W123 Inconsistant cabin heat, slight engine temp fluctuation, wire on thermostat cap? - Mercedes-Benz Forum

 
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post #1 of 7 (permalink) Old 11-25-2013, 09:23 PM Thread Starter
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W123 Inconsistant cabin heat, slight engine temp fluctuation, wire on thermostat cap?

Quick question here. I've got an '85 300D Turbo. I bought it about a year ago, and am finally getting it out of the yard and onto the road.

I knew it had this problem when I got it, but now I'm experiencing it, and not completely sure what the deal is.

The cabin heat comes and goes. Nice and hot when it works, then ice cold. When the cabin heat goes off, the engine temp goes up a few degrees, to about 85c. I unplugged the monovalve, which I understand is supposed to open up the coolant flow to the heater core and give me full heat, but it was still coming and going.

I'm kind of thinking the thermostat is a likely suspect, and a good idea to replace anyways, but there is a wire that was connected to the thermostat housing that is broken off now. I know it's not the temp sending wire, as that one comes off of the head. So what is this wire coming from the thermostat housing? Is it a temp sending unit for the aux fan?

Any thoughts would be greatly appreciated. Thanks!
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post #2 of 7 (permalink) Old 11-26-2013, 05:49 PM Thread Starter
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Well I picked up a thermostat kit today, and asked the mechanic about the sensor on the thermostat cap. It turns out to be the sensor that kicks the aux cooling fan on at a certain temp, just like a suspected. I'm not going to replace the sensor, I'm going to wire it up to a toggle switch in the cabin.

After some more reading, I ordered a monovalve repair kit as well. Of course, when I unplugged the monovalve, one of the prongs pulled out of the monovalve cover, so I get to figure out how to fix that as well.

I'll pick up a replacement hose for that short hose that connects to the thermostat cap. Might as well replace it while I'm at it.

We'll see how it goes, since the monovalve is being shipped, I'll end up putting in the thermostat first, so we'll see which one was causing the problem. At any rate, the monovalve is old and it's probably a good idea to replace it.
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post #3 of 7 (permalink) Old 12-09-2013, 08:12 PM Thread Starter
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Originally Posted by jbwolfe View Post
Well I picked up a thermostat kit today, and asked the mechanic about the sensor on the thermostat cap. It turns out to be the sensor that kicks the aux cooling fan on at a certain temp, just like a suspected. I'm not going to replace the sensor, I'm going to wire it up to a toggle switch in the cabin.

After some more reading, I ordered a monovalve repair kit as well. Of course, when I unplugged the monovalve, one of the prongs pulled out of the monovalve cover, so I get to figure out how to fix that as well.

I'll pick up a replacement hose for that short hose that connects to the thermostat cap. Might as well replace it while I'm at it.

We'll see how it goes, since the monovalve is being shipped, I'll end up putting in the thermostat first, so we'll see which one was causing the problem. At any rate, the monovalve is old and it's probably a good idea to replace it.
I replaced the thermostat and short hose. I realized there are actually two temp sending units - one on the thermostat HOUSING and one on the thermostat CAP. It's the one on the cap thats broken, so now I'm back to not being sure what it goes to.

Heater is still only working sometimes even with the new thermostat, so the mono valve is next. Engine temp still fluctuates slightly, but I'm thinking thats just normal.

So if anyone has any idea what that temp sending unit on the THERMOSTAT CAP goes to, please help me out here. I have looked all over the internet, and it seems only a handful of people have wondered about this, and apparently the ones who know aren't very talkative...
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post #4 of 7 (permalink) Old 12-17-2013, 09:07 AM
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The sensor on the thermostat cap goes to the climate control, when it sees heat in the coolant it turns on the cabin blower fan. With it disconnected you will have no cabin fan, and when you start the car cold with cabin heat selected you will not have a fan until it can blow warm air on your toes.
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post #5 of 7 (permalink) Old 12-25-2013, 06:55 PM Thread Starter
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Originally Posted by matt3140 View Post
The sensor on the thermostat cap goes to the climate control, when it sees heat in the coolant it turns on the cabin blower fan. With it disconnected you will have no cabin fan, and when you start the car cold with cabin heat selected you will not have a fan until it can blow warm air on your toes.
Thank you for the reply. I had wondered if that was what it was for, but that would only be a problem if I used the "Automatic" setting on the climate control, right? Because I have fan now, when I manually press the button. I've never even tried the automatic setting...
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post #6 of 7 (permalink) Old 12-25-2013, 08:48 PM
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I would start with cleaning and inspecting aux coolant pump.
The brushes on electric motors don't have a habit of lasting over 30 years.
Infra red thermometer would make troubleshooting extremely easy.
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post #7 of 7 (permalink) Old 12-25-2013, 09:10 PM Thread Starter
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I would start with cleaning and inspecting aux coolant pump.
The brushes on electric motors don't have a habit of lasting over 30 years.
Infra red thermometer would make troubleshooting extremely easy.
Thanks for the reply. Were exactly is the aux water pump? I've heard about it, but really don't know whey it's even necessary.

Yeah I think I'll pick up an infrared thermometer, make sure my temp gauge and sensor are accurate.

Another thing I should try is burping the cooling system. I've read that this can be done by partially removing the top radiator hose, or a heater hose. I thought about just taking the mono valve out and running the car to burp the cooling system. Any thoughts on that?
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