completely reversible if done carefully.
Use some electrical tape to insulate the bare connectors. The three terminals look like this:
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from the bottom of the generic flasher, the two parallel ones are the "input" and "output" while the third terminal is for the indicator lamp (if i remember right). The black/white wire (dash indicator) goes to the center third terminal, the green/violet (power in) and green/yellow (to switch/lamps) wires go to the parallel terminals. It's been a while so I may be off on the connections - I originally figured this out by trial and error. With the turn signal switch set to left or right (doesn't matter) and the key on, the yellow/violet wire will have power to it. There are only few combinations, if you get them wrong nothing bad happens - you may have dash indicator but no signals, dash indicator on all the time, nothing, or it works right.
Anyway, there is no need to spend $20 to $40 for a new flasher unit. If you can solder, the original flasher can usually be repaired by carefully opening it up and re-soldering all the connections on the PCB.
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