Simple Oil Pressure Alarm DIY - PeachParts Mercedes-Benz Forum

Jeremy5848 · #1 07-04-2010, 10:47 PM

For less than US$20 just about anyone can build an alarm that will let you know your engine's oil pressure is getting dangerously low. It may alert you to a problem before your engine is ruined. This alarm is designed to screw into a tapped hole in the cover of a Mercedes diesel oil filter canister but could be used in other cars with similar designs. You can also tee into the line to the mechanical oil pressure gauge in some models. It will not work in an engine using a screw-on oil filter unless you can find another place for the oil pressure switch.

I have deliberately kept this design simple and with an absolute minimum in parts so that it is easy to build and inexpensive to buy. It is not a perfect design; there are many ways to make this alarm more useful, more complicated, and more expensive. You may wish to make your own modifications.

The design does not look at either boost or engine RPMs (to see if the engine is above idle and therefore should have "good" oil pressure). This helps keeps it simple and makes it applicable to NA engines and older models without a tachometer (or newer ones with a broken tach).

The alarm activates when oil pressure drops below 1 bar (15 psi). This is lower than most of us would like. For an engine turning 3000 RPM at freeway speeds, an alarm ought to go off if the oil pressure drops below 30 or 40 psi. Without making the system more complicated (by monitoring engine speed or something) I can't do that.

One of the challenges in the Mercedes oil pressure gauge is that it sits at 3+ bars and never moves, except at idle. A gauge that never moves is a gauge that doesn't get watched. If you don't happen to see the gauge suddenly drop (because your oil pump just failed, for example), you won't know until the engine starts making bad noises. Catching a failure at 1 bar may be good enough to prevent serious engine damage.

The alarm has three parts: a bright red flashing LED, a resistor, and a Volkswagen oil pressure switch. It does not have a buzzer but one could be added. The LED is very bright; it should be visible even during daylight and absolutely cannot be missed at night. Here is the parts list:

1. Oil pressure switch, 1.0 bar, VW part number 068 919 081, about $10. This switch was used in Volkswagen TDIs during the 1990s. The one I bought tested at 13 psi. If your engine has a true hot idle oil pressure greater than 20 psi then you may wish to use the 1.4 bar version of this same switch. I found that with the 1.4 bar switch on my engine the alarm would always go off at hot idle so I changed to the 1.0 bar version. Warning: Do not rely upon the oil pressure gauge in your dash as it may be in error. Use a high-quality mechanical gauge to get the true idle oil pressure.
2. Blinking red LED, Radio Shack part number 276-312, about $2.
3. Resistor, 1000 Ohms (also called "1K Ohms"), 1 Watt, Radio Shack part number 271-153 or equivalent, pack of two, about US$1.50. You will need both of the resistors in the pack.

You will also need a 1/8 inch pipe tap and the appropriate drill to make a pilot hole (11/32 inch is good, 5/16 inch will do). A tap handle will make it easier to tap the threads straight but you can use an adjustable wrench if need be. Any good hardware store will have these items. Finally, you will need some wire, wire connectors, tape or shrink tubing for insulation, a female spade terminal, and some Teflon tape for the oil pressure switch.

Under the hood
Begin by removing the cover from your oil filter canister. You can do this project as part of an oil change if you wish; but as long as the engine is OFF, there won't be any pressure in the canister even with the sump full. Because the cover has ribs on the inside, you must drill your 11/32 or 5/16 inch hole from the inside out to miss the ribs. (This was mentioned in another thread over a year ago by Diesel911.)

If you don't want to booger up your shiny like-new cover, buy an extra from a forum member parting out an engine and put the good one away. Before you enter your car in the next concours d'elegance, put the shiny cover back.

Next, put Teflon(R) tape on the threads of the oil pressure switch and screw it into the cover. The 10 mm threads of the switch are not the same as the 1/8 inch pipe threads you made in the cover but they're close. Don't use too much Teflon tape -- you need metal to metal contact between the switch and the cover to ground the circuit. Use a circuit test light or ohmmeter to make sure there is continuity between the body of the switch and the cover. NOTE to experten: Even if you have the proper 10x1.0 mm Metric tap, don't use it. The cover isn't completely flat and the crush washer on the switch won't seal.

