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Oil sending unit circuit

Oil sending unit circuit

Oil sending unit circuit

I have an old dump truck. It had a mechanical oil pressure gauge. The line busted and sprayed oil on the exhaust. The resulting fire is something I want to avoid in the future. I bought an electronic gauge that came with a sending unit. It was a cheap gauge, so when it worked a few times and quit, I wasnt surprised. Most of the gauges on the truck dont work, so I am in the process of a raspberry pi / arduino dash for it. It is overkill, but something to piddle with.

While programming the raspberry pi, I had to figure out what the resistance range was in relationship to the pressure. I hooked an air compressor to it. At 0 psi I get a resistance of 9.3 ohms. At 100 psi I get 132.1 ohms. This was out of the truck on a workbench.

I needed to move the truck, so I reinstalled it. I ran my meter to it and to ground. I measured 9.3 ohms. When I turned the key on without starting it, I got 355 ohms. Turn the key back off, 9.3 ohms. Start it and I got a reading of 1.1M ohms.

No wonder the gauge is flipping out. Anyone have an idea why I gain a bunch of resistance by just turning the key switch on?

The ground is a block with a wire that goes all the way back to the battery. There are other ground wires on the block that go to other circuits. Do I need a filter inline with it? Would a diode isolate it and make the resistance correct?


RE: Oil sending unit circuit

Huh. Installed in the truck you're measuring from the case where threaded into the block to a single screw or terminal with a digital multimeter?

You can try a 0.47uF capacitor across the sensor.

Personally I think in an environment that noisy you're going to have a truckload of problems with the Arduino. I'd get a quality pressure gauge and have a working oil pressure gauge immediately. SW

Vehicle gauges (electrical) work via a bunch of current heating a bi-metal strip that bends the pointer in the gauge when the current heats the bi-metal. The pressure sensor is a tough rheostat that rattles around and the heater and bi-metal form an 'integrator' that heavily averages the current giving a nice steady pointer.

That rheostat may well never perform in the voltage service you are going to ask from it for the Arduino service. If you want to persist with the Arduino, pull out your wallet and buy a real pressure sensor. Real 0-150psi Sensor

Keith Cress
kcress - http://www.flaminsystems.com

RE: Oil sending unit circuit

Thanks! I was measuring with a DMM from a terminal on the sender to a ground under the dash. That is the same place the gauge is / was grounded.

I messed with it today. The sender used the threads and engine block as a ground. I connected a hose clamp around the sender with a wire under it. Then measured using that wire as the ground. I got 9 ohms not running. Then started it and read 74 ohms. This equates to about 60 psi. That is exactly what it usually runs.

Then I connected the cheap gauge back to it but used the wire under the hose clamp as the ground for the gauge. It read 0 psi while not running and 60 to 61 psi running. It seems there is a truckload of noise in the ground of the vehicle. A 2 prong or 2 wire sender would be a better choice. The one you provided the link to would be good. Pretty pricey though. Thanks for the help!

I tried lots of ground points just to see of there was a less noisy one. I found none including the negative terminal of the battery.

RE: Oil sending unit circuit

Sounds like the engine block ground is bad. You can't have enough. I ran some 4/0 welding cable (hyper flexible) from my batteries to my block and thru-bolted them to a huge support bracket for the A/C compressor. That corrected my starter issues and some gauge issues.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Oil sending unit circuit

Did you use thread tape on the sensor threads?

RE: Oil sending unit circuit

A cheaper alternative to the sensor listed above would be an oil pressure sensor off some newer OEM vehicle. I know GM has them, a google for "GM oil pressure sensor calibration" brought up an acceptable sensor and the voltages it expects. You would need to supply 5V, since it is not a resistive sensor.

As for the resistance changing when you turned the vehicle on. You only measure resistances using a DMM with the circuit off. Applying external current to the resistance you are trying to read will affect the reading. I am probably misunderstanding what you are doing, but did you disconnect the sensor from your circuit when you measured with the meter and key on/off?

RE: Oil sending unit circuit

I haven't had the chance to rewire the ground to the engine yet.

