Diesel RVs: Re: [diesel_mercedes] Re: 83 300SD Oil Pressure Gauge...fixed?

Caitlin,

NO! This is not right. The guage, if working properly, should not show ANY oil pressure until the engine is running.

While the engine is running, the gauge should fluctuate, depending on engine speed. At any speed above 'idle' the gauge should be pegged at the "3" mark. At 'idle' it can safely drop below "2".

Remember that what your are looking at is an indication (I hesitate to call it a 'measure') of oil pressure. There is NO oil pressure while the engine isn't running. Oil pressure builds because an oil pump IN the engine is pumping.

So, you have established something - that the gauge itself may be working. But you still haven't sorted out the signal to the guage. It may be that the sensor is faulty, thereby giving you a bad reading all the time.

A junk yard sensor is the cheap and easy way to experiment with alternatives. This is not high tech - all that's happening is a simple circuit that allows current to deflect a standard gauge mechanism. So you need to figure out the source of the signal variation.

I'm at the office and not near any reference books. But my simple minded thought is that the gauge gets 12vdc from the binnacle module, and the sensor in the engine provides a variable resistance to ground. So when there is no continuity, you get a "zero" reading. With increasing pressure, there is decreasing resistance in the engine sensor, this allows current flow and that causes the gauge to deflect. When pressure is high enough to provide zero resistance in the sender, the gauge will be at full deflection ("3").

This means that if the sensor shorts out, the gauge will read at full deflect as soon as power is available in the binnacle (when you turn the key to position 1).

If I am right, you can pull a sensor out of a junker and measure it with an ohm meter. If the ohm meter shows resistance from contact to case, then drop it in your pocket and try it out when you get home. (Don't forget to tell the cashier at checkout that it is in your pocket - and pay for it.)

Tom

In a message dated 11/17/2011 5:23:28 P.M. Pacific Standard Time, tinymachine@gmail.com writes:

Ok folks, I *think* the gauge is all set but I'm not sure what the correct readings are even despite the recent discussion.

I cleaned and pried apart the little plastic part that holds the connector pin that makes contact between the wire and the sensor. I cut and re-stripped the end of the wire, wrapped it around the pin and closed up the housing (I did this twice...wasn't sure the first time "held") and pushed it back onto the sensor.

With the key out and car off, gauge now reads "0" (though sometimes I have to tap the cluster to get it to drop) and as soon as key is in the first position it pegs up beyond the "3".

It stays there the entire time I'm driving and doesn't move (I tapped to make sure), even at "hot idle" if hot is around 80-85c.

When I turn off the car, needle drops to "0"...though as I said sometimes it sticks and a tap will drop it.

Is this a fairly normal oil pressure reading? Can I test this?

Thanks!
Caitlin
Santa Fe

--- In diesel_mercedes@yahoogroups.com, "Nate" <vwnate1@...> wrote:
>
>
> O.K. :
>
> A _brown_ wire , anywhere in the car , is a _GROUND_ wire and so *must* be well connected , no loose parts , frayed or broken strands , no corrosion permitted or things won't work right .
>
> The loose oil sender wire you posted a pic of , is prolly the fault , stick a pin in the open end of the wire and ground it well , turn on the key , if the gauge now reads zero , you've found the fault , gently pry open the plastic connector and re attach the wire .
>
> The oddly worn regulator brushes were because the DPO/DPM wasn't careful to hold the assembly in place as they re installed the two screws holding it in , so one brush was riding off the edge of the slip ring .
>
> You didn't show a picture of the other side of it , it *MUST* say
> " BOSCH " or it WILL NOT CHARGE ENOUGH .
>
> Period .
>
> Don't test the top end speed of the car until the tires , suspension & brakes are all tip top and then , only when no other traffic is about as things happen -REALLY- quickly at high speeds...
>
> I chicken out a little over 100 MPH , I only did that a few times to de - carbon my engine , it's safer and less nerve wracking to do Diesel Purges & routine tune ups...
>
> -Nate
> Caitlin wrote:
> >
> > I confirmed that indeed the gauge is stuck as high as it goes when the cluster is merely plugged in (no key in ignition, car obvi not running, etc) at all times.
> >
> > I'm not totally sure what to look for other than brown wires that have "loose connections" and tighten them...and maybe bare wires too...
> >
> > My electrical gremlin count is growing and I still haven't even found/fixed one...(ok, that's cuz I'm sort of avoiding them)
> >
> > Good to know I can shoot for 130MPH ;) despite what the speedo says...but the one that goes higher was orig to the car and at least i have a spare for if/when I send out mine for repair.
> >
> > Gonna go check the voltage regulator (as suggested) and also see if I can't rejuvenate my hazard switch in hopes of restoring signals (they light, inside and out, but don't blink). While after the hazard, I hope to check and dig around in the wiring behind the console in hopes of finding gremmies.
> >
> > Caitlin
> > Santa Fe
> >
> >
>