1995 S500

Car sometimes takes a mental "time out" where Climate Control display freezes. Blower motor goes to full HI and temp goes to full HEAT. Display controls are nonresponsive. This can last 5 minutes or all day. When it decides to "fix itself" it simply strats working properly.

While it is "on the fritz", if I turn on the parking lights (or headlights), the display will go completely blank (digital temp readout will go blank). When headlights are on, the red "on" LEDs on the hvac display will come on. Still remains unresponsive and hot air blasts out violently.

When it is acting up, I can remove fuse 15 and the blower motor quits. Still passes hot air thru the vents, but it is not force fed. When the fuse is out, if I turn on the headlights (or parking lights) the blower fires up and blasts me again.

The first few times this occurred, I could disconnect the battery overnight and it would correct itself for maybe a day. Now disconnecting the battery doesn't necessarily help.

Am I looking at a component failure? A relay? A sensor? A wiring harness? Demonic possession?

I know the dealer will throw high dollar parts at it till it works. Not really what I want to happen.

Ideas?
(Oh, no way to run diagnostics on the HVAC when it is on the fritz. Everything unresponsive).

My 300D and my 300SDL both had to have the proportioning valves replaced. The diaphragms would tear and as such they worked awhile, but as the tear grew control went and all you get is hot air. Was a 35 dollar fix on mine.

Ric

No, I must not be explaining myself properly.
This is not an actuator/valve/linkage/mechanical problem.

The whole darned HVAC control display "drops out" of the loop. It either freezes its display (the temp readings on the LCDs remain on) OR the whole display goes blank - and during that time the switches and buttons are NONFUNCTIONAL/ won't respond. During this nonresponsive time, the blower pegs itself on high, air comes out of ALL of the vents up/down/floor, etc. and it HOT.

To illustrate it even further, while it is in its nonresponsive state, turning the headlights on and off will make the red LEDs in the "auto" pushbuttons come on and off with the headlights.

Removal of fuse 15 will stop the blower motor, unless you turn on the headlights in which case the blower motor fires back up.

NOT AN ACTUATOR problem. Not tied into the vacuum system. Not part of a thermostat or temp sensor issue. The CONTROL unit is either losing input voltage or is buggy inside.

Was looking for somebody who had some experience or had known about a similar issue so I can know whether there is troubleshooting I can perform before replacing an expensive box or two.

Anybody?

"Blows Hot Air all the time - display blank"

I think I know her sister.

Len

Could it be the display face itself? I don't have a 140 so I am not sure about this but can the control unit "face" be replaced? I know in my car if it does nothing but blow hot air it means the control unit itself is bad but on your's with the display maybe it is the display itself causing the problem. Just a thought.

dunno if the face can be replaced.

I still find it odd that the blower will turn on and off with the headlights if fuse 15 is removed...

I'm just looking for tips and hints.

Anybody else?

McC,

1. I had similar problems with my '87 19016v. One of the techs on this site said the hot-air-all-the-time was a safety default so you would still be warm with a dry windshield if the unit failed.

2. After lots of research, i couldn't locate any troubleshooting for the unit. However, programma offers bench testing, core fees and have the best prices around. The "cheapest" way might be to take the box out yourself and send it to them for testing/possible replacement. Thats what happened to mine, it cost me 200 bucks and came with a 3 year warrentee.

cheers,
dan

dan

thanks. I'm in touch with ProgRama now.

best of luck, let us know how it turns out.


cheers,
dan