Irratic Oil Gauge

My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own!
Sometimes it's pinned right over, most times it bounces around from full
scale or some where in the middle it bounces up and down about 25% above and
below the centre mark. I'm not really worried but..... is it the meter or
the sending unit or what? Big job or an hour for a weekend mechanic?
Anything else I should know about this "problem"? Thanks in advance.

Mike
PS: If replying directly, remember to remove "nospam" from my email address.

"Mike" > wrote:

>My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own!
>Sometimes it's pinned right over, most times it bounces around from full
>scale or some where in the middle it bounces up and down about 25% above and
>below the centre mark.

The reading depends on oil temperature and engine rpm. Higher readings
if the oil is cold or the engine is revving faster.

With the oil fully heated up, the reading at idle should be well
below 30 and well above it at a cruising rpm of 4000.

Leon

> I'm not really worried but..... is it the meter or
>the sending unit or what? Big job or an hour for a weekend mechanic?
>Anything else I should know about this "problem"? Thanks in advance.
>
>Mike
>PS: If replying directly, remember to remove "nospam" from my email address.

--
Leon van Dommelen Bozo, the White 96 Sebring Miata .)
http://www.dommelen.net/miata
EXIT THE INTERSTATES (Jamie Jensen)

In article <%Ifcd.117802$a41.58196@pd7tw2no>,
"Mike" > wrote:

> My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own!
> Sometimes it's pinned right over, most times it bounces around from full
> scale or some where in the middle it bounces up and down about 25% above and
> below the centre mark.

Dunno the metric values for Canajun cars, but here are some typical
numbers:

Cold idle: 60 psi
Cold highway: 90 psi
Hot idle: 25 psi
Hot highway: 55 psi

Pressure is supposed to vary with temperature (viscosity) and rpm.

After 1994 models, the cars do not have real oil pressure gauges, and
display no variation. Presumably, Mazda did this so they'd no longer
have to answer questions like yours. :-)

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

My Canadian '94 when cold runs close to 6 Kp/cm2, when hot idles just below
2 , at 3500 or 4000 rpm at close to 4. I remember converting these
readings to psi (which is what I grew up with), and I think the readings are
close to what Leon suggests are normal.
My gauge does jitter a bit at lower readings, like when idling, but not at
all like you describe.

Pete


"Mike" > wrote in message
news:%Ifcd.117802$a41.58196@pd7tw2no...
> My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own!
> Sometimes it's pinned right over, most times it bounces around from full
> scale or some where in the middle it bounces up and down about 25% above
and
> below the centre mark. I'm not really worried but..... is it the meter or
> the sending unit or what? Big job or an hour for a weekend mechanic?
> Anything else I should know about this "problem"? Thanks in advance.
>
> Mike
> PS: If replying directly, remember to remove "nospam" from my email
address.
>
>

I appreciate the responses. I have seen similer oil pressure gauge activity
in one other Miata so I just assumed this is a common problem. But nobody
has answered my question!! Where is the problem - gauge, sender, something
else?? Thanks for all you who have told me where the needle should sit under
various conditions.
To add more detail, occasionally the gauge seems to be pinned, or sits
fairly steady somewhere left of the middle. It seems whatever it does on
startup continues until the next startup, and then it may work the same or
different.

Mike
"Mike" > wrote in message
news:%Ifcd.117802$a41.58196@pd7tw2no...
> My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own!
> Sometimes it's pinned right over, most times it bounces around from full
> scale or some where in the middle it bounces up and down about 25% above
and
> below the centre mark. I'm not really worried but..... is it the meter or
> the sending unit or what? Big job or an hour for a weekend mechanic?
> Anything else I should know about this "problem"? Thanks in advance.
>
> Mike
> PS: If replying directly, remember to remove "nospam" from my email
address.
>
>

Mike,

You don't get it! What these guys are describing is the CORRECT readings of
the oil pressure guage! It does go from almost pegged to the right (cold
oil) to below 30psi (warm oil, engine at idle) and everywhere in between.
The oil pressure really does vary that much. If you'll re-read the
responses, they do quite a good job of explainining what factors effect you
oil pressure at any given moment. If you still cannot comprehend this then I
guess I'm beginning to understand why Mazda gave up and put a 'dummy' oil
pressure guage in the later cars.

