oil pressure

Hi,

My oil pressure gauge just dropped about 25% at all rpms. I've had the
car for about 8 months, it's a '93.
at idle, 800 rpm, used to be about 1.5, now is about 1.0.
At 2000 rpm, used to be about 2.5, now about 1.9.
At 3000 rpm, used to be about 3.2, now about 2.5.
When just started, cold, it would go just over 4, now it's around 3.5.
When warm, it never gets over 2.5.

I changed the oil and filter - no difference.
I checked the pressure trasnmitter - 120 ohms with engine off.
I can't get a wrench on the transmitter to try a mechanical gauge.

The mileage has not changed (33 mpg)
The oil usage is zero.
When warm, the engine temperature is dead center on the gauge, and has
never changed.

Any ideas what might be causing the problem ?

Many thanks.

Eric.

In article <Xwyjd.116712$df2.97998@edtnps89>, Eric >
wrote:

> Any ideas what might be causing the problem ?

Poor electrical connection at either end of the wire? Unless you're
using a different oil and/or filter than previously.

--
Lanny Chambers, St. Louis, USA
'94C
the alignment page:
http://www.hummingbirds.net/alignment.html

Lanny Chambers wrote:
> In article <Xwyjd.116712$df2.97998@edtnps89>, Eric >
> wrote:
>
>
>>Any ideas what might be causing the problem ?
>
>
> Poor electrical connection at either end of the wire? Unless you're
> using a different oil and/or filter than previously.
I've been using Castrol GTX 10/30 and Mazda filters for ever.

The engine doesn't sound any different. Like I said, there's no
indication that anything is wrong, other than the oil pressure gauge. I
guess the next step is to try to fake out the dial on the dashboard with
a resistor to see if the dial is working.

Eric.

Eric > wrote in message news:<Xwyjd.116712$df2.97998@edtnps89>...
>
> I checked the pressure trasnmitter - 120 ohms with engine off.

That's about right - I think spec is 115 ohms.
What's the resistance at idle and at 3000 rpm?

L. Santer wrote:
> Eric > wrote in message news:<Xwyjd.116712$df2.97998@edtnps89>...
>
>>I checked the pressure trasnmitter - 120 ohms with engine off.
>
>
> That's about right - I think spec is 115 ohms.
> What's the resistance at idle and at 3000 rpm?

At idle, it's about 49 ohms.
At 3000 rpm it reaches 34 ohms.

I'm going to wire a variable resistor to the sensor lead and see what
the dial on the dashboard does. If that checks out ok, i'll have to put
more effort into getting the sensor out of the engine and plug in a
mechanical gauge.

Eric.

Here is some more info from the '90 shop manual:

Oil sensor:
Engine stopped - 110 to 130 ohms
Engine running - 13 to 55 ohms

Your measured resistance with the engine off is right at the median and the
running measurement at 3,000 RPM should put it shy of the 30 lb/in2 mark.

Oil Gage (as function of a fixed resistance value at the sensor terminal):
0 lb/in2 (at the mark, not at the zero itself) 52 ohms
30 lb/in2 mark 41 ohms
90 lb/in2 mark 16 ohms

I suggest you check the gage. One way of doing this is putting a 10 ohm
resistor in series with a 75 or 100 ohm potentiometer in place of the
sensor. Adjust the pot while an assistant watches the gage position. When
you adjust to any of the points, remove the pot (carefully) and measure the
resistance. Add 10 to the value (or measure the pot and resistor
together)and see how that correlates to the values in the table. I don't
know what the current is, so I suggest you use a larger pot that won't get
too hot. The purpose of adding the 10 ohm resistor is to limit the current
so you don't toast the pot or the gage.

If you decide to measure the oil pressure with a gage at the location of the
sensor, the manual says:

1,000 RPM - 28 to 43 PSI
3,000 RPM - 43 to 57 PSI

As you can see, there is a lot of inherent variance in the readings. This,
of course, does not explain the step change you have seen.

This is probably not a poor connection. A 25% negative change requires an
increase of resistance of around 9 ohms. This is quite a lot for wiring, it
is not likely that you would see this type of change without the circuit
being very intermittent. There is probably not a lot of damping in a gage
such as this, so fluxuations should be visible with a 9 ohm intermittent.

So it's likely the sensor or something mechanical with the engine. If the
sensor has gone south, my preference would be to replace it and continue to
use the factory gage.

Have at it!

Ken


"Eric" > wrote in message
news:frCkd.90919$VA5.2896@clgrps13...
> L. Santer wrote:
> > Eric > wrote in message
news:<Xwyjd.116712$df2.97998@edtnps89>...
> >
> >>I checked the pressure trasnmitter - 120 ohms with engine off.
> >
> >
> > That's about right - I think spec is 115 ohms.
> > What's the resistance at idle and at 3000 rpm?
>
> At idle, it's about 49 ohms.
> At 3000 rpm it reaches 34 ohms.
>
> I'm going to wire a variable resistor to the sensor lead and see what
> the dial on the dashboard does. If that checks out ok, i'll have to put
> more effort into getting the sensor out of the engine and plug in a
> mechanical gauge.
>
> Eric.
>

KWS wrote:
> Here is some more info from the '90 shop manual:
>
> I suggest you check the gage. One way of doing this is putting a 10 ohm
> resistor in series with a 75 or 100 ohm potentiometer in place of the
> sensor. Adjust the pot while an assistant watches the gage position. When
> you adjust to any of the points, remove the pot (carefully) and measure the
> resistance. Add 10 to the value (or measure the pot and resistor
> together)and see how that correlates to the values in the table. I don't
> know what the current is, so I suggest you use a larger pot that won't get
> too hot. The purpose of adding the 10 ohm resistor is to limit the current
> so you don't toast the pot or the gage.
Exactly my plan.

> This is probably not a poor connection. A 25% negative change requires an
> increase of resistance of around 9 ohms. This is quite a lot for wiring, it
> is not likely that you would see this type of change without the circuit
> being very intermittent. There is probably not a lot of damping in a gage
> such as this, so fluxuations should be visible with a 9 ohm intermittent.
Were you looking over my shoulder when I did the calc ?

> So it's likely the sensor or something mechanical with the engine. If the
> sensor has gone south, my preference would be to replace it and continue to
> use the factory gage.
Yup. I'm hoping it's the sensor. The 3rd gen RX7s had a similar problem
with the sensor grdaually increasing in resistance causing a false low
reading.

Thanks for your help.

Eric.