How does this pressure sensor behave?

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JoeJErnst
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How does this pressure sensor behave?

Post by JoeJErnst »

I'm trying to do some custom A/C controls and I need to know how this particular sensor works. So far it is not as I expected.

Here's what I know:
The sensor is a three-wire pressure sensor on the receiver/drier of a 2002 Mercedes-Benz CLK55. The Denso part number is 499000-7060 and the Mercedes part number is 1408300072. It takes a 5v power supply and puts out a signal on the third wire.

Here's what I don't know:
How does the output signal work? I assumed it would be a 0-5v signal directly proportional to the pressure, but when I bench-test it, I get 5.06v at atmospheric pressure. I was expecting between 0-.25v. Furthermore, when I hook it up to my air compressor (80psi), the output voltage does not change.

Do I have a bad sensor, or is it really a "switch" that will put out a steady 5v until it reaches a threshold pressure, and then goes to zero? Anyone have a clue?

Thanks in advance!
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JohnHere
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Re: How does this pressure sensor behave?

Post by JohnHere »

It appears to be a trinary switch that switches the magnetic clutch on and off at certain low-pressure and high-pressure thresholds to prevent compressor oil starvation on the low end and over-pressurization on the high end.

In a similar manner, it also switches the condenser fan(s) on and off according to a different set of pressure thresholds.

I don't understand when you say that it takes a 5-volt power supply. Everything is 12 volts unless it's a much lower control voltage from the PCM to command-on, for example, a compressor clutch relay.
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JoeJErnst
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Re: How does this pressure sensor behave?

Post by JoeJErnst »

I dug a little deeper into the wiring diagrams and found the test values that confirm my original understanding that this should give a 0-5v output signal.

Now I'm back to believing it's just a bad sensor.
AC Pressure Sensor Diagram.jpg
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JohnHere
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Re: How does this pressure sensor behave?

Post by JohnHere »

In the photo, I can just barely make out the text that says "1.16, Refrigerant pressure sensor signal input, used to protect A/C compressor when pressure drops," and "A/C pressure sensor, +5V, sens gnd, and 0-5V signal" written in by hand. There's also an electrical symbol that signifies an NPN Transistor in the dashed box labeled "B12."

What all of that is supposed to mean to an MVAC technician I'm not sure. But the device in question seems to control, in a very complicated manner, the "on" and "off" cycling of the compressor to protect it from operating only at low pressures and hence, at reduced oil flow. Apparently, it does this by means of a small signal voltage to the PCM, which in turn switches the compressor on and off.

But why does the signal voltage vary between zero volts and 5 volts? And where does the original "+5V" come from?

According to the photos, the device doesn't appear to turn-off the compressor at a predetermined upper limit to prevent over-pressurization.

It's an esoteric M-B design that I think is unnecessarily complex.
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JoeJErnst
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Re: How does this pressure sensor behave?

Post by JoeJErnst »

Sorry for the small print; that's the biggest image the forum software would let me upload.

The +5v power supply is coming from pin 7 of the climate control unit (N22)

I just bought a new sensor and tested it - it works! I am getting .45v out of it at room pressure, and a little over 1.1v when I apply shop air to it. That means it should end up being good to 300+ psi by the time it reaches the 5v max signal.

Now that I have a good sensor, since I am doing my own controls, what would be sane values for a minimum and maximum allowable pressure? The compressor is a variable-capacity unit that I'm sending a PWM signal to, if it matters.
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JohnHere
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Re: How does this pressure sensor behave?

Post by JohnHere »

Glad that you solved the riddle of the switch/sensor.

As for the high and low pressure limits, and presuming that this safety device will be installed on the R/D, they're usually ~405/30 PSI respectively.

Above ~405 PSI on the high side, the compressor circuit opens to prevent over-pressurization. Below ~30 PSI on the high side, the circuit opens again to prevent compressor oil starvation.

You'll need to figure some other way to control the condenser fans because this device apparently doesn't have a third control function, like a trinary switch does.
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DetroitAC
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Re: How does this pressure sensor behave?

Post by DetroitAC »

A sensor is used to control everything high side pressure related by the computer this sensor is interfaced to, ECM or BCM

Condenser fan control strategy will be operated off this sensor value, probably variable speed brushless fan motors, PWM signal controlled or possibly networked. I'd say that is mostly what this sensor output is used for, although they also use a combination of low side pressure, high side pressure, RPM, control valve signal to estimate the compressor torque for engine controls.

Compressor will be controlled based more on low side operating conditions, like an evaporator thermistor. Seems like you've already figured it out, but it is a 400Hz PWM at battery Voltage. I was a system development engineer back in those days and I benchmarked the compressor control strategy of the C class as much as I could for another supplier. We couldn't necessarily tell why they were sending particular PWM duties, but we could measure and document the conditions and outputs. I do remember that their PWM was right on 400Hz, whereas most other cars it's just close, typical German engineering to make an expensive PWM generator that is overly accurate. It's 400 Hz just because that frequency tends to keep the compressor control valve coil from having any noticeable stiction or hysteresis. 350 Hz is OK, 450 Hz is just fine, MB hit 400 Hz +/-1

High side pressure will only control the compressor in fault conditions, like too high/ too low pressure.

I'd suggest the following:
High Pressure Cut out 465 psia, cut back in 300 psia
Low temp disable at ~ 45 psia (30 psig)
JoeJErnst
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Re: How does this pressure sensor behave?

Post by JoeJErnst »

Thank you both for the detailed replies! I'll take all of that information into account as I configure the controls for this thing in a few weeks.
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