Trying To Find a 4 Wire Pressure Transducer


Thread Starter

Rob Armstrong

Hello all,

I am in no way an engineer or anyone even close to that, but I am in dire need of some expert assistance.

I have inherited an Ingersoll Rand screw compressor, and I am connecting it to run my shop. I have determined that the one air pressure transducer on the machine is bad (will not read any pressure on the digital display unit). Since the original replacement part is not only very scarce but also ridiculous money, I need to find a suitable replacement.

I have contacted a couple manufacturers but cannot seem to find the right one.

Here is what I know:

The transducer is a small sealed unit with 1/8" NPT connection and a fixed wire lead.

There are four wires coming out of it, red, black, green and white.

When I test for voltage between the red and black (at the control panel board Connection), I get 5 volts DC.

When I test between either the red and white or black and white, I get 2.45xxxx volts dc; and same for the red and green or black and green.

If I test between the white and green, I see nothing.

If I place a jumper wire between the black and white (or green), I can see a reading of 17psi on the display, and it tricks the compressor into thinking there is pressure and allows it to run.

I have found that the pressure range is 0-225 psi.

No matter how hard I search, it appears that no one has any 4 wire pressure transducers on the market. Does anyone know of any other alternative to replace this factory original? Could I make a 3 wire work? Is there anyway to verify what the output of this transducer should be with it not working? I know there are all sorts of different outputs, so I need to be sure I'm getting the correct one.

Any and all help would be super appreciated!

Thank you,
What is the part number or info on any label on the pressure transducer?
Can you determine the transducer output electrical range from the digital indicator? Any info or part number?

If the Tx has 4 wires and a 5 VDC supply then the output could be 1-5 VDC, 0-5 VDC, 0-20 mA or 4-20 mA. Testing the digital indicator with a power supply or calibrator could determine the electrical range.

Yes a 3 wire Tx will work, you need to determine a common connection between the 5VDC supply and the digital meter.

A quick google search finds this. Does it match?

Electronic syppliers do sell similar pressure transducers it is just a matter of knowing the pressure range, 2 or 3/4 wire, input voltage and output range.

Good luck

Rob Armstrong

Thank you for the reply. Unfortunately there are no numbers or labels left on the original transducer so I can't get any info from that.
I don't have any way to test the computer display unit to tell what the sensor output should there any other way to find that information? The link you found is for a newer machine with three wires.....I would think that could work if I figured out how to hook it up as a here wire.

Rob Armstrong

I have a wiring diagram for the existing sensor I can email to anyone if they can help......I can’t figure out how to post the photo here.

Get a pressure guage, and add an adjustable pressure switch if you need a pressure cutoff for pressure tanks.

basically an old-fashion approach.

but, if you want to preserve the units total functionality, grab your wallet.

I stand corrected on using a pressure switch with a screw compressor.

It would defeat the benefits of a screw compressor in the first place, power savings.

Since the transmtter is more than likely part of a system that controls the compressor speed and loading, any after market device not designed for your system will just bring a collection of challenges and waste your time.

good luck,
One of the first things I would do is trace the wiring from the transmitter back to where all the wiring goes. Finding out the power supply voltage specification is necessary when finding a direct replacement or when changing the loop to 2-wire or 3-wire loops. It is also necessary to determine what the output signal is and where it goes. Document what you found. It is likely that the power supply will be 12 VDC or 24 VDC as you would know if it was 120 VAC from your voltage test but be careful. It is unlikely that if you have a 5 VDC power supply that it will support a 0-20 mA or 4-20 mA signal. By hooking up a variable DC power supply to the indicator or a process calibrator to it to test for 4-20 mA signals, and you should be able determine the output of the transmitter. If you have a pressure gauge on the transmitter inlet (always a good idea), this might provide a clue as to what the transmitter should read when the compressor is running. You also block out the transmitter and hook up a process calibrator and a calibration pump, and vary the pressure into the transmitter to see what happens.

The white & black wires are likely your signal wires (positive & negative polarity is hard to determine just from the color of the wire as there is no standard but a voltage measurement can determine it). Most instrumentation is protected against hooking up the DC wiring up backwards. The green wire is likely the power common or ground, while the red wire is the loop power wire. There is no sure thing on this as some people seem to go out of their way to design or hook stuff up weirdly. When you got a reading when you put your jumper in, this may indicate that you have a current loop. Read the current through the jumper while running (or instead of a current measurement, you can put a 100 ohm resistor and measure the voltage across the resistor to get the current, less resistance if you don’t get 17 PSIG). You may be able to correlate it to your transmitter range, e.g. 4-20 mA = 0-100% = 0-220 PSIG (e.g. (Measured Current – 4 mA)/16 mA * 220 PSIG equals 17 PSIG?). If you have not done so, measure the voltage between red and green, which is likely your power supply voltage if the loop is working properly. Note that it might not be the transmitter that is creating the problem. You can then talk more intelligently to manufacturers about a replacement.

