116 climate control diagnostics

[kerry]

1979 SD with evil servo. Last year I was having problems with the blower not working. Did a lot of testing, even replaced the amp but finally replaced the blower and all was fine. A similar problem has resurfaced.
Last year when the problems were occurring it was blowing the Air Conditioning fuse. (at the time I mistakenly thought it was blowing the blower fuse). Air conditioning fuse blew about 3 times recently, I replaced it each time and blower returned. Last time I put in a heavier fuse. That fuse is holding but now the blower does not work. It's not getting 12 volts. It will run if I deliver 12 volts directly to it from the battery.
What would stop the delivery of 12 volts to the blower and be somehow related to the blowing of the fuse for the whole climate control system?

[kerry]

I'll answer part of my own question. It's partly a vacuum problem. If I either jump the connections electrically or pull a vacuum on the main vacuum control sensor in the climate control system (green vacuum electrical sensor behind the glove compartment) the blower works. The vacuum line to that sensor is blue.
It doesn't explain the fuse blowing issue but it does account for part of the problem. I think it rules out a bad amp or a bad blower.
At this point I don't know if this is caused by low vacuum in the whole system or not. I discovered yesterday that my shut off diaphragm has a hole in it. Don't know if that resulted in oil getting sucked into the whole system and damaging other parts.

[kerry]

That solution was simple. The W116 has double vacuum lines from the pump. One to the brakes, the other to everything else. The 'everything else' line had come loose from the rubber fitting at the pump. I had noticed in the past that the line seemed somewhat short and was under tension. It just pulled out. I lengthened it by a couple of inches and voila the heater blower now works.

Interestingly, this must also have been causing the shut off problem. I'm betting even with the oil leak in the diaphragm, it was still shutting off with the 'everthing else' line connected and this complete disconnection of the the 'everything else' line is what was stopping it from shutting off. So at least I was able to discover the leak in the shut off diaphragm which I might not have found had I discovered the disconnected line first.


Edit: I'll revise my analysis slightly. I think the failed shut off diaphragm was causing a slow shutdown which was shaking the engine. The shaking of the engine pulled out the 'everthing else' vacuum line which was only in the rubber fitting about 1/4". So all the problems can be traced back to the failed shut off diaphragm.

Edit 2: I hooked up the leaking shut off diaphragm and it does shut off the engine, confirming my general idea. Another lesson learned is that if I had tested the whole secondary vacuum system upon learning of the shut off problem, instead of testing just the shut off valve, I would have found the leak and solved the shut off/blower problem in one fell swoop. However, I would not have found the oil leaking into the shut off valve. I was misled into testing the shut off valve because my mind was thinking in terms of a single line vacuum pump in which total loss of vacuum will show up quickly in the brakes. In the case of the W116 dual line pump, you can lose all vacuum in the secondary systems and the brakes work fine off the other line.

[Whiskeydan]

Every problem I've found with the 'evil servo' system was vacuum related.
Still a nightmare...

We have a 116 SD here that I plan on hacking a manual system in. I don't want to have to rely on exotic replacement items again. Totally manual and NOT for the MB purist.

I wish you luck.