Heater Core Retrofit 300CD
 

This is more of a FYI than a detailed how-to.

My 1978 300CD which is nearing completion of my first pass of repairs (floor/rocker rust repair, interior cleaning, etc). On my second drive around the neighborhood since finishing the major body repairs, I noticed a bit of water on the floorboards. Investigation showed it to be antifreeze and coming from the heater box. Since I had the seats and console out, I decided to tear apart the dash and find the leak. It was tough to spot, and I almost put the old heater core back in after no leak was evident with 30psi of water. While changing the o-rings, I noticed a crack in the header opening, beneath the flange. By the way, you can split the heater box from the evaporator. There are 4 clips, a few screws, and the dash cover must be out, despite what the service manual says.

Since I was coming up empty on replacement parts (this forum confirmed that w123 heater cores were NLA), and an untested used one was 250+shipping, I decided to see if I could find something close.

The w124 heater core dimensions were online, but measuring the housing it looked to be too wide. Too bad since the piping looked identical with all three openings at the top. After searching all the online aftermarket catalogs, I came up with Carquest Heater Core #398336. It was a little more narrow, a couple inches shorter, but most importantly didn't have funky angles on the inlet and outlet pipes. It was on the shelf at all the Advance Autoparts stores in my area. I believe this goes to a late 90's Ford Taurus or Mercury Sable (not 100% of the top of my head).

To work around the stepped heater housing top as much as possible, I flipped the core and shifted the pipes to one side. I originally planned on filling the bottom with expanding foam, but found a 3"OD piece of pipe insulation fit perfectly. The top gap and side gap were filled with open cell foam blocks from packages I've saved over the years.

I thought the plumbing would be the easy part, but turned out to be the hardest. I wound up cutting off the ends of the original aluminum piping since it was the correct size and length to fit through the firewall penetrations and above the evaporator core box. I only used one inlet pipe; the straight one. The outlet pipe (larger) I cut out all but the very last straight section. To make the route possible, I used two Gates 180degree molded hoses (PN 18777) and a couple 90 degree heater hose fittings. I also transitioned from 5/8 to 3/4 after the 180 bend on the outlet side. Since the OEM pipes were a tad small (metric), I used silicone tape to take up a little room, and double hose clamped it. I used NAPA worm gear hose clamps and tightened them very well. The no-name ones from the other stores did not hold up to my torque requirements.

So far, no leaks. The heater is working as long as I unplug the Unwired Tools AAC heater valve. There are some issues that I just resolved with the thumbwheel and temp sensors, so next time I drive it I expect that issue to be solved.

Anyway, I hope this saves someone a couple hours of research if they decide to go this route. With the heater core no longer available from MB or aftermarket, I probably won't be the only one.

New w123 heater cores
 

I have 2, in the box new, NOS, W123 heater cores.
Mercedes part number: 002 835 30 01
Let me know if you need one. Pictures available.
Let's make a deal!

I'm in. Send it down. Check your PMs for details

W123 Heater Core
 

Jolly good ! .

My'78 300CD was allowed to sit with plain water in it for several years, I had to replace almost everything in the cooling system and then the damn heater core began dripping,

I didn't want to yank the dash apart so I added one tube of "Copper Seal" leak stop and a non pressure radiator cap from a 1949 Chevrolet and headed to Death Valley in July, AC blasting the whole way there and back .

The car never got hotted than 100 degrees C and didn't use -any- coolant so I called it good .

Nice to see someone still has some NOS heater cores, I bet I have one salted away who knows where .

Old post but great writeup! I am juggling this retrofit as an option for my 300CD as well, which you are correct in thinking this info will benefit future owners!

Saw in your photos some sort of rust-prevention coating on the floorboards? What product was used there?