How do you test an oil pressure sender - PeachParts Mercedes-Benz Forum

bluebird · #1 06-23-2011, 12:42 PM

How do you test the oil pressure sender and or the oil pressure guage to see which one has failed?

I started the car, 84 300SD, pegged the oil pressure guage, drove to destination. Turned off car did my computer work went to car and started it OH MY GAWD no oil pressure so turned it off and checked oil level -- full -- started car and pulled off the oil filler cap I could see oil flowing. Reached down behind the oil filter to see if the wire and oil pressure sender had parted ways. Dammmm the back of the oil filter housing was EXTREMELY hot and burned my wrist. I couldn't pull my hand out quickly enought and have a huge 2" blister.

As usual your help in this matter is greatly appreciated.

leathermang · #2 06-23-2011, 12:49 PM

I am really sorry about your injury...

Have you now driven home and let it cool ?

Given the oil level you are probably correct in your feelings that the wires are not working ... or the sending unit not working.... somewhere in the archives I am sure there will be the correlation of sending voltage and pressure ...
but first you need to see if wires are connected safely....and make some clips to attach to the sending unit...

bluebird · #3 06-23-2011, 01:13 PM

I did drive the car home and drove it up on wooden ramps, similar to the ones at loading docks, so I could get under the car instead of putting my arm in place I think it should be but shouldn"t be. I crawled under the car and the connector was off but I think I pulled it off when I was trying to get my hand off the oil filter housing. Put it back on and started the car but still not working. I did a search on "how to test oil pressure sender" but did not find any information.

leathermang · #4 06-23-2011, 01:25 PM

Someone will speak up soon...
I think I have the physical oil line to the dash... and have not had any problem with it... so I have no experience with your unit....

ah-kay · #5 06-23-2011, 01:32 PM

Most oil pressure sender works on resistive value, i.e. the resistance value varies with oil pressure. Try measure the resistance value across the sensor with an ohmmeter with engine off. Measure it again at idle and at 2000rpm if you think oil is circulating. The value should vary.

I have not worked on a MBZ oil pressure sender before so this info may or may not be applicable. But it is a start. Good luck.

http://www.clarks-garage.com/shop-manual/elect-19.htm

Jeremy5848 · #6 06-23-2011, 03:38 PM

AFAIK, the 126 and 124 use the same electrical oil pressure system. My 124 FSM gives the following resistance values for the oil pressure sender:

54-254 Testing electrical oil pressure indicator
Pressure (bar) Resistance (ohms)
0 . . . . . . . . . . . . . . . 10
1 . . . . . . . . . . . . . . . 69
2 . . . . . . . . . . . . . . 129
3 . . . . . . . . . . . . . . 184

You can test the sender with an ohmmeter. At idle when the engine is cold, the oil pressure should be 3 bar or more (~200 ohms); at idle when the engine is hot the pressure could be as low as 0.5 bar (~30 ohms).

You can test the gauge with some resistors bought at a local Radio Shack or other electronics store. Pick values similar to the above table and put them in place of the sender to see what the gauge says.

Based on my personal experience and numerous posts by others, the sender is the most likely culprit.

Jeremy

bluebird · #7 06-23-2011, 03:57 PM

I read somewhere that if the sender goes TU then the guage pegs at 3 bar but mine is at "0". I will pull the unit off my parts car and put it on my daily driver and see if it pegs at 3 like it was before this happened. I used to get 3 bar at WOT and 2 at hot idle this is with 219K on the odometer, I have yet to do a compression test. The engine starts without glow in 40 degree weather.

300sdToronto · #8 06-23-2011, 07:09 PM

You're correct that if the electrical circuit between the sender and the gauge goes open circuit then the gauge will peg at 3 bar when you turn on the ignition - before you start the engine. I know this the hard way as I replaced what turned out to be a good oil pressure sender, only to find out that the wire that connects to the sender had failed.

I'd suggest disconnecting the feed wire from the sender - when the engine is cold - and switch on the ignition. It it goes to 3 bar then the problem is in the sender - if it stays at zero, you have a wiring or ground issue or the gauge itself has failed in some way.

vstech · #9 06-23-2011, 11:34 PM

since it read 0 and you drove home with the wire disconnected, I'd say your cluster gauge is bad.

bluebird · #10 06-28-2011, 08:27 PM

Well here is what has happened, I bought a Uro oil pressure sending unit $21.99 plus shipping. Tested the sending unit that was in the car, grounded the VOM to the engine block and positive to the sending unit post 454 ohms. Tested the new unit and got 4.8 ohms. Pulled out the old unit, yes oil does come out if your up on ramps, and put in the new unit. Tested the old unit out of the engine and it tested 12.8 ohms.
Started the engine and no oil pressure on the guage.

Pulled the instrument cluster out and the multi pin round connector on the left of the cluster was off, but it couldn't have been of because the temp and fuel guages worked. So I pulled the cluster all the way our took it into the house and pulled it apart so I could resolder all the pins in the round connector as the solder joints looked like cold solder joints. I resoldered all the pins and the solder joints now look shiney instead of dull gray. Put it back in the cluster buttoned it all up. Put the Cluster back in the car hooked up everything and I have oil pressure.

While I was in the instrument cluster I disassembled the speedo unit to lube the gears that I put in after the odo quit working. I used Lubriplate grease that I believe was for fishing reels. I no longer have the click click click that you get if you do not lube the new gears.

I love being able to work on these old vehicles, on my wifes 98 Ford Windstar in order to replace the bulbs in the dash you have to drop the steering column. I would love to meet the design engineer that designed that.

Jeremy5848 · #11 06-29-2011, 01:33 AM

One of the best things about older Mercedes is that you can take them apart and fix them, put them back together and drive another 100,000 miles.

Gregory · #12 06-29-2011, 11:55 AM

I just want to say thank you once again for this forum and its contributors.

Sunday I was in western Oregon about 650 miles from home in my 300 SDL and saw the oil pressure gauge drop to zero. It was flipping up and down between zero and three, going to down to zero on acceleration.

I pulled over as soon as there was a spot on the narrow country road and pulled out my tablet/phone. I was in luck, there was a cellphone signal, so I visited this forum and thread.

I popped the hood, opened the oil fill cap to see if oil was splashing around the cam and valves. It was, and there was more oil visible when I revved the engine.

I checked the oil level, added a pint and decided it was a sender or wiring problem, and the oil pressure was ok.

I drove home about 35% of those 650 miles at 90 to 100 mph (eastern central Oregon is desolate with long, straight 2-lane roads) with the oil gauge on zero at those speeds. Finally, when driving only the speed limits, the jiggling gauge quit and pegged at three bar. Google Maps says that drive should take 13.5 hours, I made it in 11 hours.

Today I picked up the new sender from NAPA and installed it in less than 10 minutes. The oil gauge is working fine now, a $45 fix.

Cheers!
Gregory