AC Fix 1983 300D - PeachParts Mercedes-Benz Forum

BillGrissom · #1 06-29-2018, 12:29 PM

I fixed the AC in my 1984 300D. May not apply to your problem, but at least explains the compressor controls. In old GM's, a failed anti-icing switch is a common problem, which was my issue.

No AC. Compressor clutch not turning. Pulled relay (driver's inner fender, front relay) & shorted terminals 30 to 87. Clutch then engaged.

If clutch engages, the low-pressure switch (on filter-drier) is satisfied since it completes the low-side to gnd of the clutch drive circuit if >14 psig refrigerant. The manual states it is only low-pressure cut-off, as does one brand on rock. I'll look for a low & high cut-off version, to switch off if >250 psig. I used the 1983 manual, p. 307 "Compressor Clutch Control" schematic. For those w/ a 1985 300D, the Klima switch replaces the basic relay and is more complicated (rpm input, turbo boost, ... read posts).

The AC-clutch control logic is simple. The relay coil is powered via the push-button box. Any button but "off" and "EC" (economy) gives 12 V power. That feeds the Evaporator Temperature Regulator (ETR) switch which senses the outside evaporator temperature ("anti-icing switch" in U.S.). The ETR was my problem. Remove the passenger's under-dash cover to find it screwed on the side of the climate box. Jumpering the 2 wires to it (blk/red & blk/red/grn) turned the clutch on. Indeed, I might have just left it jumpered since ice build-up isn't a big problem in the dry CA Central Valley. I removed and tested in a cup of ice. It was always open. There is a screw adjustment behind the plastic side cover. I could adjust to turn on at room temperature, or even ice temperature, but it didn't respond to temperature, which suggests a leak in the "gas thermometer". PN's are given below, but none were found in a search:

RANCO T-2 6(6)A, 250V AC
0028210351
A55-H0146
18-84-2L
gas thermometer: AA104D4

The M-B PN is 003-820-24-10. You can buy the Behr brand here on P-P for $40.50. But photos show a different layout, so wasn't sure a direct bolt-up.

Since anti-ice switches are common, I checked U.S. cars. One for classic Mustangs looks similar, but costs $50. I bought one for a 78-79 Monte-Carlo (CH 28932) on ebay for $19 w/ free shipping. The box shows GPD PN 1711243 (replaces 35720A). It is shown below next to the OE. I worried if the end bulb would fit thru the metal tube "director" on my car, but slid in w/ a little persuasion. The bolt pattern is slightly closer, so I started a new hole in the side of the plastic climate box w/ a drywall screw (just slightly or risk puncturing evaporator, better than drilling). The felt blanket mostly fit and I secured w/ self-fusing silicone tape. Not sure the blanket is essential since the sensing tube is outside it. I think it just limits condensation drips.

The schematic suggests the AC compressor runs all the time until the anti-ice switch (ETR) opens. Cabin temperature is controlled by running proportionally more water thru the heater core. Seems if the ETR fails closed, refrigerant pressure could build up until the AC compressor stalls and smokes the clutch (or blows a seal). That is why a high-pressure cut-off (like many cars) seems a wise revision. The clutch in my 1985 300D smoked the R4 clutch on a hot day years ago. "Hot day" here means >110 F. We normally have at least 10 days above 100 F each year.

BillGrissom · #2 07-13-2018, 10:14 AM

Follow-up re the AC pressure switch.
A little reading found the switch in my 1984 & 85 300D cars is simply a low-pressure cut-off switch. I found that many 1976 - 2011 M-B (ex. 84-93 190E) have a binary switch which also gives a high-pressure cut-off. Apparently the same 3/8-24 thread w/ corner O-ring seal, and same male spade connectors (56 terminal). But, P-P lists 9.5 mm thread for the OE switch (which is 3/8"). Switches off at <2.1 bar or >24 bar (30.5 & 348 psig). I would prefer a lower HP cut-off ~280 psig, but hopefully that will still protect the clutch from slipping. PN's are Behr Hella 351022011 or M-B 124-820-8310. I bought 2 (~$10) and will install in my cars on a very cold day this winter, to avoid losing much refrigerant.

BTW, there are also "trinary" pressure switches. These have a 3rd terminal for the electric fan. It shorts to ground to turn on the fan relay at higher pressure. But that would require wiring changes. In a similar vein, Rollguy showed how to rewire your radiator fan relay so it turns on with the AC clutch. The downside being that the fan would run even at highway speeds when it isn't needed.