Hi Dave,

"I should almost start a new thread on this ..."

I agree. It is another "what vendor of replacement parts for 30-year old cars makes good stuff" question, which depends entirely on opinions and current experience, neither of which I can offer help on. I'm thinking maybe I'm the only one besides yourself following this old thread.

"An errant capacitor, resistor or semiconductor perhaps?"

Resistors, semiconductor devices, and even electrolytic capacitors (despite the rep they got from those rushed to the motherboard market 20 years ago) are much more reliable than simple connections. But if it is a brand-related issue, such as a workaround to a Hella patent perhaps, it could be related to components other than connections. My first guess would be the bifilar-wound current sense coils and their magnetic reed switches. Those are the pieces that make for differential current sensitivity.

"I may give it a re-solder just for the heck of it to make myself feel useful."

Yes, have a look at the soldering. That's a variable that comes under the "connections" heading. In any case, pull it apart and see what you can see that makes it different from a Hella unit. I've never seen one made by Kaehler. And today, with lead missing, solder is even more brittle.

"I always used to go with the KAE 240 fuel pump relays, in fact I much preferred them over the Volvo/Striebel (higher amp rating can better handle the increased draw of worn pumps)."

This is, in my opinion a myth. The "amp rating" is about relay contacts, and no Volvo or Stribel relay in my experience ever came close to being insufficiently rated for relay contacts in comparison to even locked-rotor current for the main fuel pump. The failures have been with the circuit board soldering, and the moisture-exposed relay socket faston connectors and crimps. The protecting fuse is rated for less current than the relays. However "amp rating" sure sells to mechanics.

Once you get the problem to be consistent, what you can try is what I should have done first, instead of jumping to the sedan tail lights and their perennial corrosion issues: Bridge the outputs to left and right at the relay pins. For the tail lights, bridge 58L and 58R. For the brake lamps, 54L and 54R. I simply short the pins together with a screwdriver tip, but for the headlights the pins are not adjacent; 56bL and 56bR. Every time I've had a warning for the headlights, a headlamp was dark.

It takes a little coordination, screwdriver and relay in left hand, and right hand to start the car, turn the light switch, poke at the brake pedal, etc., but you will force the currents to be equal and thereby prove which direction you need to investigate.

Caveat -- the nature of European street parking separates the input side of the circuit for the parking lamps, so before convicting the BFWS, make sure that corrosion on the separate fuses (240 at least) is not the trouble with the parking light bulb warning.
--
Art Benstein near Baltimore

It's always darkest before dawn. So if you're going to steal your neighbor's newspaper, that's the time to do it.