A/C part ID and how it works (78 300D)
 

Hi All,

I have been researching how to install A/C in my 1971 250 (w114). So I have been trolling around junk yards looking for late model MBZ's with all the necessary A/C parts to see how it's set up (attempting to reverse engineer).

I came across a 1978 300D. Everything seemed very straight forward (as I dissected it), but when I followed the A/C hose from the receiver back through the firewall, then under the passenger dash I saw this (see pic).

What is this, how does it work?

One of the most critical parts of an A/C system, the expansion valve. Without it, pressure would flow straight from the compressor through the whole system uninhibited. No pressure difference, no cooling! The pressure valve is what keeps the evaporator much lower in pressure relative to the condenser and as a result much cooler. You can read up on how the refrigerant boils and condenses due to the pressure differences to allow it to absorb external heat (from in the car) and then shed it in the condenser (in front of the radiator).

Thermal expansion valve - Wikipedia, the free encyclopedia
Air conditioning - Wikipedia, the free encyclopedia

Thanks Tomguy,

Follow-up: this expansion valve hooks up to the evaporator I believe. I was googling for images of a 1971 250 "evaporator", but can't get any hits?

Does the 250 have one (saw a bunch of stuff about replacing leaky evaporators in other models, but strangely nothing about my 250)?

Does anyone have pics of it?

This is what the evaporator assembly looks like for all 114/115 vehicles--you can send me an e-mail and I'll forward high-resolution photos



Tim Kraakevik
kraakevik@voyager.net

The in-car heat/AC systems and firewall-dash layouts are competely different between the W114 and W123. To fit a W123 AC in your W114, you would need to remove your entire heating-ventilation box. But then the W123 plumbing, fresh-air intakes and control setups wouldn't be compatible.

I suggest you get all you need from a W114 or W115 parts-car. Your W114, without AC, has a cover below the existing heater/air-distribution box. You simply unclip and remove that cover, then fasten the AC evaporator-blower assembly in it's place, leaving your existing heater-ventilating system mostly intact.
You would need to add the Heat-AC change-over control in the center-console, just below your existing heat-defrost panel. It's a horizontal sliding lever on early cars, or a rotary knob with green indicator-light and vacuum control on later (post '69 I think) models. And of course, all the added wiring, plumbing etc...

Happy Motoring, Mark

Thanks kraakevik, thanks Mark, thanks again Tomguy!

Still searching for a w114/w115 w/ all the goods (east coast junkyards are not as fruitful as Texas, Arizona, California areas- or dryer areas of the world). Going to hopefully canvas all yards from Baltimore to DC this weekend. Wish me luck!

Jub

I've got everything you need except for the compressor and the refrigerant lines between the various components--if that's of interest send me an a-mail or private message--thanks



Tim Kraakevik
kraakevik@voyager.net