Cabin Blower Stopped - Fixed w/ Photos

[BillGrissom]

In my 1985 300D, my son and I had smelled burning electrical and the blower seemed to go too slow sometimes, then finally stopped working. I first swapped the pushbutton unit and TempRegular box, to no avail. I then removed the blower and looked it over, even lubing the bearings as best I could, then tested out of the car and saw it blew strong. I removed the Blower Relay box and opened it, but looked pristine inside. In trouble-shooting, I measured 12.6 V (car off) at pin 12 of the blower relay box and at fuse 8, with one of the push-button connectors off, dropping to 7 V when attached. I finally found that with both connectors off I got 0 V, which means the 12 V I measured was weak and probably back-fed from the electronic controls. I usually measured 12 V on both sides of fuse 8, but I recall the data was erratic, probably from loose connections (later), and the blower did run a few times during testing. I somehow got the bright idea the ignition switch could be the culprit (stupid, since before fuse 8), so swapped it. Since I couldn't get a tool to unscrew the three mounting screws, I dropped the steering column down and removed the key stalk to finally get at the screws, which was very tedious and turned out a waste of time.

Cutting to the chase, I finally decided the problem must be in the wiring so dropped the fuse box down (detach BATT- first!). It is attached with a single nut at the bottom in engine bay. I had to remove two unknown boxes for room. The long one is held by a single bolt you get at by going behind the brake pedal with a 1/4 socket extension. When I got it down and rotated, I found 3 broken wires on the downstream side of fuse 8 (photo 1). There is slight green corrosion, but I suspect from the insulation burning off long ago and condensation. Others have thought fuse 8 connections corrode from water dripping down into the fuses, but I saw no such signs. I had replaced the melted fuse 8 with the blade fuse shown ~7 years ago. Note the downstream wire shows signs of over-heating (photo 2).

After cleaning the copper well (photo 3), I tested continuity with a multimeter. I was surprised to find infinite resistance thru the copper block. The crimped tube on the underside seemed to be the problem. Rotating the square nut back and forth gave continuity. I expect that interface is what causes people's high resistance and over-heating. I fluxed and soldered that joint, as shown in blurry photo 4. Photo 5 shows it back together w/ heat shrink so the circuits don't touch, since that fuse holder wiggles in its melted housing. Blower runs fine now, and I drove today with AC when 99 F outside.

Next task is to add a 40 A relay to power pin 12 of the blower relay. It will actuate with the wire that went to pin 12. I hope to push a 12 V feed wire (w/ fuse) from BATT+, thru the firewall. That will take the major current load off the ignition switch and fuse 8. I have had many U.S. cars where "blower high" melts switches, wires, and fuse boxes. It seems an endemic problem so I add relays when possible. Some may mention the OVP relay below the blower box in my 85 car, but that has another purpose - disconnects power to electronic boxes (smaller loads) if too much voltage on BATT+ or if reversed jumper leads.

Takeaway - clean and solder that crimped connection, or just run the wire from your new fuseholder thru the cavity and connect it (ring terminal) under the rear wires. This assumes your fuse 8 cavity is melted, but is there any W123 300D that hasn't? If not, it will without attention.