Joseph_H

Started with oil pressure gauge gone bad (dropped to 0).. Replaced oil sender and still bad gauge. Finally found gauge at eBay and replaced it. Now gauge shoots way up above 3 bar (see picture below) when oil warms up the gauge barely moves down to just above 3. What could it be? Bad gauge or sender? The other gauges (fuel and temp) seem to work just fine. During start up of engine, my oil pressure reads about ~80PSI or about 285 ohms in resistance.. it usually drops after 10-15 minutes. Is that too high?

bamberger_1

Hi Joseph, it seems this problems get out of proportion. What did you repalce, the whole cluster, only the left clock or just the oil pressure unit? To me it seems just not correctly calibrated. However, an other option is, which I dont know details about, whether or not MB has different oil gauges for the V8 and the I6. If that's the case than you'll receive false readings if the gauge belongs to an I6 (my guess only). Compared to the I6, 285 Ohm and 80PSI seems high, but it may be right for the V8. Sorry but I dont know the values for the V8's

Joseph_H

only. Although the part numbers of both my gauge and the one i bought are the same, but mine seems to be newer version 94 while the one i got from ebay is 90 version and the circuit boards are layed out differently. I think this may be the reason for different oil gauge reading. I am trying to see if I can fix mine.

I am still curious as to the high oil pressure reading (285ohm and 80PSi). what causes the oil pressure, oil pump?

bamberger_1

Hi Joseph,
sounds wierd. I just converted the readings of 80PSI to bar and it turns out to be the equvalnet to 5.5 bar. 1 bar are 14.5 PSI. It would mean the pressure readings you get, are almost double--- strange!!?? However the resistor reading is in line with your pressure reading considering the documented res. values for the lower pressures.
The pressure coming down (during idle) when the oil gets hot is normal. I have the I6 and it comes down from 3 (cold engine) to ~1.5 (hot engine) idling
High pressure readings might have different causes. But in general it calls for clogged "arteries", like if your patient has high "blood pressure". As far as I know the oil pump is mechanically driven by the timing chain and normally if the pump wheels are worn the pressure drops. If the cranck shaft bearings are shot the pressure drops. The smallest oil vessels are within the hydraulic valves and the cylinder head.
How new is your oil filter?
I'm pretty much out of thoughts what else could be the culprit.

Joseph_H

No change in pressure reading.
I think there could be a couple of things causing this: 1) I had ordered a new oil pressure sending unit and when i got it it looked different from the original one i took off, but was able to install it and plug the wire to it fine. the new one didn't have the rubber plastic peace that's supposed to pop out if oil pressure is too high or something. the new sending unit is all sealed but with correct screw fitting and wire plug. I got it form an online place so i am not sure if it is a new replacement for my older unit or not. I can never get a hold of these guys to ask them.
2) The new gauge i installed had different circuit board and it is older than my original one thus maybe the higer resistance for my V8 reads higher on this gauge (if it was made for E320 or even 400E model but with same part number as my E420 gauge). I can't think of anything else.

bamberger_1

1) hard to tell whether or not the sensor has different resistor readings than the original ones. But the resistor Ohm reading you get and the pressure reading somehow correspond to the values you would expect when the sensor resisotrs are linear growing with the pressure.
2)the different lay out normally does not chance the function it might only be necessary if electronic parts are discontinued or the supplier change properties of them. As long as the part nr. remains it should not cause any problems since the part number describes the functions of the assembly.

I tell you I'm pretty much lost in the dark.

Joseph_H

I was finally able to fix my original oil gauge. I took it all a part and spent an hour tracing every line, wire and component with an ohmmeter. I did some solder touch up in few places around the oil gauge and one of those must have been a cold solder joint. The needle now started to work. Also, before i put everything back together, i decided to do a bench test on the gauge by adding a resistor load to # 4 pin on the connector that mates with the oil pressure sending unit to simulate the resistance load that comes form the oil pressure sender. There is also a fixed value resistor of 49.9 ohms that sets across the gauge's coil. So with all that in hand, i was able to understand the circuit better and now i can manipulate (or calibrate) the gauge to make it work the way i needed it to. I added 280-OHM resister across Pin 4 of C2 connector and GND. I also left the 49.9-Ohm resister in place. Now when I apply 12VDC to pin 1 (Gnd) and Pin 6 (VDC). The oil gauge needle goes all the way up past bar 3 (similar to what you see in the picture i first submitted above). I changed the load resistance to a 180 ohm and the same thing happened. I dropped the resistance down to about 100 ohm before I got close to the 3 bar reading. So that didn't make sense because my oil pressure was giving me about 280 ohm reading and i had to stick with this value as max resistance value. I then removed the 49.9-ohm resister from the circuit. now the needle dropped down to just above the 0 bar. So with my 280 ohm load in place, i started to change resister values to replace the 49.9 ohm resister. With 100 ohm resister, and same load, i would get the needle to go up to 3 bar. So i replaced the 49.9 resister with a 100 ohm one and put the gauge back in the cluster and test-drove the car. The gauge now seems to work normal. Here is what happens now: I put the key in ignition, the turn to position 2 (battery voltage) the oil needle goes to about 1 bar. I turn ignition on and the needle goes up to 3 bar. I drive the car for few minutes until engine/oil is warm and the needle will stay at 3 bar as long as my tach is above 1000 rpm or so. it drops to about 1.5 - 2 bar when i am at idle the back up to 3 bar when i press gas and go... sounds normal?
Otherwise everything else seems fine. Of course I still don't understand why the oil pressure went up and i had to change the factory 49.9 ohm resister to a 100 ohm resister to accommodate the initial 280 ohm load that comes in from the oil pressure sender (representing ~80 PSI). Please see if you can answer the following question for me: It all looks fine to me except i don't remember if the needle should go up to 1 bar when key turned to position 2 before starting the engine. Is this expected?

