Last evening during a highway drive in a downpour, my ACC system became unresponsive to button pushing and temp wheel turning except for blowing hot air in defrost mode (MB safety feature).

After searching the archives, and fearing the kiss of death on the ACC from the auxiliary water pump / mono valve, I pulled the ACC unit from the dash to have the circuit board checked. I removed the back cover, but not the board from the unit. Fortunately, my mother-in-law who used to build custom boards for Allen Organ gave it a "looks good" verdict from what she could see.
a] What's involved in removing the board from the housing for a thorough check or isn't it worth doing?
b] Is it ok to drive the car without the ACC installed?
c] What other electrical or vacuum components are most likely to cause the inoperative ACC?

Unfortunately, I'm not good at auto electricals even with the CD manual. I know where to stick my thumb, but where to stick the red + and black - and trying to get an ohm or voltage reading - that's confusing. So I have a Wednesday appointment with an inde MB shop to diagnose it.
a] What's a reasonable amount of hours for them to find the cause? Or should I give them a limit?
b] Should I leave the ACC unit "on the seat" to prevent a faulty component from damaging it?
Or should I reinstall it without hooking up the cables and see how long it takes them to figure it out?

The head gasket, water pump, thermostat and radiator were replaced at 116k. It's now 122k and I've added about 1 pint of distilled water intermittently to match the line in the reservoir.
*] Could these recent repairs and slow coolant loss have any influence on this non-functioning ACC?

Your comments are appreciated.

Bob
89 Merkur Scorpio 76k
88 Merkur Scorpio 121k

Bob,

I've been able to purchase a replacement unit on EBAY for at or below $50 in the past.

Also, check the online junkyard inventory at https://www.car-part.com

Make sure you get your part# off of your unit first, as many of these units look identical. Also, the part# on your vehicle will be different depending on whether you have a replaceable filter or not. ... unfortunately, I'm not sure exactly what that means.

Phil at Fastlane also sells them, although the part is pretty expensive. There is also a superceeded part number out there for your unit, so I think there are actually 2 part #'s that will work.

__________________
Willy

'94 E420 White

Mods
-------
Berg Werks Keyless Entry
Buzzer replaced with Chime
Dash Light Upgrade
Burlwood/Leather shift knob
Alpine CDA-9815 Head Unit
2002/2003 16" C Class 7-Spoke Wheels
Updated "Chip" to fix Code 19 problem

I would not yet assume the pushbutton unit has failed. On your 124 chassis, unlike the earlier 123 cars, the PBU is protected against short circuits. Where the 123 PBU would melt to signify a problem, the 124 PBU goes into a kind of a hibernation state - you get full heat out the defrost and side vents, at a rather indeterminate fan speed - neither fast nor slow. The unit is unresponsive to anything but the off button, which will eventually shut it all down.

Did it work normally next time you used the car, or is it always in failure mode?

I'm guessing your problem is a shorted auxilliary water pump. The aux pump runs only when the PBU calls for heat. Even if it's shorted, everything works fine until the call for heat. Then you get shutdown mode. I'm suspecting you were cruising along in the rain, with the a/c running. Due to the low heat load, the interior started to get too cool, so the PBU decided to add a little heat to the conditioned air. Then Bam! (to quote Emeril...) the problem happened.

Try this. Find the aux water pump - it's under the hood. Not certain where on your model - I think it's near the monovalve. Look on the passenger side front fender, a bit in front of the battery. It has water pipes running to and from it Unplug the aux water pump - it has a pigtail with about six inches of wire, then a plug. See if everything works OK, especially the heating mode.

If so, you've found the problem. Given that you live in a chilly climate, you may want to replace the pump before next winter rolls around. Other than that, you probably won't miss it much.

- JimY

JimY,
Your review is correct. I know where the auxiliary water pump / mono valve is located and its +/- electrical connector. However, I don't kow how to do the electrical test on it.

Should I run a jumper wire to connect the + male plug to female connection and place ohm meter leads in plug and connector?
Bob

Bob -

In the past I've just unplugged the aux water pump and waited to see if the problem disappeared. At one time I tried to measure the resistance (ohms) of the failed aux pump on my old 123, but the results were inconclusive. The pump is currently on my 124 as well, but I've not tried to measure it. On the 124 simply unplugging the aux pump got rid of the blasting hot air problem. I've not replaced the aux pump; it just doesn't get cold enough where I live to bother.

- JimY

The way to test the aux pump is with jumper wires and an ammeter capable of measuring several amps. Connect the ammeter in series with the connections that you jump to power the aux pump. It should not read more than 1.3 amps. If it does replace it and then be VERY suspect of the PBU. High current draw of the aux pump will often blow the PBU.

Good luck,

Thanks Larry
What's a PBU and where is it?
Bob

Larry
Does PBU mean Power Button Unit?
And specifically what can be checked to determine if it is damaged or ok?
Bob