I have an 80 450 SL.
Can anyone tell me why my HVAC shuts down when I step on the gas. In every button position except defrost, the blower shuts off, even under light acceleration. Cruising everything works fine.
I thought it might be caused from a leak in the vaccuum reservoir, but I performed a leak test and it holds vaccuum fine.

I'm Lost

Bizzare as this sounds, jiggle the key when this happens. It can be caused by the ignition switch. Do you have a lot of junk on your keychain?

__________________
86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

Hi there,
You have a vacuum leak in the heater controls or air flap diaphragms. on the 76 - 80 ACII system there is a vacuum switch that controls the blower that requires a certain amount of vacuum to operate. The idea was that the heater wouldn't start until the engine water temp rose to a certain point so it wouldn't blow cold air in the winter. The defrost position bypasses the vacuum switch to allow defrost immediately. I am on a business trip, so don't have my factory manual at hand, but what you need to do is start checking the air flap diaphragms for a leak. The ACII system will not tolerate any leaks. You can get one of the CD manuals, look in the ACII system and find some troubleshooting procedures for the system. I have a '79 that had a similar problem, so I speak from experience - you will need a Mity-vac or equivalent to locate the leak, it only takes a very small leak to kill the system! You can also do a search on "ACII" for many threads on this troublesome system, originally designed by Chrysler Corp for their 1970 Imperials.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

No I only have the ignition key, no other keys or key chain.

Thanks

Hi again,
Sorry Strife, this one isn't the ignition switch problem. If it were it would fail in the defrost position as well as any of the other positions. Vacuum switch #29 or #30, (going from memory here) is the one that stops the blower from running if the vacuum is low. All the ACII climate control systems behave like this if there's a leak in a vacuum pod. The leaks can be very difficult to find. Another (slight) possibility is that the one way valve that feeds the AC vacuum system is defective, allowing vacuum to go away when the throttle is pressed. Usually the vacuum reservoir will hold the vacuum in a good system, but if the one-way valve is leaking in the reverse direction it could allow vacuum to go away. The vacuum pushbutton control has very small orifices, on the order of .040 or so, so it won't take much of a leak at all in a vacuum diaphragm to cause a failure. Besides the diaphragms, the vacuum switches themselves can leak and cause this failure. The connections to the servo unit can also leak and cause the failure. In short, ANY vacuum leak will cause this failure, the system must be tight.
There is a digital servo upgrade kit available for when your servo fails that is a good investment if you are going to keep the car. Look at the top of the SL forum main window for their advertisement.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

Thanks I'll look for the leak

I have the cd manual to do the checks, I had no idea the system is so sensitive. I WILL find the leak. Thank you for your help

i have a 75 450 sl , the blower stopped working, saw many things posted on vacuum repair, does anyone have a simple way for a modest mechanic, to check vacuum sources, ie where to start, get the parts etc. for ac and door locks. i have a mighty vac tester
also blower quit working, i am trying to rebuild. does anyone have brush source, rebuild source and or parts source. also need to know how to take out drivers window switch. thanks much willson