Climate control diagnosis stopped at step 1, any guidance? 1985 300D

[uberwasser]

Vehicle: 1985 300D

Symptom: The climate control system does nothing, period. None of the buttons elicit any response from the system whatsoever.

A friend had a spare CCU, condition unknown; I installed it and the symptoms didn't change at all.

Today I'm doing some testing with my multimeter. First thing I did was to test for a bad fuse. It passed visual inspection, so I then used the meter to test for voltage at the fuse.

What I found was that there was no voltage at either side of the fuse. *Edit* Should note that this was testing with key turned to RUN position. *End edit* Confirmed that I had a good ground by testing other fuses, read solid 12V where appropriate.

From prior reading on here and other sites I know of two potential issues...

1) A bad contact on the back of the fuse panel, due to a loose or damaged connection

2) A problem with the ignition switch

Of course there could be a problem in-line with the wiring but this seems to be the least common scenario; it's more often something wrong at one end or the other.

Are there any other potential sources for this problem I should look at?

Related to potential problem area #1, bad connection behind fuse panel, I undid the 8mm nut holding the panel to the body but can't get the fuse panel out enough to appropriately inspect the back of it. I can see the wiring, and the upper connections, but not the lower connections where the climate control wiring would connect to the panel. Any tips on getting the fuse panel out further?

Thanks in advance.

[uberwasser]

Update:

Tested all of the other fuses while I was at it and wanted to update with one other tidbit of info.

I was incorrect earlier to say there's no voltage on either side. In testing again I find approx 0.15v at both sides of the fuse.

[79Mercy]

what fuse are you checking? the blower fuse is only gonna have 12v when the climate control system is turning the blower on. usually the problem is with the push button unit or the climate control amp above the glovebox.

[uberwasser]

OK, my misunderstanding. The blue fuse (20+amp). I assumed the fuse was for the climate control system as a whole and so it would have 12v at all times.

I'll continue with diagnosis then to see what's going on at the plugs on the CCU.

[79Mercy]

do you have a wiring diagram?

[uberwasser]

I can access them. Haven't used one before, though. I've always steered clear of electrical work besides simple residential stuff. For the time I'm starting with the diagnostic steps suggested by Programa, who does the refurb's on climate control components. If that fails me I'll have to do more research.

[uberwasser]

Follow up with partial resolution:

After some more fiddling, I realized that the climate control push button that I swapped in actually did have some life. I was able to decipher that when it was turned on there was activity behind the dash (clicking noises) and the A/C compressor would kick on (engine idle change).

So my focus turned to trying to figure out why the blower motor would not spin.

I started by checking the blower motor itself. With 12V directly applied it work superbly.

I then tested and found that no matter what setting I activated on the push button unit there was no voltage at all at the plug for the blower motor. So the motor simply wasn't getting power.

In my research and studying of the wiring diagram this could be a result of an combination of problems resulting from a bad blower relay, blower resistor, temperature regulator, or push button unit.

Since I had access to them, I pulled spares for each of the above from the junkyard. I began the swapping!

In the end, it was the temperature control regulator. The culprit:



No I have a working blower, and a push button that responds to input. It turns out the A/C works pretty well by the way.

Now that it works somewhat, I can identify the remaining problems:

It seems that I only get A/C when the temp wheel is turned to MAX. Otherwise I get warm outside air or maybe even somewhat heated air.

And I have a vacuum leak somewhere in the system. Right now I have the climate control blocked off from the vacuum system as it was messing with brakes and shifting. So I only have air from the side vents. I'll need to start testing the various pods and valves.

I think in the long run it'll mean going in and re-soldering the PBU and maybe even seeing if it's possible to do the same on the temp regulator and blower relay in an effort to get them working like new. But for the next few four or five months all I really need is MAX A/C here in Tucson so I'm a happy camper for the time being!

[Zacharias]

No one ever seems to mention that piece when discussing ACC. I would not know it exists, except I have seen a few for sale on eBay.

Where does it live, behind the glovebox?

[uberwasser]

Yes, right to the left of the blower relay. Here it is dangling there after removing the 8mm screw that holds it to the bracket, and the plug.

[uberwasser]

I did a lot of reading and studying of the ACC wiring diagram in case my rudimentary parts swapping didn't help.

Along with some recent experiences by other forum members and from my friend with the green car in my signature, and the reading/studying I did, I've come to realize this part does a LOT of the thinking for the climate control. Below is a quick summary of what each major component does in laymen terms:

Push button unit - responds to selections on the panel, and the temp wheel, by simply sending signals to various devices to let them know what was selected. For example, when you press the middle button for standard vents and leave it on AUTO it tells the compressor to turn on via its relay, it tells the temp regulator what temp you want the cabin to be at, and it sends vacuum signals to the pods to open or close the appropriate vents and flaps. There isn't much thinking/logic going on here, it just passes along set responses to your inputs.

Blower relay - a glorified switching unit. It changes blower speed by switching between the various coils in the blower resistor that is behind the passenger side headlight. No brains here, just a unit that responds by switching.

Blower resistor - just like it sounds, simply a resistor that drops voltage across a coil to send a specified voltage to the blower.

Temperature regulator - this is the brain of the system, as it turns out. It is what combines all the various inputs from the push button (what you want the climate inside the cabin to be like), the cabin temp sensors (what the climate inside the car is like now), the engine lockout switch (coolant temp for heating), etc. etc. and it then controls the blower and the monovalve in concert to help reach your desired conditions.

When it came to other folks talking about this part lately, mainly it's been in regards to poor heat at highway speeds despite the rest of the system being sorted well. Replacing this unit seems to be fixing that problem.

This is the first case I've found of a bad temp regulator leading to a completely inop blower.

[JamesDean]

Originally Posted by Zacharias

No one ever seems to mention that piece when discussing ACC. I would not know it exists, except I have seen a few for sale on eBay.

Where does it live, behind the glovebox?

That is the temperature regulator control box.

Its one of three parts in the 81-85 climate control system. The three piece system is used on the 107 (at least till 87) and the 123.

On the 126 it lives on the blower box, center console side.

I've re-worked a few of them.

000 822 09 03 - 126 First Generation Temp. Regulator - Imgur

000 822 11 03 - W126 First Gen Blower Speed Control - Imgur

[uberwasser]

Thanks for sharing the pictures. Good to see the guts. I now have 3 spare blower relays and two spare temp regulators so I'll be working with a friend to see about fixing them up.

[JamesDean]

Be careful when opening the units. I believe both have fragile connections from the PCB to the pole connector. In one case I had to re-solder the connections as a few broke.

If you look in the pictures you'll see the little metal strips connecting the board to the MB connector header.

[jay_bob]

And back to your original post, about how to maintain the fuse box on a 123.

The intent is to take the fuse box and push it into the interior of the car.

Then go up under the dash and pull it down. They built the wire harness long enough that the fuse panel can be laid on the floor board for maintenance or testing.

Of course it goes without saying, pull the battery negative cable first!

[uberwasser]

Thanks! That's good to know for the future. I may pull one or two from the junkyard, knowing how now, to keep as spares in case this one ever melts.