Climate Control Unit Dead End - W124
 

1992 W124 (400E) A/C setting blows hot air from the side vents while getting cold air from center vent.

I've tested all the A/C system sensors and components. I've performed all 21 tests in the manual. Everything tests OK - except the monovalve remains open (i.e. 0v = no ground = open = water path to heat exchanger = full heat = hot air from the side vents).

I measured 12V to body ground at the monovalve connector (power to monovalve is OK), 0 ohms between the monovalve connector and LHS pin 9 on the control unit (monovalve ground wire is OK), grounded pin 9 with A/C on to force-close the monovalve and got cold air from the side vents as expected (monovalve is working OK), put 12v directly to the monovalve itself (again, monovalve solenoid engages, OK). But when the climate control unit calls for cold air, the monovalve (pin 9) never sees a ground signal (even at max detent A/C, auto setting).

I’ve investigated both sides of the pc board and all the components look to be OK (no burns, tested for cold solder, cracks, etc). Is the IC not triggering the monovalve ground signal? How & where exactly is the ground signal triggered? Stumped. Any ideas would be appreciated. I’ve searched high and low - does anyone have the pc board schematic (detailed circuit level)? Thx.

Monovalve itself
 

The climate control system almost never breaks down, the monovalve is the weakest point. If applying 12v to one of the two pins in its connector does still make a clicking sound, you likely have a tear in the internal gasket, a broken plunger, or if it doesn't click, a seized solenoid. Good news is that it's easy to replace the whole unit and it's relatively inexpensive- about $50 on eBay for one with a good warranty.

Thx Benz Mondi. I disassembled and inspected the monovalve itself. No short in the solenoid. Spring action good. Everything was spotless. No build up. No tears in the diaphragm. Movement was smooth. Bench tested, cleaned, and reinstalled. The monovalve works as it should when it sees 12v (i.e. it closes and A/C side vents blow COLD), but the monovalve just never sees a ground path from the climate control unit (i.e. no triggered ground).

I should also mention I've measured the monovalve current (~1A), and checked the voltage / current of the circ pump and blower - all OK.

The CCU in the 124 dash is known for issues, as apparently you're aware. The internet blames it on solder joints, I'm not sure if that is true but it is the "common wisdom".

I have a few around that you can use to troubleshoot if you live in the area.

OK CCU
 

Certainly seems like you've done your homework. Getting another CCU is also about the same price as the monovalve. I have a spare one myself. Easy enough to swap one in. In my case, I swapped one in but my problem turned out to be a bad monovalve. I swapped a good mono in there and kept the replacement CCU installed to test it while it was under warranty. The warranty has long expired, but the replacement CCU still functions well. When the time comes it doesn't, then I'll just put the original one back in :)

Testing the climate control circuits a bit more this weekend before I purchase a new unit. BTW Benz Mondi - Sorry you had to part with your '87. Here's my '88 (and a sliver of the 280
)

http://i1200.photobucket.com/albums/bb334/Lesliechow2/Ferrari_zpsgvkwjntb.jpg[/IMG]

Super Fun!
 

No regrets. Yours looks great. I had a blast with my blue/tan one and drove it everywhere. Found it in Ft. Lauderdale with 30K miles on the clock and shipped it out. Sold it with 60K miles. I'll send you a PM :)