MD3B - Looking to Add Oil Pressure Gauge

[stxray]

The MD3B engine in my '76 A25 Deluxe does not have an oil pressure gauge; only an idiot light and an alarm. The alarm looks to be an add-on.

I would like to add a pressure gauge but I'm bewildered by plethora of options.

If you've already done this upgrade or can help, I could use some help.

Do I need to replace the existing sending unit or can it be reused?
- If yes, how do I find the right gauge?
- If no, which sending unit and gauge can I/should I use?
If I connect up a gauge, can I keep the idiot light and alarm operational?

Any help you can offer would be greatly appreciated.
rs

[Beta Don]

The gauge you buy will require it's own matching sending unit, so you're going to need to add another one. More important than the gauge will be keeping the 'idiot light' *and* the alarm - You'll be looking at the gauge 1 or 2 percent of the time and if you develop a problem, it will be the idiot light and the alarm which will warn you before it's too late

Most fiittings for oil pressure gauges are BSP threaded - British Standard Pipe. You'll need to remove the sender(s) you have now, screw something into the engine block you can add a Tee fitting to, add your new sender to one end of the Tee and your old sender(s) to the other end

My personal belief is it's way more time, trouble and expense than it's worth - The light and the alarm are much more valuable than any info you're going to get with the new gauge. Does your car have an oil pressure gauge? I'd bet not. Detroit quit installing them many years ago and most other manufacturers followed suit. An alarm and a light afford way more practical protection for the engine than any gauge

Don

[stxray]

Thanks Don.

I do prefer a gauge to an idiot light, even in my cars. However, this sounds like more work than I anticipated. I think I'll back burner this for now.

Thanks,
rs

[Chuckatuck Ray]

if I had to choose between having the pressure gauge or having the light and alarm, I'd go with the light and alarm. My reasoning is that the odds of me having my eyes on the pressure gauge at the time of a failure in the oil delivery system is about nil but more than likely I'd hear the alarm and probably see the light. I'm lucky that my engine is set-up with the pressure gauge, idiot light and alarm. I hope you're able to get your system set up the way you like it without much trouble.
Ray

[WillieC]

I will be scrounging around for a simple gauge to screw into my spare engine block just so I can see what the pressure really is. Accuracy will get extra points for me. This is for the spare engine I am planning to install this fall. Once I know what I've got and where the light cuts in and out, I would be happy with the idiot light. As to the alarm, remember the old A25s were not so sophisticated. My audible consists of "OH SH___T! LOOKIT THAT!"
Ray, I think it is worth your while to know what you've got there. Remember, I am the fool who thought my temp gauge was fine. Until I found it was 50F off.
Running the correct wiring to a sending unit and picking up the correct wire at the instrument cluster is not a bad project considering all the work you hope to accomplish on your lovely nearly unused 25. You will learn a lot in the process. Spend some time thinking about your gauges instead of the cluster I have: VDO tach, Noname temp, Stewart Warner volts. And the first voltmeter I got didn't work out of the box. Supplier replaced with no grief, but really? SW no less. Others please weigh in on good brand suggestions. Having them all the same will give you the same night backlighting, whatever you choose to go with.
Or rip them all out and go with the multifunction display(s) (which I read many are rethinking that idea.)

Now that I am thinking about this winter's projects, having audible alarms is sounding like a real good idea. Does anyone have a simple sketch for wiring such alarms? Does each device have its own alarm piezo or bell or what? Or is one audible wired to several senders? Remember our old Pentas came with compression release for hand starting.

[Beta Don]

Sometime 30 or 40 years ago, manufacturers discovered that excessive oil pressure had more to do with rod and main bearing failures than normal wear - The oil was actually washing away the bearing surfaces faster than the engine was wearing them. Around the same time, modern oil technology produced better lubricants that didn't need the excessive flow that high pressure provided to keep the metal parts from wearing against one another and engine manufacturers began building engines which lasted twice as long using half the 'normal' oil pressure. This alarmed many of us old timers who looked at the gauge and saw only 10 or 15 pounds of pressure at idle and maybe only 15 to 20 pounds at max RPM. Complaints about the low pressure to dealers resulted in the end of installed oil pressure gauges in most vehicles - If you don't know what the pressure is, you won't be bugging the dealer because you think it's too low

Even over the road large diesel trucks run very low oil pressure these days, but the old engines which would maybe need new bearings after 3 or 4 hundred thousand miles are now going a million miles or more with the better lubricants and the lower pressures

Any newer car, truck or boat you find which does have an oil pressure gauge will be very different from what we were used to in the old days - Now the new gauges all have an 'L' on the left hand side and an 'H' on the right. Most of those 'gauges' are not really gauges at all - They're not hooked to a real pressure sender . . . . they just use a low pressure on/off switch which makes the gauge read smack in the middle (no matter the actual pressure) when the switch is closed - An idiot light with a needle on it, in essence. The engine manufacturer doesn't want you to know what the actual pressure is. Don't believe me? Look at that 'gauge' and see if the needle moves whether the engine is idling or running full tilt - Same indication all the time . . . . smack in the middle!

