'78 300D climate "control"

[chev]

In honor of $1.70/gallon gasoline, I've resolved to bring "Ms Hades," my '78 300D back from retirement. She has only 97k, drives and runs well, but is constantly in need of multiple repairs.
The list is too long for this post, but since it's summer in Fla, the control of climate has finally reached the top of the list.
4 or 5 years ago, the climate control servo was replaced, curing intermittant pushbutton operation.
Alas, the car sat for a few years, as the repair list again lengthened, although the a/c was fine. Now that she's back on the road again, after a few weeks of normal a/c operation, things have gone to "hell" again, so to speak.

Now, although the compressor turns, and the system cools, suddenly only hot (hot!!) air issues from the left,right,and floor vents.

I've ordered the MB shop manual cd, since Clymers and Chiltons are worthless for diagnosing a/c systems. I hope, when it arrives, I'll be privy to some of the deeper mysteries of Benz repair. Meanwhile, I sure could use some advice on diagnosing this problem!
How is the beloved climate control servo tested? I know I have hot water going to the heater core, since the hoses at the firewall are hot. I've checked/exchanged fuses (although the schematic diagram in Clymers show an in-line fuse going to the climate amplifier,I haven't been able to find it...), but beyond that I don't know where else to begin: eg expected voltages at the servo, how to check the pushbutton assy, vacuum vent doors, etc.
Any suggestions will be very gratefully accepted.
Now, off to hunt down door stoppers, turn signal/hazard switches, doorlock selenoids, seat cover, and whatever falls off this week.
jeeze, I love this car (BG)
ac

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97K '78 300D

[stevebfl]

That old guy is complicated enough that we built our own analyzer to diagnose it. Our tester hooks in series with one of the two electrical connectors at the servo in the engine compartment.

All the functions of our tester can be done without it but some trial and error would have to be used as there are no voltage values in the manual.

Our tester monitors the feedback pot voltage (for relative servo position), it measures the polarity to the drive motor (internal; this shows which direction heat or cold that the servo is being driven, and probably the most important it has an over ride switch which controls the motor so that the servo can be manually driven to a different position.

Without this I would move the temp wheel and listen to the servo with a stethescope (or screw driver to ones ear). The servo should whir everytime a change is made. From your description you are either stuck in full heat or defrost. I would imagine that you have a bad amplifier (behind glove box). I would have condemned the servo first but you did say it was replaced?? Find that fuse on your wiring diagram (I don't remember where it is in the 123 car).

[chev]

Steve-
Thanks for your (quick) reply:

<<and probably the most important it has an over ride switch which controls the motor so that the servo can be manually driven to a different position.

Really! does that mean that the servo could be "driven" to the no heat position and disconnected? I'd rather have cool than heat.

<< From your description you are either stuck in full heat or defrost. I would imagine that you have a bad amplifier (behind glove box). I would have condemned the servo first but you did say it was replaced??

full heat sounds about right. This puppy puts out great heat (last thing I need today!)
Servo replaced in 95 or 96. I remember it's being a fairly pricey chunk of hardware. ~$400
I don't recall the amplifier's being changed, but will try to find receipts.

<<Find that fuse on your wiring diagram (I don't remember where it is in the 123 car).

I'll check again. I have the diagram at home, but it comes off of the main fusebox (in the diagram) as a small guage wire, and heads straight off to the amplifier. I'd sure rather replace an inline fuse than an amp and/or servo. Probably the logical thing to do next is to check for voltage at the amp.
I assume I should see 12v at the connector attached to the colorcoded wire shown in the diagram (assuming it's accurate). I see no indication of any switching between the fuse block and this mystery fuse. Time to pull the glovebox.

ac

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97K '78 300D

[stevebfl]

"Really! does that mean that the servo could be "driven" to the no heat position and disconnected? I'd rather have cool than heat."

Actually, yes, one of the common problems that occurs in the servos is the breaking of a tooth from a gear and a gear lock-up as a result (this usually burns out the amplifier trying to run the motor). With our tool we have many times reversed the motor and backed up and passed over the bad spot on reapplying the cooling direction.

In Florida one can run the servo to A/C and disconnect the single connector and it will stay. Unfortunately the most common problems are servos that are dead.

To try this I think you will have to reduce the voltage and apply it to the motor (see diagram - reverse polarity to go the other direction). I think the motor might be a five volt or nine volt motor so don't put full 12v on it.

[herbct01]

There must be a virus going around. I have the same symptoms on my '80 300D. After I replaced the servo because it was killing my battery, things got worse. Now my LO/HI/BI buttons do not work. Only the DEF and though the compressor engages, it blows hot air. The fans go off and the compressor disengages if anything subsequent to DEF is selected. On top of this, my engine does not shut off immediately which leads me to believe I have a vacuum leak.
And all this because I replaced the servo? Had I have known, I may have put a quick disconnect on my battery so that the original bad servo would not kill it.
Sorry I can't help much but Larry replied to my post a few days ago (Vacuum or what?) and suggested checking the fuses and go for a vacuum leak.
I am curious about the amplififer as a cause for all this.
I've listened for a vacuum leak with a stethoscope with no luck.
I too am waiting for a chassis manual (which is on back order) which will hopefully help trace the vacuum problem.
First one with the fix should get a prize

[stevebfl]

Herb, you definitely have a vacuum problem. The system has vacuum switches that keep various part from operating till the engine is running. The defrost position by passes this condition. Check the vacuum at the bottom of the servo (two lines; in and out of a water temp vac switch). The out is the vac source for the inside controls.

Be sure your plug is pushed onto the servo all the way.

Look under your battery for corrosion. The servo vacuum harness passes through there.

[herbct01]

Thanks for your reply Steve. I am on the verge of taking this puppy in to the real mechanics but am not quite ready to give up yet. Hopefully, with more symptoms we can narrow this thing down. My door locks are slow to actuate even while driving. Only the drivers' door unlocks after the car sits for more than a day. My brakes appear to be fine. I dread to think I have to get behind the upper console to check or replace one of those vacuum switches. If I could isolate the vacuum system I could fine the source of the leak, provided I have enough vacuum to begin with. How much vacuum should I have and where should I measure it? Will a regular vacuum guage do for testing the system? Is there a location where I can isolate the door locks, etc.? My cruise control works OK also.
Any ideas?
Thanks.

Steve was right again and did I ever have vacuum problems. The vac lines in the harness under the battery were eaten away, vac connectors were defective, a vac switch was bad, and many vac lines were replaced. Even a rubber line at the ignition switch which was swollen due to oil. How does oil get into the vacuum system anyway? What a mess!
As a DIYer, I was unable to find all the problems and ended up taking it to the pros. After shelling out a few $$$, she now is like new. But, I am concerned that acid or oil may have been sucked through the system and I am driving a time bomb. Should I dump this honey with only 203K on it?