Fried ACC Board (picture) due to frozen aux pump?

[Jeremy5848]

I finally got around to removing the ACC control unit from my car. What did I find when I disassembled it but that the little pc boards had four fried traces and the PO had done a Mickey Mouse repair. See the picture -- it's gross.

To orient you for the picture -- you're looking down at the board as it is installed in the car. The left connectors are for the temp control and a cable to Nowhere. The right side are connectors for the blower speed pushbuttons and another cable to Nowhere. The pushbutton on the left is Defrost, on the right is Off, the others you can figure out.

I may be able to salvage the board. The added wires are by the PO. In the meantime, I'm mulling over several questions:

1. Are the fried traces the result of a frozen auxiliary water pump? I've never checked to see if my pump works, always assumed it did. Fried trace #4 goes to pin #6 of the right-hand "cable to Nowhere." By any chance does this wire go to the aux pump? I can trace the wire (and will do so when I add an in-line fuse to the pump, tomorrow) but am lazy and would like someone on the forum to answer the question while I get some sleep.

2. Does anyone have a schematic diagram of the ACC? The one on Braingears is illegible. I would appreciate an image or a link to one so I can figure out what the other three broken traces are for.

3. Are these fried traces and amateurish repairs related to my blower speed problem? One would think so -- what do you think? (The blower works correctly in EC mode for all three speed pushbuttons. It runs high speed in Defrost mode, also correctly. However, in the two A/C modes, it runs high speed with the High button pressed, low speed when the Automatic button is pushed, and also low speed when the Low button is pushed. In other words, Automatic doesn't work in A/C mode.)

4. It appears that the board has not been resoldered. I don't see any cold solder joints (but then, I said that about my cruise board and was wrong). I may put off resoldering everything until I get these fried traces properly rewired.

Interesting how everyone's ACC problem is slightly different!

Jeremy

[bhanson]

Not sure that it is the universal cause....but in my case, my similarly cooked ACC was indeed due to a bad aux. water pump. The standard recommendation is a slow burn 1amp fuse to the pump to prevent such chaos.
Good Luck,
Ben

[Jeremy5848]

Thanks for sharing that information, Ben. The water pump on my car must have been replaced by whoever made the repair to the ACC since, when I tested the pump, it worked fine. When I measured the current to the pump it was 0.9 Amp at 12.6 Volts from the battery. I used a 2 Amp fuse but wonder if I should change to a smaller fuse so it will blow before anything is damaged. I suppose I could try it and see...it's not like the car will die if the fuse blows.

Bottom line, fixing the ACC fixed my problem with the climate control, it works perfectly now.

For "the rest of the story," see this link:

Fixed the ACC!

Jeremy

[whunter]

Originally Posted by Jeremy5848

I finally got around to removing the ACC control unit from my car. What did I find when I disassembled it but that the little pc boards had four fried traces and the PO had done a Mickey Mouse repair. See the picture -- it's gross.

To orient you for the picture -- you're looking down at the board as it is installed in the car. The left connectors are for the temp control and a cable to Nowhere. The right side are connectors for the blower speed pushbuttons and another cable to Nowhere. The pushbutton on the left is Defrost, on the right is Off, the others you can figure out.

I may be able to salvage the board. The added wires are by the PO. In the meantime, I'm mulling over several questions:

1. Are the fried traces the result of a frozen auxiliary water pump? I've never checked to see if my pump works, always assumed it did. Fried trace #4 goes to pin #6 of the right-hand "cable to Nowhere." By any chance does this wire go to the aux pump? I can trace the wire (and will do so when I add an in-line fuse to the pump, tomorrow) but am lazy and would like someone on the forum to answer the question while I get some sleep.

2. Does anyone have a schematic diagram of the ACC? The one on Braingears is illegible. I would appreciate an image or a link to one so I can figure out what the other three broken traces are for.

3. Are these fried traces and amateurish repairs related to my blower speed problem? One would think so -- what do you think? (The blower works correctly in EC mode for all three speed pushbuttons. It runs high speed in Defrost mode, also correctly. However, in the two A/C modes, it runs high speed with the High button pressed, low speed when the Automatic button is pushed, and also low speed when the Low button is pushed. In other words, Automatic doesn't work in A/C mode.)

