W124 ACC, Finding the Source of the Problem - PeachParts Mercedes-Benz Forum

Hupomone · #1 04-14-2014, 08:59 AM

Help!

I am tracking down the issues of my ACC ssytem on my new 1987 300D Turbo. The system seems to have decided to go into "normal" mode all the time. I have heat when through the dash and side vents when the car is moving, but not when its still (no blower operation?).

I checked the monovalve, it seems good and actuates when i give it 12V and the diaphragm is good, but it is not getting any V form the car.

I checked the Y7 Switch-over Valve Unit, it too seems to operate correctly when I give it 12V and is getting 16 on teh vac guage from the engine.

All the vacuum pods work except #4 (center vents).

I guess I need to check the push button unit, but am not sure what to be looking for. Can anyone point me to a previous thread or give be a rundown on how to verify operation of the push button unit?

Thanks,

G

Maxbumpo · #2 04-14-2014, 09:15 AM

I ran the majority of the ~21 diagnostic steps in the FSM using a good digital multimeter and the wiring diagram for my car. It is a little tedious and one needs a decent knowledge of Ohm's law and how to apply it, but it is possible.

If you select the far left button (defrost), do you get full fan, full heat, and AC all at the same time? If your system passes that test, then you know the AC compressor, mono-valve, and blower fan / fan controller are all in good working order. That leaves just the push-button unit, the various sensors, vacuum pods, and the cabin-temp sampling motor, and maybe a few other components.

Maxbumpo · #3 04-14-2014, 09:18 AM

Also, PBU's have been known to develop bad solder joints, just like the cruise control amplifiers. I believe the places that offer rebuilt PBU's require a core that has not been "repaired", so if you do try to re-flow the solder for all the joints, they probably won't accept as a core, but you may fix the problem for cheap-cheap and get a few more years of life out of her.

Hupomone · #4 04-14-2014, 09:43 AM

none of the buttons seem to do anything.

Maxbumpo · #5 04-14-2014, 10:23 AM

Quote:

Originally Posted by Hupomone

none of the buttons seem to do anything.

Check your fuses, try replacing them all with quality copper fuses. If that doesn't help, and no other electrical issues (i.e. is the PBU getting any power, and is it properly grounded?), then I think it would be safe to say your PBU needs to be either repaired or replaced. Note that you may also have other problems that you cannot detect now due to non-functioning PBU.

Hupomone · #6 04-14-2014, 10:51 AM

I did already did a full fuse replacement, but will check them again to be sure. Do you know which fuse is specifically for the PBU?

Before I pull the unit is it obvious which wires are which? for testing?

Thx, G

Maxbumpo · #7 04-14-2014, 10:57 AM

Don't remember off-hand which fuse(s) are involved.

If you have a wiring diagram, you can puzzle out which wire is which. In general, Red is power, Brown is ground.

This is the time when owning a paper copy of the factory service manual is a huge help. They are NOT common at all. I bought a copy of the '87 "Introduction to Service" manual for the 124 diesel cars, and it has a fold-out wiring diagram for the earliest version of the 124 series. I've found some major differences in my '87 wagon. I also purchased from Ebay a copy of the HVAC troubleshooting manual, which also includes some one-page wiring diagrams for various HVAC components. The FSM on CD has the same one-page diagrams. If you also have Steve Nervig's web index for the CD's, life is much better...

Hupomone · #8 04-14-2014, 11:00 AM

thanks for the help. I will report back after i find out the deal. And yes diagrams and pictures would be help full.

G