How important is the circulation pump?
 

After reading a few posts regarding climate controls that get fried because of the circulation pump I decided to check mine out in my 85 300D. It doesn't look like it works. I hooked up a power source (my trusty battery charger) and it didn't even draw a current. I tried to listen to it but I didn't hear anything. I came to the conclusion that its dead. I left it disconnected just in case it decided to short out. Do I really need it? I get enough heat with the heater on.
~DJ

If you live in a moderate climate you don't need it. The purpose of the pump is to provide hot coolant to the heater matrix when the engine is idling. If you are not in a very cold climate the water pump is usually sufficent to give you the circulation you need. If you're not having insufficent heat problems with the pump disconnected then just leave it unpluged and don't worry about it.

When you are sitting in a parking lot waiting for your wife when the temp is below freezing you can, if the pump is dead, feel the temp slowly fall off as the seized impeller blocks water flow. Revving the engine brings back the heat. As a compromise one could insert a L joint and remove pump/impeller to improve flow.

Mind I did replace both the CC unit ($175 rebuilt) and the pump (used from a 91 300E, $50). That brought back a tempremental CC air flow & temp control to something that works silently with minimal air flow and holds the desired temp quite nicely. I suspect in the early 80's this would have been state of the art for regulating temp without fuss? Somehow enjoying that 20 years later is nice too.

Ok, I am not saying yours IS working.. but your test may have been faulty....
My battery charger has a circuit where it will not activate if it does not read SOME voltage from the item it is hooked up to...
So if I put it on a really discharged battery it will not do anything.. but I can hook up another with it and it will charge them right up... Better to use an actual battery for tests like that...

I'm fairly sure that if the pump was working my battery charger would have turned it on. I have even used it to check my glow plugs and they glowed HOT...ouch!

You could just remove it and check to see if the shaft will twist. Most of them are siezed. The crud from the coolant gets to it sooner or later.

Yeah I could do it that way but right now I don't feel like draining the coolant to take it out and if it is bad I'd need to have a new one to replace it with. I wouldn't want to reinstall the bad one and have to do the job one more time. I'm probably going to start looking for another one or look for a kit to rebuild my old one. I'm probably better off just getting a whole new unit since probably the motor is shot on mine.
Does anyone know if a circulating pump from a W116 would work?

A brief remark
 

The easiest and fastest way to check whether the pump is working is to simply lay a hand on it. When it vibrates it works.

Of course with ignition on, climate controls set to other than 0 and a good fuse.

I'm fairly certain then that it is dead as a door nail.

You don't really need to drain the coolant to take the pump out. I took it off and quickly plugged the hoses with a couple of short dowel rod pieces. All in all I lost maybe a cup or two of coolant.

You can buy locking pliers with a smooth area used just for pinching off the coolant hoses without damaging them. Or take vise grips and wrap electrical tape around the jaws to prevent damage and gently pinch off the hoses also. Probably wouldn't hurt to run the car with the cap off and the heat on until the t-stat opens just in case any air gets into the system whille changing the pump.

"if the pump is dead, feel the temp slowly fall off as the seized impeller blocks water flow"

Actually the impeller doesn't really impede flow at all when stopped. I thought it would but if you remove the pump and take a looksee it is quite apparent that water flows though it very easily even when its not functioning. If you have enough heat at idle then just leave it in place but disconnected from power. When my pump died I noticed that the heat would go lukewarm at best when waiting to pick my wife up at the train station on cold winter evenings. I found that just holding the idle to about 1100rpm (in park) with my right foot restores hot coolant flow and warm air. RT