Erratic oil pressure reading

[Melville]

The oil pressure gauge on my 1994 E320 (M104) wagon started acting erratically. (I posted this in off topic since it's not an E500E, but I imagine the trouble shooting is the same for M104 and M119 on this issue.)

When I bought the car, the pressure was always pegged at 3. I had the sender replaced earlier this year and that solved the problem.

However, a couple of days ago, the gauge would sometimes act normally, then suddenly drop. Sometimes all the way to 0, other times to another level. It also flutters. I've noticed that going over bumps can cause it to flutter, drop, etc.

The car runs and idles perfectly. That leads me to think that the problem is not actually oil pressure.

The lower harness/positive battery cable is original so that is my first thought. But the reading is just as likely to be at a low level than at 3 when I'm driving, and from what I gather from other posts, a bad connection to the lower harness would cause it to be pegged at 3.

My next guess is it's the gauge itself. On my first W124, a 92 cabrio, the gauge would suddenly drop to zero. A swift smack to the dash would cause it to bounce back up again. I tried that a couple of times on my wagon but nothing happened.

I suppose the new sender could be bad, but that was an MB part I bought through Naperville. Those things aren't complicated so I would be surprised if one is faulty so soon after install.

Thoughts?

 

[gerryvz]

I think it's your sender, not the gauge. If you drive the car up on ramps, while it's running, you can check the condition of the lower harness wiring and wiggle it to see if there is any change at the gauge (have someone in the car watching the gauge as you manipulate the wire). These wires do get pretty oil soaked, and heat soaked, and they are the bio-degradable wires, so it well could be the wire.....there is very little to go wrong with the gauge IMHO.

If you do have to replace the lower wiring harness on a 104, it's not a horrible job. MUCH easier than with an E500E or 400E/E420.

Everything you need to know on the oil pressure sender is here:

HOW-TO: Replacing M119 oil pressure sensor | "HOW-TO" Tutorial Articles

The oil pressure switch** sensor on the M119, thankfully, is a rather easy job to replace. Symptoms of a failing oil pressure sensor are low or no pressure indicated on the gauge (below 0.5-0.75 bar when the engine oil is hot); not jumping to the 3-bar mark when the gas pedal is pressed, and...

www.500eboard.co

It's a bit more of a PITA to do this on the M104 than the M119, but still something you can easily do yourself, Mark. Get the part and do it when you do your next oil change (or you can do it in between oil changes). I'd say 10-15 minute job, max.

Just get a new VDO sender from the normal aftermarket vendors. They can go bad even being fairly new, though I'd say it's not that common.