On a 617 SD, do yourself a favor and buy a Sanden conversion kit. The GM built R4 is junk. A member here makes them and others are available. I bought mine from Matt at Klimakit. If you do a search, you can find this information from my thread on the conversion. Dealt with three leaking R4s (even new, not reman), never again.

Early SD W126s have a fairly inefficient condenser also, a puny aux fan, larger water pump pulley, and a small main fan. A poor recipe for A/C, let alone here in Texas.

If you want to stay clutch fan, pick up an M116/7 fan with clutch and it will drop right in. BIG air flow difference which is needed for good A/C performance. A good e-fan swap will work. I have installed a Volvo 960 dual speed unit on my SD, I reused the OE Volvo relay, and a BMW 91°C low/99°C high temp switch to trigger it. Simple, cheap, works great.

The water pump pulley being smaller on the late vehicles increases low speed air flow via the engine driven fan and better cooling. No brainer. You will need shorter belts.

Up to late '88 W126s received a much larger aux fan than the early cars, this started around mid '84 I think. My 11/84 has the larger one from the factory. Plenty of good, used out there to buy.

Your condenser will not work well at all with 134A, it was marginal with R12. You and I are both in Texas, so you need to really, really consider a parallel flow condenser to deal with the R134A. I have a slight hole in my condenser and let me tell you, finding a late '84 to '87 Diesel W126 condenser is a big PITA. I gave up trying to find a good used for a good price and am going parallel flow. Matt at Klima sold me the brackets as he is out of kits currently. The condenser is only $60, you will need to build a new liquid line and high side fitting.

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'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

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