View Single Post
  #22  
Old 10-28-2014, 01:19 PM
oldtrucker's Avatar
oldtrucker oldtrucker is offline
BMW Mech (70's) Germany
 
Join Date: Jan 2014
Location: San Diego County
Posts: 131
Dunno how good you are with the soldering iron?
You see the large contact posts. They are riveted to another electrical carrier e.g. metal strip connecting to the PCB. This method for making contact was used in the very early double sided PCB's and you guessed right, wasn't durable. I guess to charge the almighty rich Mercedes Owner (Stealership price $125/open range ~$65.00) for the relay/monitor, they continued this crappy way of connecting high current traces. After 20 years of service, dust, heat , cold, moisture, a little bit of corrosion builds up at the rivet site, adding additional resistance. Perhaps enough to throw all the sensitive circuitry out of wack. I had my in dash defective bulb, go on because someone decided to put a 10W bulb into the third brake light.

The way they check the bulbs, they simply split certain circuits into groups. The known is the total consumption of power (Wattage) all light bulb watts added together total power consumption. Now they monitor the current and voltage to a predefned value. If that value is not present it triggers the in dash. Down side, you'll never know which one!!!

Back to the rivets. If you are good with a soldering Iron, you can desolder the large contact posts, so that you can get to the rivets. Once the path is clear, put some solder to the rivet side of the contact and reflow it until it looks all covered without cold solder spots (nice and shiny). On the larger posts you'll probably have to heat a little longer (cold solder), but not too long, don't want to melt the plastic carrier.
Do this for all contact posts and if all goes well it will work again as supposed to.

Blowing the fuse, is probably related to a short after the oven excursion. Normally solder with a flux core has melting point, starting commonly at 650F or less, however there are plenty of different types on the market and each has a different melting point and characteristics. Chances are, something got soft and either came lose from a solder pad or shorted out. We tend to drop hot things.
__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert
Reply With Quote