Replaced Servo - Now I have no heat - PeachParts Mercedes-Benz Forum

rgnprof · #1 08-23-2008, 12:24 AM

I have had a leaking Servo unit for several months and just got around to replacing it today with an aluminum one (used) - went well, easy install. But, now I have no heat in any settings, including Defrost - A/C works fine. It was 90 degrees here today, but I couldn't get any heat.

Any suggestions - or troubleshooting tips? Never had a problem with heat before this...

ryan

vstech · #2 08-23-2008, 12:31 AM

did you completely bleed out ALL air from the cooling system? they are VERY hard to get all the air out. you need to park the car with it's nose up as high as possible, and open the upper radiator hose, and pour all the coolant in through the hose, until coolant exits the upper radiator fitting. then attach the hose and finish the fill.

Carson357 · #3 08-23-2008, 02:03 AM

if i remember correctly the vacuum lines on the bottom of the unit go on one way only, if they are reverese you will not get heat. this is a cold lockout feature that will not allow to operate until the car is up to temp

rgnprof · #4 08-23-2008, 08:51 PM

I just tried to add coolant and the upper hose is full - I don't think I have air pockets as car seems to be warming up as it should...

The hoses on bottom are hooked up just as they were before...

Any other ideas?

ryan

Carson357 · #5 08-23-2008, 09:36 PM

possibly a bad amp? or the servo you bought may have failed, the aluminum ones also can have issues. after cycling the heat on and then turning the car off, do you hear the servo return to the park position?

rgnprof · #6 08-24-2008, 03:06 PM

I don't hear the servo running at all...

Carson357 · #7 08-24-2008, 03:17 PM

Quote:

Originally Posted by rgnprof

I don't hear the servo running at all...

if it isn't running at all i would suspect the unit is either jammed, not getting power or the amp is defective. check all fuses, i believe there are some inline fuses for this unit somewhere

rgnprof · #8 08-24-2008, 03:20 PM

Everything was working fine before the switch - except, of course, for the leaking! I guess it's possible that I blew a fuse, or the amp, during the installation process. I have checked the fuses at the panel (driver's side), but I don't know where there any other fuses..???

The AC still works fine...

Is there a way to just check the servo - to see if it will even run?

ryan

Carson357 · #9 08-24-2008, 03:22 PM

well it is likely the replacement servo is the issue, as i said earlier the aluminum ones also fail.

rgnprof · #10 08-24-2008, 03:29 PM

Yea, you're probably right - is there a way to check the thing before I just ditch it? I bought it on ebay a couple of years ago - got a good deal, but I'm stuck with it.
ryan

Carson357 · #11 08-24-2008, 03:37 PM

i have never opened one before, you could try to open it and see if the jam is obvious, there is a way to test tthe motor but i do not recall what it is

rgnprof · #12 08-24-2008, 03:50 PM

I found this post- see reply #15 where a procedure to test the servo motor is explained.

http://www.peachparts.com/shopforum/showthread.php?t=194635&highlight=checking+ACC+servo

I tried it and my wires got really hot, really fast! And the motor wouldn't run...

Not sure why they got so hot, but it appears that I clearly have issues with the servo...

ryan

Carson357 · #13 08-24-2008, 03:52 PM

Quote:

Originally Posted by rgnprof

I found this post- see reply #15 where a procedure to test the servo motor is explained.

http://www.peachparts.com/shopforum/showthread.php?t=194635&highlight=checking+ACC+servo

I tried it and my wires got really hot, really fast! And the motor wouldn't run...

Not sure why they got so hot, but it appears that I clearly have issues with the servo...

ryan

if that is the case then the replacement unit may have damaged the amp, so be prepared to possibly have to replace that unit also

rgnprof · #14 08-25-2008, 10:32 AM

A couple of operational questions - 1) does the servo motor run at all when the system is on A/C? I still have good A/C...

2) Also, if there is an air pocket, would this prevent the servo from operating?

3) Will the heat even operate in this car if ambient temps are in the 90's?

4) Would a bad auxiliary water pump contribute to this problem?

I am just not yet convinced that this rebuilt servo is bad - I tested the old one by connecting a 12v source to the leads on the servo (according to the above link), and the old motor didn't run either. Now it's possible that if the servo doesn't operate while the system is cooling, maybe that old servo is/was bad as well - since I haven't operated the heating system since Feb/Mar.

ryan

Carson357 · #15 08-26-2008, 04:24 AM

Quote:

Originally Posted by rgnprof

A couple of operational questions - 1) does the servo motor run at all when the system is on A/C? I still have good A/C...

2) Also, if there is an air pocket, would this prevent the servo from operating?

3) Will the heat even operate in this car if ambient temps are in the 90's?

4) Would a bad auxiliary water pump contribute to this problem?

I am just not yet convinced that this rebuilt servo is bad - I tested the old one by connecting a 12v source to the leads on the servo (according to the above link), and the old motor didn't run either. Now it's possible that if the servo doesn't operate while the system is cooling, maybe that old servo is/was bad as well - since I haven't operated the heating system since Feb/Mar.

ryan

1. depends on the wheel temp, if it need to warm the air yes
2. unit should cycle even if there is an air pocket
3. yes
4. no, the aux pump will only run when temp is below specific temp