I've got a 85' 300sd. It's got the climate control system with the lcd readout above the steering column. The person selling it was an honest guy and told me it simply did not work. I did some digging and found that it was a blown fuse. I replaced the fuse and it took about a day and a half for it to blow again.

When i had the fuse in it the car would blow either hot or room temp air out of the defrost vent only. I could turn the blower on or off, but there was no speed control. From what I understand the defrost vent only problem is a vaccum element issue.

When working the climate control knob could make the heat turn on or off, but the LCD seemed more or less stuck on +10 degrees regardless of what i did with the knob.
Can someone explain to me how the dual element vaccum canisters are supposet to work. While i was at pick'n'pull i tried sucking on a few of them and the back part made the lever activate, but what is the front part supposed to do on a good element?

Is this anything anyone has heard about. I've done some searching and couldn't find much about fuses blowing. Do i simply need a new panel. Is there a common place where shorts occur or do i need to pull the panel and start using my multimeter? Thanks in advance.

does anyone know what a good vaccuum can does with the nipple closest to the arm? do you think i need to replace the hvac controls or is there a weak spot or some trouble shooting i can do?

On the dual pods, the rear diaphragm does most of the work and opens or closes the door. The forward diaphragm has a fraction of the travel and "bleeds" air to the outlet.............about 20% of the typical amount when the rear diaphragm is operating.

If you have a failure in the rear diaphragm (won't hold vacuum), it definitely needs to be replaced. If you have a failure in the forward diaphragm, it's a judgment call as to whether you absolutely need the 20% bleed air that it offers.

what is on the fuse? in addition to the climate control that is...
on the older cars, the aux pump would lock and burn up the CC, perhaps this is happening?
which fuse? I have an 85 and I'll look mine over for sources of overload...

Thanks for the tip. I've got a wad of these from pick a part, some with good front diaphragms some with good back ones and some tat are just plain bad.

If it turns out i don't have enough good front diaphragms what are the consequences of simply plugging the lines that go to the front chamber(nearest the arm)?

This is the tip that really helped me. Holy !@#$ there is a lot on that little 8 amp fuse. I'm pretty sure it has something to do with the transmission kickdown switch. The fuse blows when i make the pedal click the switch.

I haven't got to poking around in that switch under the pedal but if the short isn't there does that switch control something somewhere else that could be faulty (pun intended)?

Answered in post #3.

I should have been more specific. What does the 20% bleed air do?

Presumably, the 20% bleed to the windshield might assist in preventing condensation on the inside of the glass..........but, I'm fairly certain you could live without it under most conditions.

If when you engage the kick down switch your blowing fuses, check under the vehicle for the wire that goes to the transmission. On my old 300CD it took me a year to find out that the wire to the transmission was rubbing on the body and had worn thru the insulation and was shorting out every time I floored the throttle.

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Gary
OBK #37