No heat 300SDL - intermittent
 

Hello all,

About 2-3 weeks ago, my heater started working "intermittently". Here's the data:

-Heater control set to 22 degrees (about 72 deg F). Also tried it with temp wheel clicked into 'MAX' setting.
-Button for 'ECON' selected
-Fan blower on 'AUTO'
-Engine temps seem normal (same as what they have always been every winter).

I start the car, and the heater works for a period of time. Not sure, but it seems like 10-20 mins. Then it starts going intermittent. If I stop & restart the engine, I can get heat to come out again for a while.. something is "resetting" when I turn the key off.

When it goes nuts, here's what happens: blower remains normal (usually at full speed since it's on AUTO), cool air comes out of vents. It's not outside temperature air, but it's not lukewarm either. Feels like about 40-50 deg F if I were to take a wild guess.

Any ideas? I tried unplugging the monovalve and drove for 15 mins. Seemed to still do the same thing (it may be in my head, but it seemed to go into it's nutty COOL mode more than usual)

Ready to pull the monovalve insert out & have a look.. but could this be the climate control???

Any way to bypass this for the time being? I thought unplugging the monovalve would do it (shouldn't it go into full HEAT?) but it didn't....

Thanks!
Neal

I would pull the mono out and take a look - that is the cheapest route(and mine requires a gentle blow with a rubber hammer occasionaly) However, your wild and intermittent behavior $ounds like the CCU:o. Check that mono valve and we will all light some candles and hope for the best.

What TX said.

TX76513 & Brian,

You guys are AWESOME!

Pulled out the insert and the diaphragm had a tear in it. Ordered the new part and I should have heat tomorrow.... :vbac47679

Thanks so much for your help!

Neal

Good the candles worked. Be careful, don't drop those little screws down into the cavern:P

Yep, TX knows what he's doing most of the time.:D

Help is what we do here.

B. C What a great idea. I am going to sell my Monovalve Adjusting Tool on EBAY:D
http://www.tradetiler.com/acatalog/rubber-hammer.jpg

That's perfect.

Will it come with instructions on how to use it?:D

I think I might need to buy one of these. I have no heat this morning in the 300SDL.

Stick a MB logo on there and up the price $40. ;)

Hey guys, I know that this is an old thread, but I am now experiencing the same thing with my SDL. I had the Monovalve rebuild kit installed this summer by my Indy, so that should not be the issue. Is there any way to troubleshoot the CCU? I suspect that it is the problem. It seems to work fine to control the fan speed, etc. when it is in AC mode, but no matter what I do, I can't get a drop of heat out of this beast. I also had all of the vacuum pods replaced in the dash as well, so they should all be working well too. I'm up in Indiana now and it's getting cold quickly! HELP PLEASE!

Rob

It has been almost 10 years since I got rid of my last W126 but I do remember the model quite well given how long ago it was :)

If you have rebuilt the monovalve and checked the rest of the system, it is almost certainly the climate control unit. This assumes your engine warms up properly (which means the thermostat is opening). If you aren't getting heat with a warmed up engine, it's most certainly the CCU.

Some things to check first.. is the blower motor working fine? If you turn it to cooler air and put the blower on high, do you get a rush of cool air? It could be that you have heat but since the blower is dead none of it blows into the cabin. First make sure the blower is working fine on high.

If that's not the problem, considering that winter is coming I'd look into getting your CCU rebuilt by a reputable company. I think most can do it in a week and ship back to you.

There are also CCU units on eBay for $50. You might want to buy one (or two!) and hope that one works perfectly. You can always keep the other as a spare, send it to be rebuilt, or just re-sell on eBay if you don't need it.

Check with the rebuilders if they have one on the shelf ready to go. They may sell you one with a core charge, then you send yours to them so they can rebuild it. These panels are pretty common so most rebuilders should have them.

That way you only do one swap.. you get a rebuilt one shipped to you, install it, then pop yours back in the same box and mail it back. Easy to do.

I'm sure there is a way to jump the potentiometer/rheostat ("temperature wheel") so it sends the full hot signal. But you'd have to look at the wiring diagram to figure it out. Might be more trouble/risk than it's worth. Even then, your unit could still receive the full hot signal from the temperature wheel, yet it's broken enough that you still won't get heat.

If the blower and thermostat are good (engine heating up properly) then I think this is the last part that you could realistically consider to be the problem.

Where did your Indy get the repair kit? If its a MTC brand unit that is likely your trouble. See my post titled "126 monovalves again."