1985 300D Heater Re-plumbing - PeachParts Mercedes-Benz Forum

BillGrissom · #1 01-10-2020, 06:11 PM

Some ideas that might help others, especially when we reach a day when you can't buy the proper PN heater hoses. I switched to a later W124 mono-valve, as someone else here did, since I think easier to get an affordable one which actually works (MTC not). An alternate is an electrically operated vacuum switch to a vacuum-controlled water-valve. That was used in 1990's M-B, and a company kits those parts, or you could rig parts from other cars.

Since I long-ago removed the "aux water pump" (unneeded) and thought the firewall cluttered by a Rube-Goldberg plumbing design, I installed the new monovalve at the front, sliding it into the aux-water pump bracket (needs a little thickener, could use large heat-shrink or wrap self-fusing RTV tape, I cut some rubber dust caps). I haven't yet extended the wires, but have the proper connector & pins (P-P, I recall). For now, we get full-heat but no problem in winter and my daughters drive is short.

I used silicone hose everywhere, some 5/8" "heater hose" (blue) and others from a motorcycle re-hose kit (Honda PN shown). I show another PN (blue hoses) I used for other hose things in my 300D's (tee to replace the PCV oil drain in 85 CA cars). One advantage of silicone is it stretches more than regular hose so might be coerced over fittings >5/8" like the heater flow return at the water pump housing. Though no way to fit it over the nipple on the head. Silicone can handle much higher temperatures (~400 F) and should last the life of the car. Other parts were a brass tee (5/8" recall) and a 45 deg copper "street elbow" (solder type). One side is larger OD which fit the larger hose from the rear head nipple (~11/16"D). It also helped route that hose past the oil filter and accelerator linkage. Indeed, it makes pulling off the oil filter housing easier than with the factory hose. Silicone hose doesn't like cheap screw hose clamps (slots cut into it), so I used Euro-type or Breeze smooth type.

Note that the plumbing is complicated since M-B chose a monster Behr heater core which has 2 inlet tubes and 1 outlet tube, as found in semi trucks. They had to use a custom tube w/ tee at the firewall. They also cross-over the flow - engine outlet on L side to heater core's R tube pair. My plumbing is more direct. I flow thru the heater core backwards, but probably doesn't matter for heat transfer or flow drop and air bubbles will likely get swept out eventually. No concerns for me since it rarely gets below freezing here. If you ever must replace your heater core, I would use a 2-tube all-copper one from an old U.S. car since not living in northern Germany, and OE heater cores cost ~$500. The problem is it takes days to change one. The heater core in my 1984 started leaking (fogging windshield), but I fixed it by running silicate block-seal thru it (only heater core in a little flow loop setup - pot on propane stove & aux heater pump circulation). Later, I snaked a hose down inside and blew silcone seal and let dry for days w/ air puffing thru. Hasn't leaked after ~8 years. One here guy found the leak was just from the rubber gasket and just crimped the metal fingers tighter, though that job took him 3 days.

BTW, this is a CA car with frame-mounted air cleaner, though I installed an earlier engine (1982?) when the OE engine failed. Thus, the turbo inlet sits ~2" lower than in the CA engine. I just cut the outlet tube of the air cleaner shorter and used silicone hose duct (ebay, for turbo plumbing). I ran the oil drain from the PCV tube tee over to the normal drain tube (instead of into the turbo as 85 CA engines did). I later did the same on my 1984 300D to eliminate the shaking-air-cleaner issue. I stack 2 cheap Wix air filters rather than pay for the "correct" filter (~$50).

BillGrissom · #2 01-10-2020, 06:13 PM

Last photo. Come-on Peach why limit to 5 photos in the year 2020? This isn't high-res video at 30 fps.