possible to bypass servo and have a/c? - PeachParts Mercedes-Benz Forum

kenstfelix · #1 05-18-2014, 08:34 PM

i bypassed the servo and the heat works. i was going to put a valve in so i could turn on and off flow to the heater core.

would it even possible to get the a/c working if the servo is bypassed?

uberwasser · #2 05-18-2014, 08:43 PM

Sure. Close the valve to the heater to keep the heater core cold, and then flip the Compressor switch to turn on the A/C. Of course you'd have to have good refrigerant pressure, a working compressor, etc. but the servo bypass doesn't directly affect the A/C.

kenstfelix · #3 05-19-2014, 12:16 PM

great. i will give this a try. by compressor switch do you mean the a/c on/off switch in the car?

according to the service records that came with the car, the compressor was replaced and it was converted to 134a in 2001.

uberwasser · #4 05-19-2014, 12:30 PM

Yes, that switch. I forget, because on my '79 the switch says "COMPRESSOR" above it. But on most it just says "A/C" or something similar.

All that switch does is trigger the compressor relay, which is located under the black plastic box on your driver's side fender under the hood, to send 12v power to the compressor.

On later W123's they changed it so the climate control would turn on the compressor automatically unless you specifically told it not to, by using the "Economy" setting. On the earlier style like you have you had to manually turn it on if you wanted anything cooler than outside fresh air.

Graplr · #5 05-19-2014, 10:00 PM

Quote:

Originally Posted by kenstfelix

i bypassed the servo and the heat works. i was going to put a valve in so i could turn on and off flow to the heater core.

would it even possible to get the a/c working if the servo is bypassed?

Yes, this is currently how my 79 TD is set up. I used 2 3/4 PEX tees and a 3/4 PEX ball valve in place of the connections at the servo. I teed the two main hoses together and still had the flow running to and from the heater core. Then I installed the ball valve in line to the heater core. A/C works fine. I think there is some sort of newly developed delay for the system coming on but if I really need A/C I hit defrost (since no heat it just blows cooled air) and then switch to another one. I assume it is because the servo sensor can't detect the heat from the coolant.

If I want heat I just have to open the hood and open the ball valve. Of course if I want the heat to stop I would have to open the hood again and shut the valve but I only drive in the summer so it is not a big deal to me.

cooljjay · #6 05-20-2014, 04:21 AM

You may be able to get ac as mentioned above by using the deforst but I have heard that if your servo system isn't working, that it won't direct out the cool air....

Graplr · #7 05-21-2014, 12:25 AM

Quote:

Originally Posted by cooljjay

You may be able to get ac as mentioned above by using the deforst but I have heard that if your servo system isn't working, that it won't direct out the cool air....

Mine does direct out cold air. It works on Auto Hi and Lo as well. Just not in the first few minutes of driving. It seemed similar to me to the amount of time it took for the servo to kick the heat on (which I believe is a thermostat in the bottom of the servo) so that is why I assumed it had to do with that delay.

cooljjay · #8 05-21-2014, 02:52 AM

You may also be getting a delay in the system because your sensor tube is gone and it is unable to sense the air to get a temp reading. The delay in heat is totally normal, as you say its due to the thermostat. I have read, if you don't want this delay with heat....simply connect the two lines that run to the bottom of the servo together.

Graplr · #9 05-21-2014, 02:56 PM

Quote:

Originally Posted by cooljjay

You may also be getting a delay in the system because your sensor tube is gone and it is unable to sense the air to get a temp reading. The delay in heat is totally normal, as you say its due to the thermostat. I have read, if you don't want this delay with heat....simply connect the two lines that run to the bottom of the servo together.

My tube is there. A PO replaced it with rubber hose.

kenstfelix · #10 05-21-2014, 11:09 PM

i added a valve today and it seemed to work! it was about 60 degrees out so it might just be the vent but before i had only hot air all the time so that's nice. definitely doesn't blow very powerfully. the center vents don't do much at all so it's mainly that left vent cooling my arm. i felt it the most with the defrost setting. we'll see what happens when it's hot out...

Graplr · #11 05-22-2014, 11:10 AM

Quote:

Originally Posted by kenstfelix

i added a valve today and it seemed to work! it was about 60 degrees out so it might just be the vent but before i had only hot air all the time so that's nice. definitely doesn't blow very powerfully. the center vents don't do much at all so it's mainly that left vent cooling my arm. i felt it the most with the defrost setting. we'll see what happens when it's hot out...

Get a cheap thermometer from Harbor Freight and stick it in the vent. I have them in all my cars. It is an easy cheap way to monitor the climate control system.

1'' Pocket Thermometer

Was the A/C working before you bypassed the servo? Visually check that your compressor is coming on by popping the hood and looking for the clutch to engage while you have the AC on. Why did you bypass the servo in the first place? When you put the valve in, specifically where did you put it and how did you plumb the bypass?

Graplr · #12 05-22-2014, 11:29 AM

Here is a diagram of how I plumbed mine. I removed the coolant flow from the servo (due to a leaking servo for the 3rd time! And I only drive it in the summer. If I drove it every season I may have done something else) Now it is plumbed so the coolant will always flow from the tee on the right to the tee on the left and then circulate back to the engine (or I guess radiator upon full operating temperature). The ball valve, when closed will not allow coolant to flow into the heater core giving no heat in the cabin. When the ball valve is open the coolant will flow both towards the heater core and follow the path to the other tee giving heat to the cabin.

kenstfelix · #13 05-23-2014, 11:55 PM

When I got the car the servo was leaking, so I immediately bypassed it, by just connecting the hoses on either side of the servo. I added a valve to the hose on the right side. It definitely did something because I do get cool air now and it was always hot before. Still hasn't been hot enough outside to tell if the compressor is working.

Where do I look to see if the clutch is engaging? I've looked at the compressor but didn't know what to look for.

On my old 300d the servo was hooked up and not leaking, but the air was always incredibly hot.

unkl300d · #14 05-24-2014, 01:45 AM

Perspective. If your servo leaked into the head and oxidized the armature , it may be permanently on and causing a battery drain or permanently off.
If the servo head is dry inside and operational then it will still operate in conjunction with the AC amp behind your glove compartment. You will have AC.
If the amp is bad, then no AC as well.

Correct me if I am wrong. Best.

Graplr · #15 05-24-2014, 06:01 PM

Quote:

Originally Posted by kenstfelix

When I got the car the servo was leaking, so I immediately bypassed it, by just connecting the hoses on either side of the servo. I added a valve to the hose on the right side. It definitely did something because I do get cool air now and it was always hot before. Still hasn't been hot enough outside to tell if the compressor is working.

Where do I look to see if the clutch is engaging? I've looked at the compressor but didn't know what to look for.

On my old 300d the servo was hooked up and not leaking, but the air was always incredibly hot.

Be careful... I think you may have put the ball valve in so that you stop the coolant flow? I can't tell from your picture but is the coolant returning in another place? I teed my two hoses together so that when the ball valve is closed the coolant can still flow.

On the front of the compressor, when the A/C is NOT turned on the "front middle plate" also known as the clutch will not spin. When the A/C is ON the clutch will spin with the pulley. There is an electromagnetic charge which attracts the metal plate on the clutch to the pulley and thus engages the pump of the compressor. So if your A/C is working the clutch will be spinning. That is what you look for.