Low oil pressure alarm. - Page 11 - PeachParts Mercedes-Benz Forum

Jeremy5848 · #**151** 02-07-2010, 06:30 PM

Here's something that just came to me . . . what if we use the alternator idiot light as the "trigger" for the oil pressure alarm? A 12 Volt reed relay (drawing very little current) could be wired across the alternator light as shown in the following diagram. The "normally closed" (NC) contacts would be pulled apart as long as the alternator light was on. The low current of the reed relay would not upset the charging circuit. The advantage of this alarm is that it is applicable to any car that uses an idiot light for the charging system. I used a W124 diagram because it was handy but the same system should work for other cars, gas or diesel, Mercedes or not. The only exceptions would be cars that have an ammeter instead of an idiot light, and very new cars that use a completely electronic system.

When you first turn on the key switch and the engine (diesel or gasser) is not yet running, the alternator light [a battery symbol is used by Mercedes] is on. This pulls in the new relay, keeping the oil pressure alarm from sounding. Once the engine starts, the alternator light immediately goes out; since it will take a second or two for oil pressure to build up to 1.4 bar (or whatever), the alarm will sound. Once the oil pressure is up, the alarm stops. This is good -- it is a self-test for the system, every time you start the car.

With the engine running normally, you drive off. Should something happen to cause loss of oil pressure, regardless of throttle position, tachometer reading, boost pressure or anything else, the alarm will immediately sound and you will have a precious few seconds to shut down and pull over.

Note that if you have a true alternator failure, the charging light will stay on and your oil pressure alarm will not work. OTOH, if the alternator bulb burns out, the oil pressure alarm will still work but the charging system may not -- it depends on the current drawn by the reed relay and whether it is enough to self-energize the alternator. Hopefully you will recognize an alternator failure!

Jeremy

Edit: Add diode in series with relay coil.

compu_85 · #**152** 02-07-2010, 07:46 PM

You could also rob the water level switch out of a washing machine and use that to switch to the higher pressure switch... the ones that I've messed with can be adjusted to trip at almost no pressure. I had the idea to connect into the idiot light for low oil level on my car... seems fitting.

-Jason

Jeremy5848 · #**153** 02-07-2010, 08:37 PM

Most of the warning lights in the instrument cluster could be used; they all are powered through the alternator in "test" mode when you first turn on the ignition switch. Once the engine is running and the alternator is delivering power to the various consumers, all of the warning lights go out (assuming their circuits are OK, and excepting the emergency brake warning).

Low oil level won't work in the W124 series as there's a transistor in the circuit. The charge light itself actually shouldn't be used as there's a series diode that would be hard for the novice to wire around. Fuel Reserve shouldn't be used, you might actually be on reserve some time. Brake Fluid/Parking Brake is likewise out. That leaves Low Coolant, Low Windshield Wiper Fluid, and Brake Pad Wear Indicator. The last seems best to me. In the W124 instrument cluster, pins 6 or 8 in the big round 15-pin plug can be used for one leg of the relay and pin 10 for the other leg. Optionally, you can still use the charging system pin 9 if you put a diode in series with the relay coil.

barry123400 · #**154** 02-08-2010, 09:16 PM

Quote:

Originally Posted by compu_85

You could also rob the water level switch out of a washing machine and use that to switch to the higher pressure switch... the ones that I've messed with can be adjusted to trip at almost no pressure. I had the idea to connect into the idiot light for low oil level on my car... seems fitting.

-Jason

That is a good ideal to try. Looks like about 34.00 for the new one to three pound adjustable sensor in single quantities delivered. I got a response today.

Just hooking up a water sensor and temporary circuit will let me know if the low tubo pressure is activating it early enough .We have to keep this very cheap.

I hope and expect the water level pressure switch has a small hose connection . I know it starts the timer and motor when the tub is full. If I remember as well it is custiom adjustable by the washing machine user for different levels of water or they have something that is. Should have good contacts as it may shut the water solinoids off as well. I will get my hands on one.

Thank you for the insite. I suspected there might be something usable out there and this might be it.

I should not disable my wifes washing machine though. If I did I would be in no shape to install anything.

Jeremy5848 · #**155** 02-09-2010, 01:00 AM

Quote:

Originally Posted by rs899

Here's another VW off-the-shelf switch, this one closer to 1 bar. I can't find it from Phil, but its used on Vanagons:

http://www.busdepot.com/details.jsp?partnumber=068919081A

Bus Depot also has the 1.4 and 1.8 bar switches:

Oil Pressure Switch
.3 Bar $3.24 028919081D

Oil Pressure Switch
1.8 Bar $3.68 056919081E

Oil Pressure Switch
1.4 Bar $4.52 068919081

Oil Pressure Switch
.9 Bar $4.18 068919081A

I'll check the local foreign car parts place, see what they have.

