So, why is it that my MB and my MG have "real" oil pressure gauges, meaning they provide a real time oil pressure reading while driving and it fluctuates. Then, more modern, particularly US cars, seem to have oil pressure gauges that just go to a certain point and stay there, and at least are always in the "N" zone, which is pretty much the whole gauge, rendering them useless unless the damage is likely already done. Can anyne explain the difference ion how thse work, and how the "US" gauges can still be called oil pressure gauges?

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1984 300D turbo, 250K
1971 MGBGT, 101K
2007 VW Jetta, 4K

Hi,

Good question. My old Volvo P1800E had a mechanical gauge. The ones I've seen inside have a Bourdon(sp?) tube which is this coiled up tube that unwinds as you pump oil pressure into it. This motion is geared up to move a needle. The downside of these things is when they spring a leak, you get oil in the dash which is common in W123s. It happened in mine.

My guess is that the electronic ones work using a sensor that has a small diaphragm that deforms under pressure that has a strain gauge rosette in it among other signal conditioning electronics. I saw some made by Honeywell Microswitch and Motorola (now Freescale?) that worked this way years ago. Very cheap. They have good performance and they are "real time" enough that they have to have some filtering on them to damp out the bounciness. What I think you're seeing is poorly designed gauges and electronics in the dash...but hey, what do I know, I've never had enough money to own a car new enough and modern enough to have one of these things.

They are idiot "gauges". Much like their water temp gauges as well. Most will stay in the normal range until you are a few degrees from boiling over.

i also prefer mechanical temp gauges too.just don't trust electronic ones.

I believe the last mechanical temp guage I've seen was on an old tractor! Mechanical temp guages have a bourdon tube that extends all the way from the guage to the block. They are very delicate and the slightest kink in the tube requires guage replacement. The other alternative is a bi-metallic coil guage mounted directly on the engine.

Electronic guages come in various ranges but nearly all are basically a "voltmeter". The difference between most of the guages, similar to ones on our MB's from the 80's, and the new type is that the older type is more linear through out the range. The newer guages are non-linear. This is easily done with electronic circuitry. Both are very reliable. More than likely, the reason most of the newer guages are non-linear is to sort of passify the temperature questions from customers. Have you noticed how many threads pop up this time of year when folks new to these older MB's are a little unsettled when there temp. guages read 98C in 100F degree heat?

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Sam

84 300SD 350K+ miles ( Blue Belle )

If you ever spring a leak at the gauge, you will wish you had an electronic one.

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1982 300CD Petrol/Black Leather

My answer would be the latter are CHEAPER gauges .They really arent much better than an idiot light,mbz gauges are much more accurate,thus you can see the pressure change with rpm and temp. Johnny

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85 300sd SOLD ,85 745i,95 740i,1972 Suburban,1938 International

My electric one has a leak. Lucky for me its only 12 Volts but I keep having to fill up my battery

Trust me the electrics are much more reliable. They work just as well, I believe in 86 MB went to electric as well BTW.
Your debating behavioral designs and not the type. They don't read constant because they are electric, but because thats what the manufacturer designed it to do.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section