Put the cover back onto the oil filter housing. Don't forget the o-ring and make sure you didn't leave any metal chips in the cover. Now put the spade connector on the wire and push the spade connector onto the switch, then run the wire into the passenger compartment through any convenient hole. Start the engine and make sure you don't have any leaks; you're now finished in the engine bay.

In the driver's seat
You will want to put the LED where it will point straight at your eyes when you are driving. It is not bright enough to be dangerous but bright enough that you will not miss it when it blinks at you. (The LED is made of clear plastic but the light is red.) I installed my LED in the instrument cluster next to the oil pressure gauge. This is not recommended for beginners as there isn't a lot of working room and you could damage something expensive. Here's what it looks like (I didn't use the Radio Shack LEDs).

The following diagram shows how to wire the LED and the resistors to the oil pressure switch. Notice that the resistors are in parallel so the effective resistance is 500 Ohms and the power handling capacity is 2 Watts. This is necessary because of the way the switch works: it opens the circuit when it sees low oil pressure. A close-on-low switch would have been more convenient but we have to work with what we have.

When the oil pressure in your engine is above 1 bar (15 psi), as it should be, the oil pressure switch closes and shorts out the LED so it cannot blink. Current will still flow through the resistors and they will get a little warm to the touch but not hot. The current is only about 30 milliamps at 14 Volts, less than a medium-sized bulb would use, and it does this only when the engine is running, so there's no need to worry about the battery. When the LED is blinking the circuit uses even less current

The following is a note on polarity. If you wire the LED backwards it won't work. The resistors don't care, they have no polarity.

Connect the components as shown in the diagram. Insulate the connections with tape or shrink tubing. You may add a 1 Amp fuse to the wire marked (+) if you wish but it really isn't necessary. Connect the wire marked (+) to anything that has 12 Volts when the ignition switch is in "position 2" (pre-glow or engine running). Connect the wire marked (-) to any convenient chassis ground. That finishes building the alarm.

Using the alarm
When you turn the key on in pre-glow, the LED will blink until you crank the engine, it starts, and the oil pressure comes up. Then the LED will go out. You may consider this a "self-test" of the alarm.

If the LED starts to blink while you are driving, it means that the oil pressure has apparently fallen below 1 bar (15 psi). Check your oil pressure gauge. Is it below where you are used to seeing it? If it is safe to do so, turn the engine off immediately and coast to a stop. Remember that without the engine you will not have power-assisted steering and the vacuum-assisted brakes will last for only a couple of pushes on the pedal, then you will have to press much harder. Otherwise, stop as soon as it's safe and shut the engine off.

OTOH, if the oil pressure gauge says everything is OK, you get to ask yourself, "Do I feel lucky?" Remember that if the oil pressure gauge sender or its wire fails, the gauge may say 3 bars all the time. IMHO this is a design failure. Oh well. If the oil pressure switch fails or if its wire breaks, the LED will flash. This is a form of "fail-safe."

That's all there is. I will add additional posts to this thread with "options" (complications, really): keeping the alarm off until the engine starts, adding a buzzer, and adding a mechanical oil pressure gauge.

Enjoy and be safe!

Jeremy

Cadin · #2 01-19-2015, 02:14 PM

This is excellent; kudos!

I know this thread is Ancient, but just a note: For my system, I plan to run a VSV in parallel with the light and the resistors. It will allow vacuum to the vacuum shut-off on the rear of the IP, cutting fuel to the motor.

BillGrissom · #3 01-19-2015, 02:58 PM

Cadin,
A few possible problems with your plan:

1. When the key is in "run" or "start", the vacuum switch vents the shut-off valve tube. Your VSV would need to be a similar SPDT vacuum switch to disconnect the path to the key switch.

