Heat all the time, even with CCU off (87 300D) - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 06-24-2012, 10:51 PM

Newest problem in a string of electrical problems I may or may not have caused by trying to solve the first electrical problem. I have hot (really hot) air coming out of the vents all the time, even with the CCU switched off. Once the car is in motion, it trickles out of the vents, enough to be pretty awfully uncomfortable when it's already 90 degrees out. When I turn the unit to the "EC" setting with "auto," it immediately starts blowing hard -- including out the center vents, but the air stays piping hot. This is the W124.
I have some theories about what could be wrong but wanted to see if you guys think I'm on the right track. My guess would be the monovalve is stuck open, allowing heat to be pumped into the vents at all times. When I turn the CCU on, I think it's trying to cool down the cabin (hence the center vents being opened) but since there's coolant cycling through the core, the air is coming out is hot. From looking at the wiring diagram, it looks like the monovalve is connects directly to fuse 7 with one wire, and directly to the CCU post 9 with a green/yellow wire. When the CCU is off (or detects cabin area does not need more warmth) it provides a ground to the green/yellow wire, which provides 12v to the monovalve and shuts it. Is that correct?
If I take the harness off the monovalve and run the engine (or put the key in the "2" position?) with the CCU turned OFF, and put a multimeter across the harness (with the + lead on the black/red wire and the - lead in the green/yellow wire) I should get 12V, correct? Or does the harness have to be plugged into the monovalve to test it? In which case I could try the light bulb trick Jeremy used in post 17 here: Monovalve which fuse?.
If I do the first test (run a multimeter across the monovalve's harness) and don't get 12V with the CCU turned off, I would assume that means I fried part of the CCU (or it coincidentally went on it's own). If I get 12v at the harness (but the monovalve is obviously open anyway), that would mean the monovalve is bad. Is my logic logical?
Sorry if this ground has been covered (no pun intended), but I just want to make sure my interpretation of the FSM and previous threads is on the right track and not on track to mess up something else.

EDIT: The A/C has been inop since I have owned the car.

qwerty · #2 06-24-2012, 11:06 PM

The monovalve is controlled by a switched ground. So…simply having 12v present is not the whole story. With the CCU off and key on, you should have 12v at one terminal and continuity to ground at the other.

If you need a quick solution for the summer, just plug or clamp a heater hose.

BodhiBenz1987 · #3 06-24-2012, 11:20 PM

Quote:

Originally Posted by qwerty

The monovalve is controlled by a switched ground. So…simply having 12v present is not the whole story. With the CCU off and key on, you should have 12v at one terminal and continuity to ground at the other.

If you need a quick solution for the summer, just plug or clamp a heater hose.

I meant put the multimeter between the power lead and the switched ground lead, which would effectively test the continuity to the ground, right? I guess I would need to test each seperately anyway to narrow it down though. I'll check each lead now.

Jeremy5848 · #4 06-24-2012, 11:40 PM

If the temperature sensor in the dome lamp assembly has a broken or disconnected wire (rare that the sensor itself fails) then the CCU will do nothing but blow heat. Check that sensor wiring first.

Second, install a small lamp (like one of the warning light lamps) in parallel with the monovalve. When the CCU switches on the monovalve, the lamp will glow. If the lamp never glows then something is wrong with the CCU or the wiring to the monovalve. If the lamp lights but you still get heat, then the monovalve is stuck open or the coil (that closes the valve) has failed.

Jeremy

qwerty · #5 06-24-2012, 11:59 PM

Quote:

Originally Posted by BodhiBenz1987

I meant put the multimeter between the power lead and the switched ground lead, which would effectively test the continuity to the ground, right?

It would. If that is okay, you either have a poor connection or the valve is stuck open. Disassembly and cleaning will sometimes solve the latter issue.

BodhiBenz1987 · #6 06-25-2012, 12:27 AM

Quote:

Originally Posted by Jeremy5848

Check that sensor wiring first.

