I’ve slowly come to the conclusion, over decades of BMW ownership, that Bosch has become König der Dunkelheit, overthrowing Lucas as the “King of Darkness”. I’m sure that Behr also shares some credit, but it’s difficult to tell those two apart. Long live the king!
I was inspired to write a page by this post over at Bimmerforums. While helping out a fellow enthusiast, another member suggested clamping a failed main relay’s terminals closed with a spring clamp. That’s a great little slice of advice!
Personally, I use an entirely different method…make a dedicated set of relay socket jumpers for each car. For most Motronics, that’s just three short bits of spare wire, crimped into an “i” and a “V” shape, with a male 1/4″ spade on each end. They stay in the tool kit tray for the next owner/victim. Since experience is such a harsh mistress, she’s taught me to carry a few known-good spare relays & bulbs in the toolbox too.
This phenomenon has been such a common failure with new-to-me project cars that I spent a little time doing a failure analysis. Just like E24 window switches, main relays fail gradually *and* for the same reason. Cause is the same in both cases: the ‘load’ side of the switch carries fairly high wattage without any protection. The electrical contacts are gradually eroded; vaporized by high-wattage spikes. These relays are usually rated for 30 amps @12VDC, almost 400 watts continuous current. That’s a fair amount of juice…
As the relay contacts gradually vaporize over time, resistance increases until the relay can no longer carry the current spike. Ironically, the relay may still test out OK. I used to test them at 12VDC, they’d always work. When I started using a decently sized load, the results are far different. A halogen ‘sealed beam’ headlight is ideal for this purpose, BTW.
Chris Wright & Brucey had a few excellent responses in this thread over at BigCoupe about the subtle differences between several OEM relay styles. There are quite a few variations of resistor- and diode-equipped versions; some were weatherproofed. Many gave superior performance to the plain Bosch units typically installed. I hope they don’t mind me sharing their conclusions, but this info is incredibly thorough and relevant. The habit of posting firsthand knowledge along with part numbers AND pictures is far too rare these days in my humble opinion.