You idiot light should be lit and the alarm buzzer sounding off before you start the engine - If the light goes out and the buzzer quits when the engine starts, you're good to go. Use a good diesel rated oil - RPM Delo 400 or Shell Rotella and know that your antique diesel is much better lubricated now, no matter what the pressure is, than it ever was when it was new. Seriously, I wouldn't give it a second thought. Knowing what the actual pressure is will cause you more worry than not knowing. If the light is out, the pressure is just fine

Don

[WillieC]

Well said, Don. Great synopsis of oil history. And right on with Delo 400 or Rotella. I started using Delo in my ancient VWs back in the day, but they started leaking so I went back to Havoline. Good stuff.
Idiot light it is! So how do the audible alarms work? Same contacts paralleling the light? A relay using normally closed contacts?
Thanks again for your knowledge and advice.

[Beta Don]

On ancient systems like you'd find on a 35 or 40 year old boat, the light and the alarm are usually tied to the same pressure switch on the engine. The switch opens around 5 psi and turns out the light which also silences the buzzer

Later vehicles may use different variations of the light/buzzer. Here's a good explanation of how things work on VW vans made after 1986

http://www.gowesty.com/tech-article-details.php?id=42

The light tells you one thing, the buzzer something completely different

I have a 1994 Mazda Miata sports car which came with a real oil pressure gauge, perched right at the top of the dash between the speedometer and the tach. It reads from zero to 90 psi and of course it reads less pressure at idle than it does at any other RPM. In 1999, Mazda made a big change and it took the enthusiasts quite a while to catch on to what had happened. Same gauge (or at least it looked the same) and in the same location, but now no numbers - Just reads L to H instead of the actual pressure, just like the temp gauge does BTW, and the 'pressure' always stays right in the middle of the L and the H - No idea what that pressure actually is

Somebody got to tinkering with it and discovered why the 'gauge' never moves - L when the engine isn't running, smack in the middle when it is. What they discovered was it's just a low oil pressure switch and when the switch is closed, the 'gauge' reads smack in the middle. Since it's a car with a major enthusiast following, someone worked out a way to turn that OEM pseudo-gauge into a real gauge by installing a real sender on the engine which varies with pressure changes and then modifying the 'gauge' in the dash cluster so it can track the changes reported by the new sender. Still no idea what the actual pressure is though

Miatas are driven by lots of techie types who aren't hesitant to make changes to the car - Here's one guy who not only changed his pseudo-gauge to the real thing, but he added an idiot light as well

http://www.miata.net/garage/opg2/Miata_ ... ng_LED.pdf

The 'adapter bushings' mentioned in his parts list are needed because the engine block is threaded with BSP threads and the VDO sender is threaded NPT, so you can't screw the sender directly into the engine

These pseudo-gauges are very common on pick-ups and the few other car models which actually offer a gauge, or rather what looks like a gauge. On trucks, which seem to have more 'gauges' than cars, they're probably only there because there are guys who say "I would never buy any truck which didn't give me a real gauge! - I don't care for idiot lights!" If only they knew how their 'gauge' actually functions!

You know if a modern sports car doesn't have a real oil pressure gauge, there's not much hope for regular cars and trucks! I'm an old fashioned guy who kinda likes to see a real gauge sitting there on the dash, but on a modern engine it doesn't really tell you much anymore - The pressure relief valve on the mechanical pump in the engine is now engineered to respond to the weight of oil you're using and the temperature of that oil to more or less keep that new, lower pressure right where the engine manufacturer wants it, but it still seems scary low to many of us

On an antique boat diesel which now has much, much better oil in it than was available when the engine was designed, the engine is much better served with a light and a buzzer because it will alert you far sooner than a real gauge which you only occasionally look at. As much as *I* like gauges, if I have to choose between a gauge and a light, I'll take the light any day

Don

[WillieC]

Don, this is from the article you cited.

", while on my back in a parking lot, with some basic tools. "

This is why I no longer own VWs. At least in the A25, I am on my knees, working below my feet. A vast improvement.

[stxray]

Wow, what a great discussion.

Don, I guess I didn't realize how "old school" was I am. Yes, I see the advantages of a light and buzzer and that was why I was looking to keep them while adding a gauge. The gauge, I believed, would allow for better insight into the health of the engine; kind of like a doctor taking your blood pressure.

I think the other reason I liked a gauge is the absence of data when all is well. A gauge gives a continuous readout of the engines status where as a light and buzzer only speak up when there's a problem. But, you make a good point. What good is a gauge if the data it's providing isn't a good indicator to begin with?

I guess it comes down to trusting the system by first making sure the light lights and the buzzer is buzzing before starting and then trusting all is OK if all is quiet.

Thanks to all for taking the time to comment. You saved me a lot of work and have made me a better, more knowledgeable boater.
rs