Low oil pressure alarm. - Page 13 - PeachParts Mercedes-Benz Forum

Jeremy5848 · #**181** 06-09-2010, 04:00 PM

Initial tests with the VW 1.4 bar switch suggests that my factory oil pressure gauge reads high. The VW switch was first tested with my Mityvac and switched at roughly 1.4 bar as designed (using air pressure, not oil, I hope that doesn't make a difference).

When installed in parallel with the factory sender, the VW switch acts at about 2 bar on the factory gauge, suggesting that my factory gauge reads too high. (The mechanical gauge in the picture is defective and will be replaced with a new VDO gauge, allowing a more accurate comparison.) I may need to use a 1.0 bar VW switch instead of the 1.4 bar switch.

I looked first at installing the switch package down at the oil filter housing but there isn't a lot of room to work so I ran an oil line (see first picture) up to the left inner fender next to the blue flying saucer. This puts all of the components where I can get at them (see picture #3).

This oil line comes with some risks which I have hopefully minimized by running the oil line inside protective tubing. In any case, the risk is no higher than those of W123s and others with mechanical oil pressure gauges, plus the oil line does not go into the passenger compartment. I do not see any difference in the factory oil pressure gauge except for slightly slower response due to the length of the tubing.

The second picture shows the components that I assembled, using a modified brass 1/8" pipe thread 4-way fitting. The factory sender, VW switch, and mechanical gauge all receive pressure from the oil line that connects at the oil filter housing. One leg of the brass 4-way must be retapped 10x1.0 mm for the VW switch and the opposite leg must be drilled out and tapped 12x1.5 mm for the factory sender.

It still might be possible to install the VW switch and the factory sender in a tee at the oil filter housing and I will try that once I have the correct fittings. My main concern is that shifting of the engine due to acceleration or worse, collapse of a motor mount might send the modified plumbing into the side of the engine compartment. There just isn't a lot of room down there.

Jeremy

barry123400 · #**182** 06-09-2010, 09:28 PM

Looking good. This may become the general basic installation for many. Tedious perhaps but well worth the effort. Congratulations.

It certainly has to be easier than dealing with the results of running with no oil pressure because you missed looking at the gauge. Those external oil lines to and from the oil cooler are perhaps the weakest part of the oiling circuit design. It probably takes very little time for the engine to pump itself dry if one really bursts. Or some road hazzard cuts into the oil cooler or oil pan. Two gallons of oil at 60 pounds pressure through lines or holes that size might be less than a minute. Plus the oil pump chains are getting pretty old on many of these cars.

I am pretty well certain as time goes by you will further refine it if required. It is an american expression I believe. You are the man.

Jeremy5848 · #**183** 06-09-2010, 10:05 PM

My primary concern for the DIYer is the three- or four-way splitter for the oil pressure line. Drilling and tapping for the metric threads must be perfectly straight or the crush ring on the VW switch and the Mercedes sender won't seal properly. It can be done in a garage with an inexpensive drill press and a companion vise but it takes some patience. I may look into having someone make up a bunch of fittings, see if I can get the unit price down to something reasonable. Everything else is pretty simple.

funola · #**184** 06-09-2010, 10:11 PM

Another souce of a signal to control the oil pressure alarm is the EGR computer. Some models have it and some don't, which will add to the confusion. I know the 83 240D has an EGR computer because I have tapped into its tach circuitry to drive a 300D turbo tach. An EGR computer typically looks at RPM and use the signal to control the EGR solenoid.

funola · #**185** 06-09-2010, 10:17 PM

Quote:

Originally Posted by Jeremy5848

My primary concern for the DIYer is the three- or four-way splitter for the oil pressure line. Drilling and tapping for the metric threads must be perfectly straight or the crush ring on the VW switch and the Mercedes sender won't seal properly. It can be done in a garage with an inexpensive drill press and a companion vise but it takes some patience. I may look into having someone make up a bunch of fittings, see if I can get the unit price down to something reasonable. Everything else is pretty simple.

The VW pressure sensors will thread right into 1/8 pipe. Clean the thread well with brake clean and use epoxy. It will seal and will be fine. Keep the number of fittings/adapters to the absolute minimum to keep the leverage down.

Jeremy5848 · #**186** 06-16-2010, 12:59 AM

My VDO mechanical 100 psi oil pressure gauge arrived today so I installed it in parallel with the factory electric sender. There definitely is a problem with my factory gauge. I don't know whether it's the sender, which is relatively new, or the gauge (I've tried two with similar results).

At idle with the engine warmed up (but not "hot") the factory gauge reads 2.5 bar (~37 psi) but the VDO says 20 psi. I tend to believe the VDO because it agrees with the VW 1.4 bar oil pressure switch.

Has anyone else had experience comparing an "external" oil pressure gauge to the factory gauge in any Mercedes?

Jeremy

Diesel911 · #**187** 06-16-2010, 03:22 AM

I can only speak in general that the Gauges that come on regular cars from the Factory are not as accurate as add on Gauges or when you connect up a test Gauge.

Even the Cheap add on Gauges are not precise.

On old Mechanical type pressure Gauges the Boudon Tube inside becomes less flexiable over time and sometimes even crack.

Also if you had a handfull of New Sending Units made by different companies and tested them you would find some that will not show the same reading on the Gauge as others.

