My meter probes were too big to fit down into the fuse cavity. I did put the probes onto the top of the fuse (where you can check the fuse without removing it) and I did not get any reading. Time to head out for the day so will resume this repair attempt next Tuesday.

Merry Christmas!

 

Bob, go back to George's post - you MUST have 12 volts at Pin 2 at all times. If you dont have 12 volts there, THAT is your problem. With the car shut off, there will be 12 volts at ALL three pins and when you start the car, pins 1 and 3 will go to zero or close to zero IF the heat controls are at "full cold". At "full hot" they should go very close to 12 volts.

Now, the diagram shows a direct connection from the fuse to the connector at the valves - i'd caution you to take that less than literally. I've found, over the years, that they took a lot of shortcuts in writing the wiring diagram, there may be a connector/splice/etc. somewhere in the path that has corroded, gotten wet, come unplugged, been chewed by a rodent, etc.

But know this: DO NOT waste any time on any thing now, but getting 12 volts to pin 2.

 

Merry Christmas Mark, Thanks for your explanation. That does help clarify things for me. I'll do more research and testing tomorrow . . . Will post and update on what I discover . . .

Bob Davis (Red Dog)

 

UPDATE 12/26/17 . . . More progress but still not fixed.

I DO have power at the fuse #15 circuit . . . Sorry had not been testing it correctly. I still have no power to the #2 pin at the heater control valve connector. As I had been previously advised, that fuse #15 circuit his hot all the time and also so should the center #2 pin of the three female pin heater control valve connector. Somehow by SSB roadster is not getting power to that #2 pin.

I will next do a continuity check of the red/yellow stripe wire from the fuse box to that three pin connector. If no continuity (and I presume there will not be), I will try to that trace the routing of that wiring harness to see if I can find a break in that wire. Will advise further when I get a chance to do more.

Happy New Year!
Bob Davis (Red Dog)

 

If all else fails, come off the box in front of the battery with a inline fuse wire to the #2 pin.
( true redneck solution )

 

Some years ago Graphite Ghost experimented with these valves and found that they pulsed.
I doubted that at the time but he proved that they did. He s a persistent person by nature obviously.
It may be of some help at sometime, if not you then someone else.
CLICK

 

They do pulse, I mean, it's like this:

The valves' natural, relaxed state is "open", they must be energized in order to close.

You want full cold/no heat? Then pins 1 and 3 are grounded. The valves are energized, which means they are closed. No water flow, no heat.

You want full heat? Then pins 1 and 3 are open/disconnected, that is, at full voltage. The valves are not energized, which means they are open. Full water flow, maximum heat.

You want medium heat? Then pins 1 and 3 are pulsed with a ground - the valves open and close at a rate designed to give enough water flow to make the right air temperature in the ducts. There are two thermistors, heat sensors, in the heater core - one for driver, one for passenger - they tell the climate control what the temperature is, and it then sends pulses to the valves to 'modulate' the temperature in compliance with what you have dialed on the two wheels.


Merry Christmas, Bob. Wish I'd see this thread earlier, but I'm in Michigan (where it is, now, right now, 3F outside) visiting my kids.

 

Mark, Bob has no heat and Bob has no 12v at pin 2.
Given this, why doesn't he have full heat instead of no heat as you describe.
I think pins 1 and 3 go to ground and energize the solenoids to OPEN the valves.
Only way it makes sense

( I will go out and unplug mine in a bit and see what happens )

 

His first sentence in post number one said he had full heat when the engine was warm.

 

Thanks Dave, missed that little nugget.
That puts validity to Mark's conclusions, and makes mine pure poo poo.
( I had always thought it worked that way, relaxed = ON, energized = OFF. )

I apologize to Mark for ever even thinking he might be wrong.

 

Like me he is never wrong, it’s just that sometimes I am not quite right. That’s what I tell my wife anyway.

 

George, you forget, I've been divorced since 2010 - never been wrong in seven years now.

 

[QUOTE=pizzaguy;899295]George, you forget, I've been divorced since 2010 - never been wrong in seven years now. [/QUOTE
Never wrong but you have been hopeless ever since.

 

UPDATE: Hello again, Didn't want to leave everyone "hanging" I still have not been able to fix this problem. To review I have FULL heat which overwhelms the AC system. The heater control valves have been checked and they are working perfectly BUT there is no 12V power getting from fuse #15 (circuit is live) to the center pin of the three pin heater control valve connector.

Now here's something I've been told that surprised me . . . My OAT (outside ambient temperature reading is also not working and displays -36 F. The sensor unit in the front grille has been verified as working properly (swapped out to another SRT6). I went to a local independent Mercedes garage to talk with them and the owner tells me that the OAT system not working is probably related to my AC / Heat system issue. He say's "Why would you need AC when the computer thinks that the outside temperature is -36 F??" That outside temperature system is tied to the computer - it's not just to let you know the outside temperature."

Can anyone confirm that information is correct?? Any new suggestions before I take the car in for "professional assistance"??

Thanks, Bob Davis (Red Dog)

 

The 12v to the solenoids is NOT dependent on any computer module. ( see post #22 )
Is there 12v at fuse #15 ? If so, then da mice ( or in your case, da crabs ) done chewed da wire.
As to the OAT sensor, I don't have a clue.

 

Hi Dr. George, That's what I thought too. That doesn't inspire confidence in the local independent Mercedes repair shop owner. I'll take the car up to J S Benz in Miami. That's where they do all the V8 conversion work with Rudy. They know their stuff . . .

Will post the outcome here when I get the car back.

 

The ambient air temp sensor may be perfect but the system it plugs into may have a fault in it. I would follow the instructions in the Service manual to check this sensor out. Also test the solenoids as explained in the Service Manual. The solenoids must shut down or the heat will overcome the AC cold air.
Have you considered putting a clamp on the hose where it goes behind the overflow tank? That should stop the heat and prove the AC system is working fine.
The air compressor works even though we have the AC system OFF, it is used to dehumidify air used inside the car. If it is not used moist air is blown on to a cool windshield which then fogs up.
To understand the AC system better you should read the Student Handbook CLICK in the documentation thread. The Service Manual does not explain too much at all.

I have not studied what has been going on here much as I have been sick with the flu, but have you studied the control panel on the console? If this is not working then all sorts of problems can occur. A member said poor contacts cause most of the problems. CLICK

 

Bob, the power ( 12v ) to fuse 15 comes direct from the honking big 200 amp bolt-in fuse in front of the battery, as does most of the other non-switch fuses :
( point "B" is that big fuse )

 

Well . . . I took my SSB SRT6 roadster up to J S Benz in Miami so they could track down the gremlins and root them out.

Seems that the wires connecting the OAT (outside ambient temperature) sensor to the instrument cluster had a break somewhere. Also I did find out that if the OAT system is not working (reading -36 F) it WILL disable the air conditioning as well as affecting other systems in the car. That OAT system is not just to let the driver know the outside temperature. It communicates with the ECM computer and affects fuel mixture and other things!

Also the wiring harness connecting fuse #15 to the heater control valves had a break somewhere.

They ran new (nicely and carefully hidden) wires to both systems and everything is now working as it should and looks factory fresh. NICE WORK!

Only problem is that my wallet much lighter!

 

Is this a bad time to mention that $5 Torque Pro can display & log temperatures (I normally have IAT and Coolant on display. Had a TPS go to zero when you hit the brakes and the engine stalled. Without instrumentation I never would have found it.


Besides TP is a great tool for checking out a used car.