I was driving the car earlier with a/c on, all was well. I parked it for about 45 minutes then got in, started it up, and even though the hvac setting was set to off, fairly hot heat began to blow out of the vents.

At first I thought it was just a bit of accumulation of hot air in the vents but it wasn't. It just kept coming.

I pushed the 'off' button again to make sure it was fully depressed but.... still more heat came out. I put it in a/c mode and it was heat.

This went on for about 15 minutes until I noticed the middle fan speed button was ejected too far out so maybe the low and middle button were in the on position at the same time. I fully depressed the middle fan button and shortly thereafter all was well. But I think it'd be weird for a fan speed button to make the hvac put out heat instead of ambient air or a/c air.

Any thoughts on this?

1991 300d, 222k

Having the fan speed switch is a non-registered position puts the controller in "random" mode essentially. The fan tends to run on high speed and the monovalve will be de-energized (full heat). One of the many quirks of the ACCIII system.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I think that middle button comes too far out because I previously had to take all the buttons off once to get something fixed. It never got re-seated back properly and even though I press it in firmly, occasionally it pops back out again too far when I press a different speed.

Is there any way to get that button back into its proper place?

Thanks.

Give it a firm poke and see if it re-seats on the peg. If it's seated, but still comes out too far, the switch may be broken internally. It happens, it's also easy to hit the buttons lightly enough with your knuckle when grabbing the gear shifter in Park to disengage all the fan speeds, but not actually engage any single mode (I've done it more than I want to admit, then wondered why I was cooking!)

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Most of the time it's seated properly but occasionally it hyper extends outward if I go from the mid button (when pushed in) to another button (when the mid has to come back out). Instead of popping back up just flush where it should be with the high button, it hyper extends out too far.

Years ago that happened to me. Turns out that the tab on the inside of the button had broken. Replaced it and have not had another problem.

This default to heat might by design. Every vacuum operated system regardless of brand defaults to heat / defrost with loss of vacuum in order to keep the windshield clear.

With this particular electronic system, putting the fan sw into an undefined position apparently signals the controller that something has failed and reverts to a safe mode by dropping all outputs.

As a side note, something I do to all cars without a heater valve is to add a manual one. The difference in cabin temps is huge once you shut off that hot thing under the dash. My 97 SL320 and 97 C280 also got a manual valve, this keeps hot water out of the core when the car is shut off after a drive.

Get a 3 / 8 " pipe ball valve and a piece of 3/8" x 6" pipe. Cut the pipe in half and insert the valve in the heater supply line. At least on my 97 MB cars, the factory valve is on the heater core outlet not the inlet like most cars so be sure to follow water flow.

3/8" pipe is slightly larger than 5/8" ( 15.8 mm ) and slightly smaller than 3/4 ( 19mm ) making it a pretty good one size fits all.

Did you just yank it out when it hyperextended and pop in a new one without having to take the whole climate control thing out of the dash?

Yes, it just pops out and pops back in.

Sl320, can you explain the thinking of this more? I was under the impression that the monovalve kept it all closed until it it was called to action.

Anything to help cool the cabin of my 380sl more than it already does!

The mono valve needs power to close.