300 cd servo nothing but heat

[bfowler71]

Late last summer I installed a rebuilt AC compressor, Expansion valve and drier. I had cool air after installing new compressor late last summer.
Now here is the problem. I installed a rebuilt servo last winter. It is now warm enough to check performance of the AC for cool air. I get nothing but full heat (hotter than ambient temp). The AC system is fully charged and clutch is running. It appears to me it me that it must have a bad Servo. That is only thing I have done to it since late last summer. What do think?

[unkl300d]

Could just be the climate control amp. If its bad, it won't allow the AC to go on.

Check to make sure the contacts are clean and that you don't see 'burn' marks on the outside.

If you replaced the servo with a new one, I doubt it would go bad suddenly. Normally you get no heat with a bad servo. At least to my experience.

[bfowler71]

Thanks for your reply. I had Cataract surgery last week and can't bend down until next week. So, I am, doling a little desk debugging for now. If the climate control amp is bad, would the AC clutch come on? Or would hot air override the cool air? I assume the CC amp sends a message to the Servo to direct hot water or no water or cool air to the cabin. is this correct? If not, you will not make me angry to say...you or wrong...I welcome it.

[LUVMBDiesels]

I have been doing some reading on the servo from back issues of "Star Magazine" it says that they usually fail in the cold position if the valve gets stripped.

When you can take a look at it check it for leakage. if you can pull it, there is lots of troubleshooting info available. I can probably scan some pages from the magazine articles for you.
if worse comes to worse I have a servo that I do not need

[rocky raccoon]

If you have a model with a monovalve check that first. It is the easiest, least expensive and highest probability. Occam's Razor applies.

Failure mode for a monovalve is usually a failed (torn from age and heat) diaphragm. That results in full heat all the time and will overwhelm the air conditioner. I had similar problems on both an 88 560SL and an 83 300CD. If the diaphragm is good, check that the valve solenoid has voltage and is operating. No voltage or failed solenoid also results in full heat.

Repair cost is about $40.00 in parts and 15 minutes of your underhood time. One of the easier fixes you will ever do.

Good luck and come back with your story when fixed. It strengthens the data base.

[bfowler71]

I have a 1978 300cd. Last fall I changed the compressor, drier, expansion valve. Everything worked fine. The servo was leaking so last February I installed a rebuilt aluminum servo. The weather was cold and I did not check for AC. The problem must be intermitting. Last week I drove it for the first time in about four weeks. I drove 25 miles and could only get hot air (hotter than ambient temp). Today I drove it twice, heat and AC worked just fine. I set my temp wheel to 69 degrees and got cool air. Moved it 85 degrees and got hot air. I did this three times and it worked every time. I suppose I will just keep an eye on it for now.
I don’t use the car for primary transportation. Perhaps it is telling me to drive it more often. Thanks for your reply.

[ScooterABC]

Originally Posted by rocky raccoon

If you have a model with a monovalve check that first. It is the easiest, least expensive and highest probability.

+1. I think it's $19 from auto haus.

[unkl300d]

AMP its the amp when my servo broke I did not have hot air.

Prior to that my amp was replaced and the AC came alive. no cool air prior to that.


my servo broke later again and I had no hot air.

I bypassed it.

experiences may vary.
1979 300D

[79Mercy]

With the key in the 2nd position turn the temp wheel and go listen to see if the servo is running after moving the temp wheel from full heat to the full cold, or visa versa.

If it is not running unplug it and briefly run some wires from battery to pins 4 and 5, reversing the polarity just changes the direction of the motor. It should run smooth back and forth, if nothing happens then the servo is dead. When the servo dies it usually takes out the amp as well, but it doesnt happen in all cases.

[unkl300d]

Also if the servo dies and the internal rotating switch stays engaged in the ON position, you will have a battery drain issue.

One needs to use the servo often even if its academic in order to keep it from freezing up.

[cooljjay]

Welcome to the evil servo world!

Your problem is the amp behind the glove box liner.

Your problem is also a disintegrated temp sensor tube, behind the glove box. There is a temp sensor in the passenger side dash speaker, there is suppose to be a tube that runs from the sensor to the blower...it was originally foam and its now dust..

And also there is an aux water pump connected to the servo under the hood, this is 99.9% frozen....this is what fries the amp behind the glove box.