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  #16  
Old 02-28-2003, 12:48 PM
JimF's Avatar
'94 S500: only 793 sold!
 
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Mercedes specs that. . . . .

the oil pressure should follow the table below:

RPM ---------------- Oil Pres (bar) -------- Oil Pres (psi)
600 - 750 ------------ 0.3 - 1.5 --------------- 4.4 - 22.0
1000 ----------------- 0.7 - 3.0 -------------- 10.3 - 44.1
2000 ----------------- 1.6 - 5.0 -------------- 23.5 - 73.5
2500 ----------------- 2.0 - 5.5 -------------- 29.4 - 80.9
3000 ----------------- 2.5 - 5.5 -------------- 36.7 - 80.9
4000 ----------------- 2.7 - 5.5 -------------- 39.7 - 80.9
5000 ----------------- 2.8 - 5.5 -------------- 41.2 - 80.9

So as you can see, the oil pressure has a wide leeway!

The oil pressure gauge in the IC, has a 'microprosser' (more like a regulator) than controls the meters range on the high end. On the low end, there's a peg that stops the needle.

I suspect you "broke" the meter when you removed, fixed or replaced it. It should limit at 3 bar, since as you can see, the oil pressure can reach some pretty high values.
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Last edited by JimF; 03-02-2003 at 11:37 AM.
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  #17  
Old 02-28-2003, 02:08 PM
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Since you said your mechanic measured 80 psi, I think your gauge may not be incorrect. While this may be in-spec, I know of no other MB that runs that high. I may be concerned that there is a blockage down line of the oil pressure sending unit. Thus causing the high pressure. This may have the effect of not providing enough oil to the rest of the engine.

Anybody elses thoughts on this?
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  #18  
Old 02-28-2003, 02:22 PM
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That the table I am looking for!

Thanks Jim. Would have been nice if this table also included the resistance values you’d get out of the pressure sender to see how mine correlates with the spec. My resistance reading at 80 psi is ~285ohms. In any case, what started this whole issue for me is that I had my needle drop to “0” bar few weeks ago while I was cruising at 80 mph. Without going back to bore you with details, It wasn’t oil pressure problem, nor was it oil pressure sender problem either, it came down to the oil gauge itself went south on me! After fiddling with the gauge for days, I was able to get the needle to sweep again after taking a soldering iron to few solder joints around the meter’s coil. (good ole shotgun approach). Of course I had to remove the needles and gauge faceplate to get to the pcb and can’t see how I could have broke the meter if needle sweeps, but again, Murphy’s law always applies in these scenarios. You gave me few things to think about and it’s time to hit the bench again for more work. Just to let you know, in the mean time, I noticed that whenever I change the value of a resister that sets across the meter’s coil I could control the upper sweep of the needle. So with 285 ohm resistor load I was able to figure that it would take approx 100 ohms resister to go up to 3 bar. The original resister value was 49.9 ohm. So I removed the 49.9 ohm and soldered the 100 ohm resister in and now I “pretend” the gauge is back to normal… and it is behaving like it should with bar reading changing as per oil pressure and all. I am still looking for root cause of this and my only concern up until this point was if 80psi was acceptable, and your table answered that one for me. I can live with the fix I have, but knowing myself I gotta find out exactly what happened… by the way, I learned a lot from you guys about many other things related to oil viscosity, pressure, oil change intervals etc… that is way this site is top #1 for me and others.

And by the way Jim, Your Cool Harness is doing its thing just like a charm... My aux fans are about to get going now that we're getting some hot weather.
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  #19  
Old 03-01-2003, 01:34 PM
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'94 S500: only 793 sold!
 
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I think you have the right . . . .

combination with the resistor (100 ohms). If it pegs at 3 bar, then 'button-it-up'!

Personally I've never taken one apart so I don't know what the 'regulator' is; that device that keeps the meter movement pinned above 3 bar.

At this writing I can't see how a resistor can serve as a regulator but can see how a transistor would perform this.