No thread tape to interfere with the connection. It does have a brass reducer that takes it from 1/4 NPT to 1/8 NPT. I dont think the brass is the issue though. It reads the correct resistance with the key off and probing the sender terminal and any ground location.

I only measured with the sender unplugged when the key was on. The only thing common to the key being on was the ground and that was any ground source on the truck.

Thanks for the info on the GM sensors. I will look into that. I have an old (2005) junk GM car. I had thought about stealing the sensor off of it, but I think it is just a switch. That car never had an oil pressure gauge. It just had an oil light. The Arduino can supply 5v. That should be a perfect fit for the GM sensors.

Thanks again guys!

RE: Oil sending unit circuit

The meter measures resistance by applying a voltage across the circuit. It doesn't take much voltage to mess up the meter. A ground bus for multiple items with a wire running to the battery really isn't that great a ground source.

RE: Oil sending unit circuit

Agreed; you absolutely want a solid, clean, and heavy earth to the chassis from the block. And then the same from chassis to battery. Many more modern vehicles use a woven wire strap for this purpose, sometimes several, specifically because of these sorts of issues with sensors. Especially once everything's soaked in oil, maybe a bit or corrosion seeps in, etc.
Also might be a wise idea to tee in a simple pressure switch as well, such as the one off your scrap car, and hook it up to a light as a backup system. Maybe a buzzer too, if you want to get fancy. It's easy and very cheap insurance.

RE: Oil sending unit circuit

When you wire in the 3 wire sensor to your Arduino, make sure not to share the grounds for that with any other systems on the dump truck. Run all 3 wires back to your PCB. This will be a cleaner signal, since it doesn't use the engine as a ground "return" for the signal like your current setup does.

If you need the connector for it, they are Delphi GT150 series if you want to hunt it on Mouser, or you can find them easily from Ballenger Motorsports or other places.

RE: Oil sending unit circuit

The light and buzzer are a good idea. I hadn't even noticed the gauge quit working before the oil caught fire. Thanks for the idea and thanks for the info on the ground bus. I was trying to clean it up with the ground bus.

@bradrs This is exactly what I am looking for. Do you happen to know a part number or vehicle with a 3 wire oil pressure sensor? Id rather walk into a local parts store and buy what I need for a sensor. Every time I search it, it gives me an oil pressure "switch". I assume that is just a 2 wire to a dash light.

Thank you everyone for the help.

RE: Oil sending unit circuit

Not an electrical solution, but can you replace the lines with stainless braided hoses (or similar)? I used to work in mining trucks where broken hoses causing fires is a huge issue. Engine/Truck OEM's won't change from the rubber hoses, but (at least in Aus) operators were strongly encouraged to replace the factory hoses with more robust ones. The pressure gauge won't prevent the leak from happening without additional intervention.

I can see you're doing the whole dash which makes sense, but maybe the hose upgrade would be a good idea as well.

RE: Oil sending unit circuit

On the older vehicles it was common for a heavy ground cable to run from the battery to the engine block to provide a return path for the starter current. The engine block should be a good ground. Pipe tape on the sensor threads may be insulating the sensor and causing problems.
The motor and transmission mounts do not provide a good electrical connection from the engine block to the body.
Typically there is a flat braided cable from the engine block to the firewall to provide a ground return for the body.
Dim headlights may be a sign that this braid has been damaged.
The old style oil pressure systems that Keith mentioned had two matching heated bi-metal strips. One in the sendor and one in the gauge driving the needle. In the sender there was a bourdon tube with a contact pressing against a contact on the bi-metal strip. The higher the oil pressure the further the bi-metal strip had to bend to "get away" from the bourdon and open the contacts. The bi-metal would then open and close as it moved way from the bourdon and moved back. The higher the oil pressure, the longer the on time of the heating element.
The similar heated bi-metal in the gauge unit would match the action of the sender bi-metal.
The same current flowed through both heaters, making the system immune to voltage variations.
The instantaneous resistance will be either very low or very high depending on when in the duty cycle the measurement is taken.
Some newer systems use a variable resistance. These systems often use a voltage regulator and a lower voltage to make them immune to voltage variation.

"Why not the best?"
Jimmy Carter

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