Do you know where the wiring routes? I'm assuming an A-pillar, but does it go behind the cluster or glovebox? Guess I can try to trace them from the harness on the back of the CCU. The FSM shows a procedure that involves an expensive-looking tester on the CCU, but can I just check continuity across the two wires under the dome light? Would that sensor (or its wiring) cause the problem even with the CCU turned off?
I have a spare courtesy light from a door I can turn into a little tester for the monovalve harness; will do that tomorrow.

whunter · #7 06-25-2012, 01:38 AM

Quote:

Originally Posted by BodhiBenz1987

Newest problem in a string of electrical problems I may or may not have caused by trying to solve the first electrical problem. I have hot (really hot) air coming out of the vents all the time, even with the CCU switched off. Once the car is in motion, it trickles out of the vents, enough to be pretty awfully uncomfortable when it's already 90 degrees out. When I turn the unit to the "EC" setting with "auto," it immediately starts blowing hard -- including out the center vents, but the air stays piping hot. This is the W124.
I have some theories about what could be wrong but wanted to see if you guys think I'm on the right track. My guess would be the monovalve is stuck open, allowing heat to be pumped into the vents at all times. When I turn the CCU on, I think it's trying to cool down the cabin (hence the center vents being opened) but since there's coolant cycling through the core, the air is coming out is hot. From looking at the wiring diagram, it looks like the monovalve is connects directly to fuse 7 with one wire, and directly to the CCU post 9 with a green/yellow wire. When the CCU is off (or detects cabin area does not need more warmth) it provides a ground to the green/yellow wire, which provides 12v to the monovalve and shuts it. Is that correct?
If I take the harness off the monovalve and run the engine (or put the key in the "2" position?) with the CCU turned OFF, and put a multimeter across the harness (with the + lead on the black/red wire and the - lead in the green/yellow wire) I should get 12V, correct? Or does the harness have to be plugged into the monovalve to test it? In which case I could try the light bulb trick Jeremy used in post 17 here: Monovalve which fuse?.
If I do the first test (run a multimeter across the monovalve's harness) and don't get 12V with the CCU turned off, I would assume that means I fried part of the CCU (or it coincidentally went on it's own). If I get 12v at the harness (but the monovalve is obviously open anyway), that would mean the monovalve is bad. Is my logic logical?
Sorry if this ground has been covered (no pun intended), but I just want to make sure my interpretation of the FSM and previous threads is on the right track and not on track to mess up something else.

EDIT: The A/C has been inop since I have owned the car.

Call me Monday.

.

Jeremy5848 · #8 06-25-2012, 12:40 PM

Quote:

Originally Posted by BodhiBenz1987

Do you know where the wiring routes? I'm assuming an A-pillar, but does it go behind the cluster or glovebox? Guess I can try to trace them from the harness on the back of the CCU. The FSM shows a procedure that involves an expensive-looking tester on the CCU, but can I just check continuity across the two wires under the dome light? Would that sensor (or its wiring) cause the problem even with the CCU turned off?
I have a spare courtesy light from a door I can turn into a little tester for the monovalve harness; will do that tomorrow.

The sensor wires go down the right A-pillar and behind the glovebox. If there is a problem, it is likely to be at the dome light itself -- a wire pulling out of a connector would be a good example. The sensor is a heat-sensitive resistor and can be tested with an ohmmeter. The FSM says that at 20C the sensor should read from 11.5 to 13.5 thousand Ohms.

With the CCU turned off the sensor would not cause the problem. A stuck-open monovalve would.

Jeremy

whunter · #9 06-25-2012, 06:51 PM

Quote:

Originally Posted by BodhiBenz1987

Do you know where the wiring routes? I'm assuming an A-pillar, but does it go behind the cluster or glovebox? Guess I can try to trace them from the harness on the back of the CCU. The FSM shows a procedure that involves an expensive-looking tester on the CCU, but can I just check continuity across the two wires under the dome light? Would that sensor (or its wiring) cause the problem even with the CCU turned off?
I have a spare courtesy light from a door I can turn into a little tester for the monovalve harness; will do that tomorrow.

I hope our discussion helps.

.