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.

[funola]

Bill, nice pics and attempt to fix. I am wondering if it will be good enough long term? Surely cleaning and soldering upside down was challenging. The original terminal block connections were crimped? and soldered and held on with screws and they failed. Do you think the solder repair you made will hold up? From your pics, it appears there was a lot of localized heat which caused the soldered connections to melt and also the insulation to melt. The same thing could happen to your solder repair. I think bypassing the fuse box with a new relay is the way to go. The relay inside a bad glow plug relay is perfect for the task.

I plan on adding an electronic blower control to get infinite blower speeds. I find the 2 speeds blower on the ACC III too noisy.

[BillGrissom]

I didn't solder upside down, I pulled the fuse block down and rotated the bottom up. Even then, solder dripped and burned a finger. Solder seems to have gotten into the round crimped joint well (photo 4, sorry for the blur), but a 40 A separate relay near the "blower relay box" (my term) is the long-term solution. I will use a standard U.S. "auto relay" (flat blades) in a holder, since they work for a radiator fan. As I understand, crimped connection are claimed as good as a solder joint, if very tight and "oxygen free". My guess is that round crimp (top side of square block, as installed) is more of a "tube swage" and probably doesn't have enough contact area to keep oxygen out. By the time M-B noticed the corrosion problem, there were probably 8 yrs of 300D's in the field, plus out of warranty so "pech, Kustomer".

[Diesel911]

Crimped connectors can indeed turn bad over time.

Pics of 2 crimped connectors on My Blower Moter. One good and one bad. Soldering over the Bad crimped Connector fixed the Blower Motor.

Also notice that in the pic if the bad connection that the Copper coil is partly over heated as well as the Brush wire while the one with the good connection looks new.

[BillGrissom]

Melted connections is a common problem when dealing with high currents. I fixed my 10 yr old AC in Atlanta after finally finding one spade terminal on the compressor (outdoors) had melted away. All it takes is a little corrosion and the heat generated at the terminal goes way up (P = i^2 * R). I soldered the wire to the stub left coming out of the compressor, then cleaned the other terminal and coated w/ di-electric grease (as factory should have done).

[BillGrissom]

Got around to adding the relay mentioned above to power the blower relay box (officially "Electronic Unit for Blower Speed Control") in my 1985 300D. This takes the major current load off the "ignition switch" and fuse #8, plus makes a much shorter path from battery to box. The terminals on the later usually over-heat and melt the plastic holder. I found that came from excessive resistance in the terminal itself (post #1). I fixed, but still wanted a relay for a "belt & suspenders" fix. I used a standard "30 or 40 A automotive relay" which you can buy at Radio Shack, with plug-in base (qty 10 from ebay years ago). The flat spade connectors are "56 terminal" or "Packard 56", used on old GM & Chrysler cars and appliances.

I expected the job would take about an hour, but overly optimistic since electrical mod's take time to collect tools & materials and strategize. Probably 3 hours. Following my plan could save time. This mod is definitely optional, but I have done similar in my classic cars since they also melt switches and wiring from the high blower current.

Photo 1 shows the blower relay box. Its job is to switch in the under-hood resistors (behind passenger headlight). If you pop open the box, you will find a board with many relays and a few chips (digital I think). One simple way to check your blower is to jumper pin 12 (+12 V power) to pin 6 (to blower), as shown. That should give max blower if pin 12 has power (use multimeter). It gets power with key in "run" and any climate push-button but "off". My problem was no power after the fuse box.

The relay I added intercepts the pin 12 wire (red/yell, 14 awg) to actuate the relay coil (term 86). Relay output (term 87, 14 awg) goes to pin 12. Relay coil- (term 85) goes to ground (stud at bottom of dash by passenger's right knee). The relay is powered (term 30) from BATT+. It was simple to push a 14 awg wire thru the firewall grommet beside the battery and pull out under the dash. I used PVC-105 sheathing to protect it.

Photo 2 shows the rewiring of pin 12. Immediately after popping off the connector cover, secure the wires w/ rubber bands or you will fuss getting them back in the correct holes (how would I know?). I removed the factory wire from the terminal using a gun-type soldering iron, cleaned the hole w/ solder wick and soldered the relay output wire in the terminal hole. The yellow wire is soldered/heat-shrinked to the factory red/yell wire and goes to relay coil+. I sheathed all wires w/ PVC-105. After the photo, I secured the wires w/ nylon ties. I secured the relay by stuffing behind the thick wire harnesses.

Photo 3 shows the under-hood power tap. The new wire to the relay is the red wire at the bottom. I will buy an in-line fuseholder (blade type) to wire to BATT+ (big screws in terminal block). For testing, I used a jumper wire.

If, like me, you use the pdf document for 1983 300D climate control (pages 302-312) often posted in this forum, note typos on page 303:
pin 12 "power" should be "2.5 RD/YL" wire (14 awg)
pin 6 should be "2.5" mm^2 wire (14 awg)
pin "8" label should be on the wire under pin 9.


Sundry comments on my photo 3:

Grey box at top is "Battery Brain", which I have in all my cars. It trips off if the battery voltage gets too low (not while in "run"), hopefully leaving enough charge to start the car. I prefer the model with remote off.

The round connector at lower right goes to the alternator's "boot me up" terminal (D+). Several dash lamps (alt, brake pad, fuel low) normally provide the necessary current. In a pinch, you can apply +12 V thru a 1000 ohm resistor (I carry one) to bootup the alternator. That connector w/ the junction block allows access all alternator I/O wires. Not blower-related, but the question often arises, so I mention since the photo shows the connector.