Once I have a 1.4 bar oil pressure switch and a normally closed reed relay I can build a prototype. Total cost should be about $10-20.

Jeremy

barry123400 · #**156** 02-09-2010, 02:04 AM

Those sensor prices seem very reasonable. Get something going is a great ideal.

I had forgotten about the 1.8 bar sensor availability as well. Whatever will work and is inexpensive is great.

mplafleur · #**157** 02-09-2010, 07:32 AM

My oil pressure is routinely below 1.8 bar and probably below 1.4 bar at times. I'd have to go with a 0.9 bar at a maximum. That warns me of low oil pressure, but still above minimum. This is for idle.

But at running rpm, we won't have an alarm for "less than spec oil pressure", just a warning if we loose pressure if I am correct.

Jeremy5848 · #**158** 02-09-2010, 11:31 AM

Quote:

Originally Posted by mplafleur

. . . But at running rpm, we won't have an alarm for "less than spec oil pressure", just a warning if we loose pressure if I am correct.

With a 0.9 bar switch you won't have an alarm for low oil pressure in on-the-road driving conditions. For that you will have to use a 1.4 or 1.8 bar oil pressure switch plus one of the ways to detect that the engine is above idle. At the moment, the contenders are the washing machine fill sensor, to detect the onset of boost as the turbo spools up, or a mechanical switch in the throttle linkage. Either one would keep the alarm from tripping if your oil pressure drops below 1.4 bar (but still above 1 bar) at idle.

mplafleur · #**159** 02-09-2010, 09:15 PM

Quote:

Originally Posted by Jeremy5848

With a 0.9 bar switch you won't have an alarm for low oil pressure in on-the-road driving conditions. For that you will have to use a 1.4 or 1.8 bar oil pressure switch plus one of the ways to detect that the engine is above idle. At the moment, the contenders are the washing machine fill sensor, to detect the onset of boost as the turbo spools up, or a mechanical switch in the throttle linkage. Either one would keep the alarm from tripping if your oil pressure drops below 1.4 bar (but still above 1 bar) at idle.

The current method proposed will active the alarm as soon as the engine is running, so the pressure sensor will have to be sized such that it won't alarm when my oil pressure gets to 1.4 bar during the hot summer at an intersection.

The 0.9 won't give me low pressure at highway speeds, but the spec is 3 bar at 3000 rpm anyway. Anything under 3 bar is low pressure.

For a warning that pressure is about to become zero, which I thought wsa the original intent, the 0.9 works without adding circuitry to detect engine rpm, or load.

Jeremy5848 · #**160** 02-09-2010, 09:22 PM

Quote:

Originally Posted by mplafleur

The current method proposed will active the alarm as soon as the engine is running, so the pressure sensor will have to be sized such that it won't alarm when my oil pressure gets to 1.4 bar during the hot summer at an intersection.

The 0.9 won't give me low pressure at highway speeds, but the spec is 3 bar at 3000 rpm anyway. Anything under 3 bar is low pressure.

For a warning that pressure is about to become zero, which I thought wsa the original intent, the 0.9 works without adding circuitry to detect engine rpm, or load.

"About to become zero" may be too late. I understand your challenge; perhaps a method that looks at boost or tach and won't alarm unless the engine is at (say) 1500 RPM or greater AND the pressure falls below 1.8 bar. It's a more complicated way of doing it but will take care of your semi-low idle-speed pressure.

Jeremy5848 · #**161** 02-09-2010, 10:05 PM

The 1.4 bar VW oil pressure switch is on order. All I did was to go to this link, print the page, and take it to the local foreign auto parts store. They have it in their warehouse and I'll have it tomorrow. Their price $6 and change.

While waiting for tomorrow, I redesigned the circuit after discovering that normally-closed reed relays are harder to get. The new circuit (my edited post #151, above) uses a diode, a normally-open reed relay, the VW oil pressure switch, and an alarm buzzer. I'm not positive the diode is even needed; I put it in for CYA purposes.

I tested the reed relay and diode and alarm in my '87 300D Turbo (W124, OM603) this afternoon. With the key on and all of the idiot lights illuminated, the relay sees only about 2 Volts, not enough to activate it, so the alarm remains silent. Once the engine starts and the alternator begins delivering current, the voltage across the reed relay rises to about 14 volts and the alarm sounds. If the VW pressure switch were in the circuit, the alarm would turn off as soon as the pressure rose above 1.4 bar. While driving, if the pressure ever goes below 1.4 bar, the alarm sounds. It is really irritating, you won't ignore it.