My tech has some older ICs available. Next week I'll check the oil gauge out and let you know what I find.
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Last edited by JimF; 03-01-2003 at 01:39 PM.
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  #20  
Old 03-03-2003, 05:44 PM
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'94 S500: only 793 sold!
 
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Today, I found an older IC from . . .

a '87 -' 89 560 V8 at my tech's shop. Note the 'bad oil gauge' hand written on the back.

The pic shows that it appears to be an electrical coil (large white area) with a 'trimmer' coil to set the 'calibration'.

Have not yet found a 'schematic' of it so am at a loss to explain. First one I've ever seen. Looking closely, it appears that the trimmer coil (shunt?) is burned.

Does yours look like this?

The other IC was from '82 Mercedes and it was mechanical with a direct connection on the back of the IC for a line from the engine.

The failure in this one (also bad oil gauge!) was leaking oil!! Imagine the complaint . . " I'm getting oil dripping down my leg. . "!

The oil pressure line directly hooks to the oil gauge and drives a small plunger which works against a large coiled spring to drive the indicator needle via some mechanical step up linkage. Almost like a watch!!!

Good idea except for the direct high pressure oil connection to the back of the IC!!! Could see why Mercedes engineers re-designed that idea.
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Last edited by JimF; 03-04-2003 at 12:19 PM.
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  #21  
Old 03-04-2003, 11:45 AM
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Good Pix Jim.

My camera can't get close up pix like that! Other wise i would have attached a picture worth a 1k words . In any case, My gauge is different from the one you're showing in a couple of things: 1) mine is electrical gauge and no oil pressure cable connected to it; it simply have a wire that comes from the oil pressure sender that carries resistance value that correlates with oil pressure and the gauge needle sweeps up/down according to resistance value. 2) My gauge does not have the trimmer coil like shown on your picture. The trimmer coil was removed after 1992 As i have similar gauge made in 1990 looks like the one you show, but my 95 gauge has a resister soldered on the main gauge coil and the original value of this resistor is 49.9 ohm and changing this resister value with your max load (in my case was ~270 ohm representing max oil pressure at initial start up) would result in needle calibration. So yesterday, I removed the 100 ohm and added a 108 ohm resister and now I get a perfectly lined up needle on 3 bar at initial start up where i was getting a little higher than 3 with the 100 ohm and a way higher than three (see picture on my initial post) with the 49.9 ohm. So I can live with this fix (0.07c). As for the question of why my oil gauge started to act this way to start with, that's another issue I will deal with it later after i test the oil pressure myself mechanically. My tech already did that for me and told me it was ~80 psi then drops to normal range after warm up. I don't see leaks or any engine abnormalities.
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  #22  
Old 03-04-2003, 12:26 PM
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'94 S500: only 793 sold!
 
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Still haven't found a . . .

schematic of the unit. I'm pretty sure that your unit has a 'transistor' whereas the gauge pictured above does not.

But still would like to know how a 'resistor' can serve as a regulator. A resisitor is a linear device, so how it can function as non-linear device (as a transistor), escapes me.

If I come across a modern day schematic, I'll post it here. Glad you fixed your problem. It does appear that all oil gauges have a definite failure problem. Who'da thunk it????
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Last edited by JimF; 03-04-2003 at 05:38 PM.
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  #23  
Old 03-04-2003, 01:07 PM
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I'd love to get a copy of the gauge schematics if you ever find one.

The part number for the gauge module is 124 542 43 01. The only schematics I have is for the whole cluster P/N 124 542 12 69 and not for a specific gauge. It kinda helped me find my way around to power the unit on the bench to perform the calibration without having to keep plugging it back and forth in the car ... Let me know if you need to take a look at the schematics I have and I can send you email with it... as soon as i find the soft copy. By the way, do transistors come in a resister-like form? Meaning do they have color bands etc? The 49.9 ohm resister had about five color bands, and then i took it off and measured it and took it to parts store and they confirmed it was 49.9 fixed value resister.
Take care.
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