4. It appears that the board has not been re-soldered. I don't see any cold solder joints (but then, I said that about my cruise board and was wrong). I may put off re-soldering everything until I get these fried traces properly rewired.

Interesting how everyone's ACC problem is slightly different!

Jeremy

This is caused by a seized/shorted auxiliary water pump...

There is an easy DIY to prevent this issue.

* Cut the auxiliary water pump positive wire
* Install a one amp fuse
* When the fuse blows = bad auxiliary water pump, and no other damage

:EDIT:
Here is what one member sells to prevent damage
1 amp fuse link for w123 Auxiliary Water Pump – Plug and Play!

Here is another good idea
Inline Fuse on a HVAC Blower Motor


:EDIT:


Have a great day...

[DeliveryValve]

Here is a recent shot a fried CCU from my friend's 300d. The aux pump seized. A fuse in the line would of saved this and prevented a possible fire.

[WhoWhat]

Originally Posted by DeliveryValve

Here is a recent shot a fried CCU from my friend's 300d. The aux pump seized. A fuse in the line would of saved this and prevented a possible fire.

WOWSERS! Glad I unplugged my aux pump...

[compu_85]

Glad the 86+ system has built in short protection!

Might it be a good idea to put a small fuse in there anyway? What size?

-Jason

[JimmyL]

Good gracious that is one fried CCU unit!!!
That really could have been a bonifide car fire. Sheesh......
I need to check both wagons now. I have meant to do the fuse in the past but other issue, and of course the complete lack of a memory have kept me from getting that done. Can't remember if I unplugged them in the mean time, though.
Gotta love 240's, eh........

[rcounts]

OK, I'll admit my newbie-ness.

What and where is this auxiliary water pump you all speak of?

[compu_85]

On your car it lives under the air cleaner area and pumps coolant into the heater core at a constant rate, so that the cabin heat is even and doesn't change with engine (and therefore water pump) speed.

-Jason

[pawoSD]

I will attest to the fuse. I installed one being suspicious of my noisy aux pump.....sure enough, two months later I had silence when starting the car, I checked the fuse, blown. Replaced the aux pump with a good spare I had acquired, and a new fuse....still working to this day, nearly a year later. Nice smooth hum when starting car...

[daw_two]

Does the Aux Water Pump exist on w126s? I was under the hood last night due the ohm check of my glowplugs and forgot to look. I'm having an interesting heat situation.....like my heater does not warm up like Elegant Madame did. MonoValve, maybe? It's like ...the air is barely not "outside air" temp; turn the fan to low....and then all of sudden the blower air is VERY hot.

Bumps in the road can be helpful to these old cars.

Speaking of glow plugs, Coda has good news and bad news --- I get to take all 6 back to AutoPlace; hers all ohm out fine; so the rough starting issue is something else. Ugh. I love driving an old Mercedes.

[rwthomas1]

If you really want a bulletproof way to fix the aux pump issue look at my solution in this thread. Arrrgh! Auxilliary coolant pump leaking..... I haven't had any issues since this mod, everything works perfectly. RT

[pawoSD]

Originally Posted by daw_two

Does the Aux Water Pump exist on w126s? I was under the hood last night due the ohm check of my glowplugs and forgot to look. I'm having an interesting heat situation.....like my heater does not warm up like Elegant Madame did. MonoValve, maybe? It's like ...the air is barely not "outside air" temp; turn the fan to low....and then all of sudden the blower air is VERY hot.

Bumps in the road can be helpful to these old cars.

Speaking of glow plugs, Coda has good news and bad news --- I get to take all 6 back to AutoPlace; hers all ohm out fine; so the rough starting issue is something else. Ugh. I love driving an old Mercedes.

Yes, the W126 does have one. Its beneath the monovalve behind the firewall (near the battery). Your lack of heat at higher speeds is a torn monovalve diaphram. Order the repair kit and replace it. Cost $25-30ish....time to replace: about 5 minutes. Reward: full heat!

[daw_two]

Originally Posted by pawoSD

Yes, the W126 does have one. Its beneath the monovalve behind the firewall (near the battery). Your lack of heat at higher speeds is a torn monovalve diaphram. Order the repair kit and replace it. Cost $25-30ish....time to replace: about 5 minutes. Reward: full heat!

Thanks a million??? Sorry for the hijacking.