Short version: My A/C compressor wasn't engaging when told to by the ACC pushbutton unit, and testing seemed to narrow it down to the Klima relay, although readings from several sensors were also suspect. I took the gamble and swapped in a new relay ($120 from Rusty) and now everything's fine. Yay!
Long version: The new Klima relay unfortunately is sealed with silicone around the bottom. I was hoping to open it up and see if it's any different from the old one internally. I did try resoldering the old one, with no change in behavior (except that I got a headache, despite decent ventilation, from the melting of the epoxy coating or whatever it is that's on so many of these old boards). I don't know enough about electronics to get much out of the broken relay, so I'm willing to send it to anyone (Jim?) who wants to dissect it or attempt a component-level repair. There sure are more components in it than I expected---ICs, diodes, and caps, oh my! I'll even pay the postage in the interest of Science. (But if you get it working and plan to sell it, you have to offer me right of first refusal!) I'm still not happy with the fact that my compressor speed sensor and engine speed sensor were giving weird readings when I went through all the tests of the compressor control circuit in the FSM. I worry that they are drifting out of spec with age, and sooner or later will give readings far enough apart that the Klima will see the mismatch as a locked-up compressor and refuse to trip. The engine speed sensor looks easy to change, so I might do that preventatively. (The hardest part is just running the cable... kind of like putting in a block heater cord on models with the heater preinstalled, but much worse.) And I cleaned the contacts of the compressor sensor---which are right on top of the compressor and hard to see unless you look straight down past the serpentine belt, to the right of the vacuum pump (as you're standing in front of the radiator). They were pretty grimy, presumably because of an old power steering leak. Hint to anyone contemplating doing the same job: unless you have smaller hands than I do, you're not going to be able to reach the terminals directly. But you can easily pull the connector apart from the top (I had to wiggle it and pry at it with a long flatblade screwdriver to get it started), and then work an old toothbrush saturated with contact cleaner in from the side and give things a good scrubbing. Cleaning did make some difference: the resistance across the sensor is a little lower (though still not within the spec in the FSM), about 800 ohms instead of 900. I also cleaned the compressor clutch by squirting most of a can of Brakleen down into the gap, and then checked the gap too as Marshall suggested. It's right about 0.6 mm, so just within spec. (As well as I can tell with a rather old, beat-up set of feeler gauges. I really had to dig to find them. When was the last time I worked on a car with points? Must have been some old girlfriend with a muscle car. Too fast for me!) Marshall also suggested checking the evaporator temp sensor. I know it's working reasonably within spec, because it's read not by the Klima but by the pushbutton ACC unit, which is consistently sending the right signal to the Klima (it's grounding pin 10 in the Klima socket). But if the evap temp sensor is as easy to get at as 83-646 in the 124 manual suggests (right there under the right side of the steering column), and changing it for a new one (or even just adding a resistor) will improve cooling, I might do so anyway. Anyone BTDT? (Dave M.?) Alex Chamberlain '87 300D Turbo '93 Isuzu Trooper
