1987 300SDL heat issues... - PeachParts Mercedes-Benz Forum

MercFan · #1 11-28-2010, 12:35 AM

Now that it's plenty cold here in CO I've discovered that my SDL has some heat issues. For starters, it only has heat when I click the defrost button (left most button on the control) and with the dial control on MAX heat.

The heat starts flowing and soon it's boiling hot. If I then click any other button (leaving the dial alone) the heat will quickly subside and turn cold pretty quickly (with fan either high or low).

So to keep the temp comfortable inside the cabin I have to continuously keep cycling between Defrost button (most left) and Off button (most right)...

I've searched and found references to the Monovalve - I plan on inspecting that first. Beyond that, it seems that there were some references to the vacuum system - specifically the pods behind the dash (sure hope that's not the case, but it seems probable).

Can someone suggest some other checks beyond the monovalve?!

I should also mention that the dash has an aftermarket plastic cover on it - could that be affecting the cabin temperature sensor and confusing the Climate Control Unit?!

Thanks,
James

Brian Carlton · #2 11-28-2010, 09:55 AM

The monovalve is working properly if you can get full heat in any of the positions.

Therefore, your issue is most likely the CCU itself.

MercFan · #3 11-28-2010, 10:48 AM

Does it make sense to take apart the CCU in hopes of finding a broken connection somewhere in the circuit board?!

Also, does the CCU from a 300SD fit the SDL - if so, any particular year?

vstech · #4 11-28-2010, 11:18 AM

the ccu made a change from II to III in the second gen 126's not sure what year it took place. the buttons are further apart on the III's
you can sometimes pull the cover off the ccu and resolder the cold joints and get them working. lots of pics and info on here found with a search I'm sure.
basically, you pry the cover off, and heat and touch a scoch of rosin core solder to each connection.

Brian Carlton · #5 11-28-2010, 12:28 PM

Quote:

Originally Posted by MercFan

Does it make sense to take apart the CCU in hopes of finding a broken connection somewhere in the circuit board?!

Also, does the CCU from a 300SD fit the SDL - if so, any particular year?

Sometimes you win at this game.............sometimes you don't. If you don't, you cannot use the CCU as an exchange if you decide to get a rebuilt.

The CCU from the early W126 is different from the later version. It will physically fit, but the function isn't going to be the same.

LUVMBDiesels · #6 11-28-2010, 12:30 PM

The 1986 and up CCU has more pins in the connectors so the older ones will not work.

The temp sensor in the SDL SHOULD be in the headliner so the dash cover should not be a problem. Have you fiddled with the temp control knob?

Does the temp stay set when you are using AC?

Here is a thought, the temp sensor is driven by a small fan attached to the right side knee bolster under the dash. Take the screws out and and drop the panel and see if the little fan is turning. If it does not move air, the sensor could become confused and think the car is warmer than it really is...

Good luck!

MercFan · #7 11-28-2010, 03:07 PM

Quote:

Originally Posted by LUVMBDiesels

Here is a thought, the temp sensor is driven by a small fan attached to the right side knee bolster under the dash. Take the screws out and and drop the panel and see if the little fan is turning. If it does not move air, the sensor could become confused and think the car is warmer than it really is...

Ok - I've pulled three screws out of that passenger-side knee bolster along with one plastic peg but the panel won't budge... it still feels pretty tight in there... are there more screws or are there some snaps behind it that have to be dislodged before it comes out? I don't wanna yank on it too hard - I've broken things before like that...

MercFan · #8 11-28-2010, 04:23 PM

Quote:

Originally Posted by Brian Carlton

The monovalve is working properly if you can get full heat in any of the positions.

Correct - I double checked it - just in case, and you were right - the monovalve looks fine - at least the diaphragm isn't torn - so I just cleaned the contacts (they were a bit corroded, but that's all).

Not that I didn't believe you... just easy access and I wanted to be sure.

MercFan · #9 11-28-2010, 09:42 PM

Quote:

Originally Posted by LUVMBDiesels

... the temp sensor is driven by a small fan attached to the right side knee bolster under the dash. Take the screws out and and drop the panel and see if the little fan is turning.

LUV -

I've been trying to remove the passenger side panel under the glove compartment - I hope this is what you meant. I's kind of a bear to get it - I've removed all screws that I can see and it's still jammed in there. I'll try removing the glove box first.

brandlj · #10 11-28-2010, 09:42 PM

It sounds like you have a CCU problem and possibly an aux water pump problem. 3 years ago my 86 300sdl had the same issue that you have. I would get heat and then it would just disappear. I could turn the car off and then back on and I would have heat, for about 2 minutes, then cold again. Here is what I found. My aux water pump was drawing too much amperage due to its old age. It would then make my CCU overheat and on the 86 and up 126 CCU's, they shut down and go into defrost mode. These units have a circuit breake built into them and turning off the car and restarting it would reset the circuit breaker till the unit got too hot again.
I fixed this issue by first checking my monovalve, it was good. I replaced the aux water pump, but the damage was already done to the CCU. I replaced the CCU with a used one from a wrecking yard. It did not work. I ended up going through 3 used CCU's and they all had issues. I finally bit the bullet and bought a remanufactured CCU and all was well and continues to be to this day.

MercFan · #11 11-28-2010, 09:54 PM

Quote:

Originally Posted by brandlj

I ended up going through 3 used CCU's and they all had issues. I finally bit the bullet and bought a remanufactured CCU and all was well and continues to be to this day.

Thanks brandlj -

1) Is there a way I can tell if my CCU is having issues?

2) Where did you end up buying the remanufactured CCU and approx how much did you pay for it?

THx,
James

MercFan · #12 11-28-2010, 11:27 PM

I suppose that's the general location where I should start 'digging' for the aux. water pump, eh?! Looks like all those brackets and hoses need to come off along with the battery/tray? Don't even see the plug.

Wonder if these hoses will survive the 'surgery' in that area...

Brian Carlton · #13 11-28-2010, 11:44 PM

Quote:

Originally Posted by MercFan

I suppose that's the general location where I should start 'digging' for the aux. water pump, eh?! Looks like all those brackets and hoses need to come off along with the battery/tray? Don't even see the plug.

Wonder if these hoses will survive the 'surgery' in that area...

That's the place. The pump is directly below the two hoses in the photo. The plug is over by the battery. The pump is supported by a bracket that is attached to the firewall which is just barely visible in the photo.

The battery and tray do not need removal, although removal of the battery will give you better visibility from the side.

Jeremy5848 · #14 11-28-2010, 11:48 PM

If you are getting full heat in Defrost, the aux coolant pump is probably working.

MercFan · #15 11-28-2010, 11:50 PM

Quote:

Originally Posted by Jeremy5848

If you are getting full heat in Defrost, the aux coolant pump is probably working.

According the the other poster above the pump may be drawing too much current... which has an affect on the ACC. Not sure I can get in there to measure the current it draws...