If you still don’t understand the hookup, look at these websites for some information on 4-wire transmitter hook ups.

You might also look at the below transmitter website below for a 4-wire pressure transmitter (I have not used this manufacturer’s transmitters). Some other manufacturers that might have 4-wire pressure transmitters are Wika, Setra, Dynisco, & Ashcroft. Talk to them once you have figured out what is going on.

If you are still lost, find yourself a competent instrument engineer and he/she will figure out what is going on and if you have to convert to 3-wire or 2-wire loop, he /she will know how to do this. There are many third parties out there that can provide this type of service to help you for this type of thing.

You should document your loop with a loop diagram with all the pertinent instrument information and how the loop is hooked up. This way if you have a problem later on or need to periodically calibrate the instrument, you will have the information to do so. This is true for all your instrument loops. Documentation is one of the more important things to have to achieve successful maintenance.

Hope this helps out.

William (Bill) L. Mostia, Jr. PE
ISA Fellow
Principle Engineer
WLM Engineering Co.
Provide your email address and I will send you a link to a dropbox folder where you can upload the wiring diagram.

>I have a wiring diagram for the existing sensor I can email
>to anyone if they can help.
Some here are assuming it's a regular instrument e.g. 4-20 mA. I suspect it's no more than a simple strain gauge 5 Volt feeding the bridge on Red/Black with millivolts out White/Green. It may be one of the cells that produce a strain gauge like output from a silicone chip. Perhaps the strain element has become unstuck (gone to zero). Have you measured the White/Green wires from the sensor while it's under pressure?

You will need to match the cell exactly. Somehow we need to find out what the mV to Volt range is for the display can you borrow or make a millivolt source? A wiring diagram would certainly help.

Re reading the thread it seems you need a millivolt source to inject the display. A suitable source can be made using a 1.5 Volt battery a 10k resistor and a 1k potentiometer from local electronic supplier. With the potentiometer and fixed resistor in series across the battery you will get 0 - ~136 mV from the potentiometer, use that to inject the display reading the millivolts with your meter. Perhaps get several resistors in case you need to have a different mV range.

With this simple apparatus you will be able to figure out how many millivolts is equivalent to 225 PSI then calculate the range of cell you need.

Rob Armstrong

THANK YOU Everyone for so much great information!!!!!

I have some good photos of the sensor, wiring diagram and wires etc. if you would like me to upload them to a Dropbox I am happy to do so to give you some better information.

My email is [email protected]

I will definitely do a bit more testing per your advice to get all of the data for you all.
The sensor plugs into what is called an “intellysis” control unit which controls all of the compressor functions, speed, loading/unloading etc. AND it somehow does all of this with only two sensors in the entire machine, a temp sensor for the oil and this pressure sensor which is connected to a valve so it can switch between sensing sump pressure or output pressure,,,,thus the importance of this silky sensor.

I was also thinking like some of you had mentioned about injecting some type of power into the control unit to see what the computer is looking for as an input to change the psi numbers.......I’m not sure I have your savvy to make a power there something simple I can get that would allow me to test the voltage without blowing it up?

A 4-wire transmitter is usually noted as having a separate power (i.e. plus and minus) supply and an a separate analog output (i.e. plus and minus). I work with all of these all of the time.

What I understand you're describing to me is either a 2-wire (4-20 Ma) or a 3-wire voltage transmitter. If it is a 2-wire it will be using only two wires(typically the red and black) and the green wire ground, and the white wire is not connected. Some manufactures do this so they can use the same housing and wire, they just terminate it differently in the housing depending if it is a current or a voltage output.

If it is being used in the voltage output configuration all wires will be used. Red for positive supply, black for minus supply, white for signal output, and green for ground. NOTE: The green is a drain wire or shield and is not connected to anything but helps shield eternal electrical interference.

If all 4 wire are connected then this is a voltage transmitter (sensor). These come in a variety of outputs. 0-5, 1-5 0-10 are the most common.

I agree with the previous comments the best way to find out is to simulate the input with a voltage input. Probably just wire a potentiometer to the input of the control board / terminals.

one source below:

0-225 PSI is a strange range. It may be using a larger range but only limiting the top end. I do this all of the time.


Terry Wilfong

This is probably a little late...
Your OEM IR Part# should be 39853809 if your in a bind.
I’m Looking for a replacement for mine now.
I refuse to pay 550 bucks for something that should cost $100 to $150.