My parts (3rd photo below) all came from my junk box or the surplus store but I designed the circuit so that Radio Shack parts can be used, except for the VW switch. The Radio Shack parts are:

1. 12 Volt reed relay, part 275-233, US$2.99 [reed relays draw very little current; I used one specifically to avoid changing the parameters of the charging circuit]

2. 1N914 diode, part 276-1122, US$1.49 [almost any diode will work]

3. Buzzer, part 273-055, US$3.49 [ditto]

Construction hints:

1. Use a small plastic box to mount the alarm. The diode and reed relay can go inside the box. The whole thing can go behind the instrument cluster.

2. The diode must go in the right way or it won't work. In the diagram of post 151, the diode is shown as a symbol with an arrow and a line or bar. The 1N914 diode is a little glass thing with two leads; one end of the glass is painted black -- that is the bar in the diagram.

3. The alternator indicator (a battery symbol) in the instrument cluster of the W124 is pin 9 of connector 3 ("X3" in the diagrams) for the W124. I don't have a picture of the back of a W123 instrument cluster but it is supposed to be pin 7. For other Benzes you will have to get the wiring diagram for the instrument cluster and find the right pin.

4. The easiest way to connect to this pin without doing a lot of rewiring is to use the method shown in the first two pictures below. Twist a piece of solid wire into a loop, put it over the pin, put the connector back in place. You can also take plug X3 apart and solder to pin 9 but it is a 15-pin plug; be careful some of the pins don't come out of their holes and get lost!

I'll address mounting the VW pressure switch later. Does anyone have a good photo of the base of a Mercedes oil filter housing that shows the oil pressure sender (for electric gauges) or oil line (for mechanical gauges)?

Jeremy

compress ignite · #**162** 02-10-2010, 04:05 PM

Here ya go!

[Orange micro-fiber towel used for "contrast"...Sorry about focus on #3]

compress ignite · #**163** 02-10-2010, 04:17 PM

'Seeing "How Much Fun" the OP sender is to access,[and you'd have to fit a
"T" fitting in where the OP sender is now , to be able to keep your OP sender
and the New VW switch]

I'm sorely tempted to look at the Oil Feed Line for the Turbocharger ,as an
install point for the VW switch...(Nightmares about No Turbo Lube!)
(Yeah,scratch that idea ...if anything happened to the VW switch...
the Turbocharger is suitable as a fishing lure.)
(#65 in the Pictogram)

What about one of the Oil Gallery plugs in the front of the engine...
Are they accessible from the EC or buried inside the Timing Cover?
('Thread that baby and pop the VW switch in!]

compress ignite · #**164** 02-10-2010, 04:31 PM

Noble!

Jeremy,

Zwei Fragen nur...

'Warm engine (AND lubricant) @ Idle...will not the OP be below 1.4 Bar ?
AND your excellent,elegantly simple device be alerting you to the fact?

Jeremy5848 · #**165** 02-10-2010, 05:05 PM

Quote:

Originally Posted by compress ignite

Noble!

Jeremy,

Zwei Fragen nur...

'Warm engine (AND lubricant) @ Idle...will not the OP be below 1.4 Bar ?
AND your excellent,elegantly simple device be alerting you to the fact?

Speaking only for my '87 300D Turbo (W124, OM603, about 150K miles), with the engine at "operating temperature" (whatever that means), coolant temp 80C, outside temp about 50F, at idle the oil pressure gauge in the instrument panel reads just below 2 bars, say about 1.8 bars minimum.

Obviously, I could have a flaky gauge and/or sender, but that's what I have at the moment. YMMV, as usual. If the VW 1.4 bar switch thinks my idle OP is below its threshold then yes, I'll get unwanted alarms. We shall see what happens when I get it installed.

I suspected that there might not be a lot of room under there, thanks for the photos C-I. Only way to be sure is to "get out and get under." Wonder what thread the sender is? Guess I'll have to pull it to see.

I'd also like to add an additional mechanical oil pressure gauge, both because I'd like to see what the high-end pressure actually is (since the factory gauge pins at 3+ bars) and also because I just happen to have an 80 PSI (5.6 bar) gauge, brand new ("highest quality," made in Taiwan

) and sitting around in the garage since I bought it 10 or 20 years ago.

What will happen this summer when it's hot outside? Stay tuned!

Still waiting for the call to say my VW switch has arrived.

Jeremy