Jeremy5848 · #**188** 06-23-2010, 09:04 PM

My post #174 in this thread is in error (but why can't I edit it?). Turns out the oil filter cannister DOES see full system oil pressure -- I thought it didn't because my cheap mechanical oil pressure gauge (now in the garbage can) was defective!

My new VDO 100 psi mechanical gauge works great in the filter cover of my 1987 300D Turbo (W124, OM603). At cold startup, system pressure is 60 to 70 psi (4+ bars) while at hot idle it is about 20 psi (1+ bar). Give the engine some throttle and the pressure immediately jumps up to at least 50-60 psi.

As several others have noted, the factory gauge is more of a "serving suggestion" than a "real" oil pressure gauge. Assuming the new VDO gauge is accurate (and how would I know?), I plan to adjust the needle of my cluster's pressure gauge so that it reads more like the VDO. That and the low-pressure alarm (I'm almost finished!) will give me a bit more peace of mind.

Jeremy

whunter · #**189** 06-24-2010, 12:43 PM

Quote:

Originally Posted by Jeremy5848

My post #174 in this thread is in error (but why can't I edit it?). Turns out the oil filter cannister DOES see full system oil pressure -- I thought it didn't because my cheap mechanical oil pressure gauge (now in the garbage can) was defective!

My new VDO 100 psi mechanical gauge works great in the filter cover of my 1987 300D Turbo (W124, OM603). At cold startup, system pressure is 60 to 70 psi (4+ bars) while at hot idle it is about 20 psi (1+ bar). Give the engine some throttle and the pressure immediately jumps up to at least 50-60 psi.

As several others have noted, the factory gauge is more of a "serving suggestion" than a "real" oil pressure gauge. Assuming the new VDO gauge is accurate (and how would I know?), I plan to adjust the needle of my cluster's pressure gauge so that it reads more like the VDO. That and the low-pressure alarm (I'm almost finished!) will give me a bit more peace of mind.

Jeremy

Technical Edit per your request.

Jeremy5848 · #**190** 06-24-2010, 03:39 PM

Quote:

Originally Posted by whunter

Technical Edit per your request.

Thank you.

Jeremy5848 · #**191** 06-27-2010, 07:22 PM

Quote:

Originally Posted by Diesel911

I can only speak in general that the Gauges that come on regular cars from the Factory are not as accurate as add on Gauges or when you connect up a test Gauge. . .

Agreed. I just completed testing both the electric oil pressure sender and the gauge on my '87. The sender (relatively new - installed a year or so ago) measured resistance values close to what the FSM said it should for various pressures, using my Fluke 8800 DVM.

The oil pressure gauge (meter movement), however, (originally out of an '88 gasser -- I wanted the vacuum gauge) reads about 1 bar high across most of its range, based on a comparison with my new VDO mechanical gauge. This is confirmed with the VW oil pressure switch, which switches at 1.4 bar when the electric gauge reads about 2.5 bar.

All of this assumes that my Fluke and VDO are accurate, of course, but you've got to start somewhere. Even the possibility that the gauge reads high makes me want to have something else watching the oil pressure.

The alarm design is pretty much done. I want to have a working version in my car before publishing it to the forum. It uses three parts from Radio Shack and the VW switch. Total cost about $15.

I'm also working on a coolant high-temperature alarm that will use only two components at a cost of about $3. Sixto, among others, feels that it's even more important than a low oil pressure alarm, at least for the 603 engine.

Jeremy

samboyellowsub · #**192** 06-30-2010, 12:02 AM

has anyone tried wiring the low oil pressure alarm up to the factory buzzer that is behind the dash? it seams like it could be easier than installing a new one from Radioshack (that would probably be made in china). Any thoughts on this?

Jeremy5848 · #**193** 06-30-2010, 02:46 AM

Quote:

Originally Posted by samboyellowsub

has anyone tried wiring the low oil pressure alarm up to the factory buzzer that is behind the dash? it seams like it could be easier than installing a new one from Radioshack (that would probably be made in china). Any thoughts on this?

An interesting idea but the people who flew Apollo 13 would not agree with you. Having one alarm for several functions leaves you wondering which problem is being announced. It also leaves you with the risk of having the alarm on for one problem and then another problem occurs. You don't know about the second problem because it is trying to use a buzzer that's already buzzing. Better to have a separate alarm or at least have an alarm buzzer coupled with a set of lights to tell you which problem is causing the alarm.

In any case, the alarms that I am about to publish here use bright flashing LEDs; they should be bright enough even in daylight, especially as they blink about 2X per second, that you shouldn't miss them. A buzzer can be added but I'm trying initially to keep the circuit as simple as possible for the non-electronic types in the forum.

The factory buzzer behind the instrument cluster already has several functions (key in ignition, lights left on, seat belt). Although anything is possible, it would be a complicated job to add another function. If you really have to use a Mercedes part, get a spare "N2/4 Warning Module" from a junkyard car and use its buzzer.

Jeremy

samboyellowsub · #**194** 06-30-2010, 09:04 AM

if after hooking an alarm up to the factory buzzer, I just can't imagine riding down the highway, hearing the alarm, and thinking, "oh crap! did my seat belt come undone or did my oil cooler hose break!?!" What I'd probably do actually, is use the terminal for the seat belt alarm! I really don't need a machine to tell me my seat belt is on or off.

is that warning module another buzzer from a mercedes? the same part that I'm talking about, maybe?

Jeremy5848 · #**195** 07-04-2010, 10:49 PM

I've just posted the design of my low oil pressure alarm here.