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Old 02-03-2002, 12:36 PM
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Unhappy switchover valve 3 getting less than 11 volts?

1985 300sd (126 body); climate control unit problem...

switchover valve #3 only getting 8.79 volts in heating mode. Gets 11+ in cooling mode. I've swaped CCU's (refurbished) with no change. I've swaped Elec Temp Control box (an old one I had) with no change. Installed new Aux. Water pump. Monovalve works fine.

Where else should I look?

Wife's car, can't put it back together till it's fixed (or I'm dead!)
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Old 02-05-2002, 08:43 AM
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Okay, Guys......

Maybe I was being too technical.

I need help with the Climate Control Unit (CCU) or Push Button Unit, whatever it's called.

Everything was fine for a long time. Had replaced the CCU a while back. My wife kept telling me that she heard a hissing noise. I never heard it. Then one day I did begin to hear it. It gradually got louder. I knew it was a vacuum leak, probably in one of the servo's. I took things apart and found the leak at the #3 swithover valve. It was leaking from the vent hole. Replaced the valve, same thing. Replaced the CCU, same thing. Tested the voltage to the #3 and found only 8.79 volts. Supposed to be 11+. Further testing revealed that I get 11+ at #3 when in cooling mode, but turn the wheel to heat and it drops to 8. Why? Where's the problem? CCU? Temp Control Box? Where?

Now, all the other switchover valves are either on or off. They have a + and ground only. However #3 ground goes through everything. The temp wheel, the temp control unit and the CCU. I'd pull my hair out but it's already gone. I've come to a impass, with no where to go.

Any suggestion? Anybody? (set it a fire and call the insurance company?)

Is there a place in Atlanta that has a MB CCU testing unit? Maybe I got a second bad unit? Maybe there's something else that's frying the units when I hook it up?

Two wires, red/black and brown/blue go to #3. Red/black is straight power. Doesn't go through the CCU. Brown/Blue goes through everything. For some reason I'm getting 2+ volts to the brown/blue when hooked to ground. Weird? Should be only (-), right? Red/black to body I get 11+, Red/black to Brown/blue I get 8+ volts.

Somebody shoot me!!! Should I try to find a brand new CCU ($300+ !!!), or is it the Temp Control Box. How do you test a temp control box, anyway?

I'm so confused......

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Old 02-05-2002, 11:09 AM
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Common sense troubleshooting time.
1) B+ is 11 volts at unit? disconnect the wire from unit and see if you then get 12.5 volts. Is your voltmeter accurate? Connect it to your battery terminals to see if you get 12.5 minimum voltage - if you do then it is good.
2) B+ of 2 volts on a ground? Disconnect battery and using ohm meter meter your grounds. If it is ground - you cannot have B+ of 2 volts. Suspect bad solder joint or bad ground connection ( could be open ground in computer)
3) Where voltages are lower than should be "For some reason I'm getting 2+ volts to the brown/blue when hooked to ground. Weird? Should be only (-), right? Red/black to body I get 11+, Red/black to Brown/blue I get 8+ volts" cut the wires and see if the voltage goes up - isolate what is pulling the voltage down. If all roads lead to the computer then------ otherwise you could find a bad solder joint, a bad crimp joint - anything. If you take a deep breath and look at ONE circuit at a time and follow it through - you should be able to diagnose it. Without a schematic diagram with voltages marked as a roadmap though - it may be impossible. Patience & Logic Patience & Logic - helped me many times. Just had a case on my son's car - alternator went dead. Checked at battery got 11.2 volts with motor running or not. Tried charging battery - overheated charger - battery had internal short. Put in new battery and alternator - voltage at battery now 12.56 volts car running or not. Scratched head thought it through. Checked voltage at alternator with car running 14.6 volts. The battery melted the wire open from the alternator inside the harness. Put in new wire -problem solved. Don't get frustrated - think it through - I'll bet you will find it.
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Old 02-05-2002, 02:18 PM
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The book says there must be 11 or more volts to the switchover valves, which I get on the red/blac wire to ground. The brown/blue wire goes from the CCU through the temp reostat (wheel) to the temp control box back to the CCU. I'm at a loss as which is bad. Common sense tells me it's the CCU. But I switched that at first, with the same results. It's possible that I got a bad 2nd CCU also. But funny how it gave the same problems. I've had CCU problems before but mostly with servo's not open/close right.

I'm afraid that if I buy another CCU and hook it up that whatever is the problem it will discombooberate the new one also. Can a bad Temp Control Unit cause CCU damage?
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