So, my oil pressure gauge pegs to 3 when I engage the key
but before the engine even starts. What gives?

Jeff

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1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

Does it change when your engine warm up? if no, you've got a ground issue, or a crap sender unit. if it does fluctuate while you drive, accept the quirk and dont worry about it.

*Fluctuate, IE Pegging on acceleration and moving down at idle, somewhere around midway

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

Your oil pressure sensor is probably bad.

I believe 1st gen 126 uses an electrical oil pressure sensing system like found on the 124s. If so it's a silver can on the base of the oil filter tower.

A couple tips:
- Be sure to also get the sealing washer that goes with it.
- If the starter with the big + cable is right below where you are working, disconnect the battery first.
- The sensor is not under pressure when the car is not running so it is not necessary to drain the oil first. You might lose a couple ounces of oil so be prepared for that.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

yup... the 126 Oil pressure gauge fails to peg the meter... a new sender should straighten it out, and it's on the base of the oil filter facing the firewall... not terribly difficult to change.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks. Don't see that part in PP under "sending unit".

Jeff

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1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

it's there on the engine sensors section
1983 Mercedes-Benz 300SD Base Sedan - Switches, Motors, Relays, Fuses & Wiring - Page 3

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Thanks!

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1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

Yes, on most Cars I have driven when you first turn the Key the needles on all of the Electrical Gauges should move from what ever position they were in before the key was turned.
That is a quicktest on them.
Along with the above all of the Warning/Idiot Lights should also turn on to test the see if the bulbs are OK.

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84 300D, 82 Volvo 244Gl Diesel

Ground on wire to sender = 0 psi on gauge
Open on wire (disconnected) = max psi on gauge

so make sure the wire is connected and making a good connection. If it still does the same thing then disconnect the wire and connect it to a good ground and see if the gauge goes to 0 psi when the key is turned on. If it does then change the sending unit. If the above test fail then the problem is in the wiring or the gauge.

Paul

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84 500 SEL (307,xxx miles)