Heat

[Feather535]

I know there have been a lot of threads on climate control, but I'm hoping the group can steer me to the right sources for the problem I'm trying to solve.

The car is a 1977 280SE with ACC II.  The AC doesn't work, but I'm not concerned about that right; with cool weather coming soon, I just want to get some heat in the cabin. The push buttons for climate control work as they should (the fan blows, and air comes out the right places), but the air is always cool, regardless of where the temperature control wheel is set.  The car still has the original regulating valve with 14 vacuum hoses and an electrical harness, but the electric hot water pump is missing.

Where to start?

[daantjie]

If you push DEF you need to get max hot air out of the outlets at the windshield.  That would be the first test.

Also, you say the auxiliary water pump is missing.  So do you know how the hoses have been rerouted?  Maybe a previous owner bridged the hoses to bypass the heater core.  If the heater core is leaking perhaps this was a shortcut fix.  Hopefully not the case as that is a huge job to replace the heater core.

[Feather535]

Yeah, when I push the DEF button, it blows cool (ambient) air.  I also read on this site that unplugging the ambient temperature sensor should make the system to full hot, but that doesn't work either.

On the routing of the hoses, it looks like someone ran the hose from the hot side (cylinder head) directly to the inlet of regulating valve instead of through the auxiliary pump.  So I don't think the heater core is bypassed.  BUT the flow through the core looks to be reversed compared to the picture in the manual https://handbook.w116.org/cd/Climate/83-630.pdf.  The hose closest to the firewall is connected to the right (passenger) side of the core and the one closest to the engine is connected to the left (driver) side.  I don't know if this makes a difference, but the manual shows water flowing right to left through the core, then back to the valve.

[daantjie]

Hmm.  One more test is to turn ignition ON no start.  Then run the temp wheel on the panel from full cold to full hot.  Works best in garage so it's nice and quiet.
 
With the hood open you should hear the servo running to open and close the water valve.  Should be pretty clear whirring noise, same noise as when the servo parks itself when you turn the ignition off.

If no noise then you either have a problem in the amplifier or servo or both.  You can test the servo in situ with a 9v battery and some jumper wires for basic function and resistance with a multimeter.

[Feather535]

Thanks for the suggestion, I'll do the first test and report back.

More info on testing the servo would be appreciated: where to connect the wires and what to look for.

[daantjie]

I'll try and upload this doc I have, unfortunately the quality is crappy:

[daantjie]

Ok I actually found a much better doc for the servo test.  It shows how to connect your multimeter and battery feed.  I just use a 9v battery but be careful not to leave it connected after the motor comes to a stop or else you'll burn out the motor.  Also I'm not sure why the doc mentions a 3 minute wait time, but the movement from full hot to full cold on the motor with power connected is maybe 5 seconds:  Have fun

[Feather535]

Got it, thanks.  The document says the factory test harness supplies 1.5 volts, so maybe something like a D battery is sufficient?

[Feather535]

The servo doesn't make any noise when I turn the temperature wheel with the ignition on, so I rigged up a crude facsimile of the factory test harness with some wires, alligator clips and a 9-volt battery.  The ohmmeter reads about 1300 regardless of the temperature setting and reversing polarity on the battery doesn't make the motor run. 

I guess this means the servo is toast.  So now what? 

[daantjie]

Yup she's done like dinner by the sounds of it.

You can find used ones on ebay but it's a gamble.  There is a guy on ebay selling the aluminum bodied ones which tend to hold up better as the bakelite bodied original ones crack over time due to all the heat cycles.  Crappy design but it is what it is as they say 

Here is the link to the seller, he also sells on ebay:

https://www.mbdiesel1.com/