mleskovar

Can't resist this thread anymore. Do I understand it correctly that you fixed the original gauge by adding resistance to the loop? Have you tested and measured the pump pressure/output?

Joseph_H

he tested the oil pressure mechanically and it is around 80 PSI and goes down when oil warms up. I am in process of ordering a mechanical oil pressure gauge and the fittings to do it myself. However, i did test the resistance output myself (as you know resistance is direct result of pressure in this case) and reads about 285ohm initially and then drops as oil warms up. It still doesn't make sense why the gauge would pig all the way up beyond the 3 bar reading and me having to change a resister value to fool the gauge and make it not go beyond 3 bar. The fix i put in place seems to work fine and the oil pressure gauge goes up and down as it supposed to... I am lost. But at least know i have oil pressure. I have no leaks or any indication that my oil pressure is too high outside normal other than the readings i describe above!!!

mleskovar

"It still doesn't make sense why the gauge would pig all the way up beyond the 3 bar reading and me having to change a resister value to fool the gauge and make it not go beyond 3 bar." I agree, unless 1. The new sending unit is the wrong one, or 2. The original gauge is faulty.

Joseph_H

Do you have a way (access) to a spec indicating what is a 4.2L engine supposed to put out in terms of oil pressure and the corresponding resistor value reading from the sending unit? That will tell me if the 80~ PSI and 285 ohms are acceptable or not. Otherwise i am still planning on testing the oil pressure mechanically myself without the sending unit and if it still shows 80psi then it is not the sending unit and i'll live with the adjustments i made to the gauge. thanks.

mleskovar

Nope, I don't have any special access to tech data but the independent mechanic should have known if it was OK when he tested it.

Joseph_H

but was surprised for it being that high. Good/bad, i don't know.

bamberger_1

Hi it's me again.
I tryed to understand more of the issue, and here is what I gathered in the mean time. The gauge shall show about 1.5 bar at idle (warmed up engine) could drop to 1.1 bar when engine is really hot (5 hour driving in summer 85 degrees). It shall be at 3 bar when speed (rpm) goes above 1500 rpm. What could be the problem with yours is, that the 3 bar mark shall also be the mechanical STOP for the dial, even the pressure still rising. So in case the dial once was removed from the instrument and put back together w/o taking in account the mechanical limitation at 3 bar, then the dial overshoots to its mechanical limit. In your case what the picture shows. Having a reading already when engine not running may be caused by the resistor was changed from 50 Ohm to 100 Ohm.

Joseph_H

Good input Bam!

What I am about to do (once I have nice weather!) is to mechanically test the oil pressure in the place where the sender unit goes. I already have the proper fittings to do so. I will do it the way MB is suggesting (i.e warm up engine to 80 degrees etc) then I will see how that corresponds to bar reading (1 bar = 14.5 psi). Then since I still have the original sending unit I will put that back and see how the resistance numbers compare to the new sending unit (both being different in the packaging; the new one doesn't have the diaphragm that allows for relief of oil incase too much pressure). And finally, If all readings seem within spec, I will make the proper tweaks to the oil gauge now that i managed to get various resistors with values that will allow me to calibrate the gauge to precise. I will make sure I provide an update here once all done. With my current configuration, the needle will drop to about 2 bars at idle and behaves the normal way when rpm goes up etc. I found out after many trials with another spare gauge that the 100 ohm resistor really needs to be 88 to 90 ohms to get perfect 3 bar at initial start and also will allow for the 1.5 bar at idle as you state. Weird problem... but with these gauges, anything can go out of tolerance and if the pressure mechanically tests okay, then i can fix the electronic part without worries.