The air conditioning works intermittently. When the outside air is really hot and the cabin temp is up, it does not work. Have been told it is the controller Bosch XXXXXXXXXX that is the problem but not sure. Where is the temperature sensed that tells the a/c to go on/off? This is a 1992 400E.
Country: United StatesMake: Mercedes-BenzModel: 400E
Had the system checked for pressure leaks etc and OK
Hi, the a/c temperature is sensed with the evaporator temperature sensor in the evaporator. Click here for a description of the sensor and the location. Your part number on the controller did not come through. Can you post it again with the letter A at the front of the number?
A XXXXXXXXXX Bosch
You need to put the A as part of the number.
Ok, that's the pushbutton switch.
Has the shop determined that the compressor is turning off when it starts blowing warm?
Yes, and thumbwheel that sets the desired temperature
I think so.
What determines that the compressor goes on/off. Is it the controller I listed?
No, it's not the controller. Let me pull up a wiring diagram.
Ok, it gets a little complicated on this model.
The pushbutton switch sends an a/c request to the Base Module through the refrigerant pressure switch.
The compressor has a speed sensor in it as well that also influences the compressor operation.
Is it possibly the Evaporator Temperature sensor that is faulty and causing intermittent on/off?
Wher is the base module located?
Yes the evaporator sensor is a possibility but does not usually fail on this model.
The base module is located under the hood on the passenger side in the module box.
Has the shop completely evacuated and recharged the a/c system yet?
Ok, it's very important that they evacuated and recharged the system and didn't just check the pressures and assume it to be ok. Low refrigerant is the number one cause of erratic cooling. The next thing to be absolutely sure of is that the compressor clutch is not engaged when it's blowing hot. If the compressor clutch is engaged you are chasing the wrong problem.
No the compressor clutch is not engaged when blowing hot.
Ok, then they need to jump the refrigerant pressure switch in the drier and see if the clutch engages.
Recently I drove 250 miles and the a/c was on the whole time. I stopped for a visit where the oat was 102 degrees. When I restarted the car, the a/c would not come on. This is quite typical.
And they also need to test for power at the compressor clutch to see if there is power to the clutch wen it will not engage.
This does not sound like a bad evaporator sensor.
This sounds more like a bad compressor clutch or refrigerant pressure switch.
Or possibly the control unit. They just need to see if they have a compressor request signal to the refrigerant pressure switch when the compressor will not turn on.
I think they did jump the switch.
It was thought the control unit, the number I gave, you was the problem, but would it be intermittent like I have stated?
It can be intermittent, especially if the interior cabin temperature goes way up.
So, there is a sensor in the unit?
No, just an electrical contact.
Then why does it not function when the cabin temp goes up? Should that Bosch controller be replaced?
They need to test for the signal from the controller at the pressure switch. If there is no signal when it's acting up they should replace the controller. They tend to act up in extreme heat due to the circuit board in the controller expanding.
OK, I'll go with that. Thanks for your help.
You're quite welcome.
Bosch Certified Master Tech. Over 35 years experience.
Replaced the controller and the problem still exists.
Hi, sorry that you are still having trouble and sorry for the delay in replying. I'm a little under the weather today. Did they do any testing on the controller or did they just try a new one? Have they determined that the compressor is not running when the air is blowing warm?41103.995287963