2. Your VSV should not actuate while cranking the engine, before oil pressure has come up.

An easy indicator would be to tap into an existing warning lamp, such as the "brake fluid low" or "clutch pads worn" sensors. I recall those simply short to ground to actuate the bulb, so the "low oil" switch could be another short to ground. That is how most 1-wire switches, like in my 60's Chryslers work. Of course, a future owner may never figure out the lamp.

leathermang · #4 01-19-2015, 03:31 PM

On really important warning indicators... and oil pressure is about at the top of the list..on a car.... that it should be AUDIO.... way too many people would not notice a flashing led on their dash until way past the time it would do any good... airplane warnings on serious things are AUDIO.... for this reason...
And I think the driver should be in charge of when the engine turns off....
might mean the difference between getting to the side of the road and out of the way of a big truck have a warning between losing pressure and the car actually not having power....

Junkman · #5 01-19-2015, 06:32 PM

Do you have a source of different pressure switches? I'd like one for fuel pressure on the diesel Ram set to 9 or 10#.

Diesel911 · #6 01-19-2015, 08:24 PM

I used a Buzzer on mine.
Low oil pressure alarm for 617.952
Low oil pressure alarm for 617.952 - PeachParts Mercedes ShopForum

Things people objected to:

They did not like the Sending Unit Drilled into the Oil Filter Cap.

A 7 psi sending unit that trips if the Oil Pressure goes below 7 psi may be too late if you are on the Freeway because the Oil Pressure is way up there at higher rpms and I guess needs to be that high. That means that at 7 psi when the Alarm Buzzer goes on you could still be having Oil Starvation; in particular the Pistons on a Turbo Engine are not getting sprayed with Oil.

In my case the Car operates like 95% of the time within the City and on City Streets so there is a good chance the Alarm could save Me.

CTD · #7 01-19-2015, 08:40 PM

I think an alarm like this could save the engine even when turning over fast--at least in an old naturally aspirated motor. Thinking of it brings me back to a time when I accidentally placed two O-rings (placed the new one over the old one, which I forgot to remove) on the oil filter canister in my 220D when doing an oil change. I did not become aware of this problem until I was climbing a hill in third gear round 45 miles per hour, when I noticed that the oil pressure registered less than 15 psi on the gauge. I took it out of gear to idle, and the oil pressure dropped to 0. I then promptly shut off the engine and coasted to a parking lot, where I was greeted by over a gallon of thick, black, gloppy oil all over the underside of my vehicle. All of this--seeing the low oil pressure at high RPM and shutting down the motor too--occurred in a matter of seconds and the engine made it out OK in the end with no damage I could tell (no changes in oil consumption, idle oil pressure, etc).

Makes me think I'd really like to put together a warning light of this type just in case if my oil cooler blows up or something. Only thing is the engine in my 220D typically idles around 15 psi when the oil is fully up to temperature--I'd like a pressure switch that registers at say 4 or 7 psi so it won't be lighting up all the time.

300D85 · #8 01-19-2015, 09:04 PM

i'd say 'must be' aural and as far as shutting off quick, yes

be aware tho that you could be at a highway cruise speed, and you can end up locking your steering

Junkman · #9 01-19-2015, 09:41 PM

Quote:

Originally Posted by CTD

Makes me think I'd really like to put together a warning light --I'd like a pressure switch that registers at say 4 or 7 psi so it won't be lighting up all the time.

Something visual and audible would be better than just 1 indicator. You may not be looking at the gauges when the light goes on. You may be playing the radio when the buzzer goes off.

My Ram is more likely to have low fuel pressure and lose the $1,000 IP than it is to lose oil. The oil cooler lines on a 617 are 1 possible cause of sudden oil loss that isn't on a Cummins.

There is room to mount a light in the A-Pillar on the Ram. Making or modifying an A-Pillar mount for the SD would be interesting. There is a list on hybridZ that lists alternate pillar mounts that can be modified. I can compare the SD to the Z as I have both.

I've also thought about making a wooden replacement for the ash tray and coin tray for use as a gauge mount. This would work but wouldn't be as visible as A-Pillar gauges.