Chris
92BB&T

"Mike" > wrote in message
news:kxEcd.745719$M95.195850@pd7tw1no...
> I appreciate the responses. I have seen similer oil pressure gauge
activity
> in one other Miata so I just assumed this is a common problem. But nobody
> has answered my question!! Where is the problem - gauge, sender, something
> else?? Thanks for all you who have told me where the needle should sit
under
> various conditions.
> To add more detail, occasionally the gauge seems to be pinned, or sits
> fairly steady somewhere left of the middle. It seems whatever it does on
> startup continues until the next startup, and then it may work the same or
> different.
>
> Mike
> "Mike" > wrote in message
> news:%Ifcd.117802$a41.58196@pd7tw2no...
> > My recently acquired 1990 MX5 oil gauge seems to have a mind of it's
own!
> > Sometimes it's pinned right over, most times it bounces around from full
> > scale or some where in the middle it bounces up and down about 25% above
> and
> > below the centre mark. I'm not really worried but..... is it the meter
or
> > the sending unit or what? Big job or an hour for a weekend mechanic?
> > Anything else I should know about this "problem"? Thanks in advance.
> >
> > Mike
> > PS: If replying directly, remember to remove "nospam" from my email
> address.
> >
> >
>
>

Mike, if you're saying this bouncing around of the needle happens at a
given rpm and engine temp, then I'd be inclined to suspect the sending unit
rather than the gauge itself. If I remember correctly, it is located near
the base of the oil filter... not very easy to get at, but if you've got
small hands, not a big job to replace.

sorry if we initially mis-interpreted your query.

Pete
--
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.760 / Virus Database: 509 - Release Date: 10/9/2004

"Mike" > wrote in message
news:kxEcd.745719$M95.195850@pd7tw1no...
> I appreciate the responses. I have seen similer oil pressure gauge
activity
> in one other Miata so I just assumed this is a common problem. But nobody
> has answered my question!! Where is the problem - gauge, sender, something
> else?? Thanks for all you who have told me where the needle should sit
under
> various conditions.
> To add more detail, occasionally the gauge seems to be pinned, or sits
> fairly steady somewhere left of the middle. It seems whatever it does on
> startup continues until the next startup, and then it may work the same or
> different.
>
> Mike
> "Mike" > wrote in message
> news:%Ifcd.117802$a41.58196@pd7tw2no...
> > My recently acquired 1990 MX5 oil gauge seems to have a mind of it's
own!
> > Sometimes it's pinned right over, most times it bounces around from full
> > scale or some where in the middle it bounces up and down about 25% above
> and
> > below the centre mark. I'm not really worried but..... is it the meter
or
> > the sending unit or what? Big job or an hour for a weekend mechanic?
> > Anything else I should know about this "problem"? Thanks in advance.
> >
> > Mike
> > PS: If replying directly, remember to remove "nospam" from my email
> address.
> >
> >
>
>

The oil pressure meter is a fairly simple gage that is electrically in
series with the sending unit. Current through the gage is determined by the
resistance of the sending unit, which changes with pressure.

According to the shop manual, the sensor resistance should be between 110
and 130 ohms when the engine is not running and 13 to 55 ohms when it is.
From this, I would assume that 13 ohms is probably the value when the meter
is around full scale (maximum pressure).

So I would suggest doing a simple resistance measurement first, assuming you
have an ohmmeter and access to the sensor. From the shop manual, it appears
to be on the driver's right side of the engine, behind the alternator,
slightly above and just behind the oil filter. The information isn't all
that clear, but it is someplace in this area. It will have a single wire
attached to it, probably with a connector, as Mazda does this right.

Any cheap ohmmeter will do; they sell them at Harbor Freight for about $5.
If that's not it, then it's the wiring or the gage in the dash. A way of
checking this out is to put about a 47 or 27 ohm resistor (get one at Radio
Shack - go for 1/2Watt or greater) between the single wire that goes to the
sensor and ground. The meter should go someplace towards the middle and not
jump around.

In any event, it looks like a simple job to replace the sensor.

Good luck!

Ken



"Pete Breemhaar" > wrote in message
...
> Mike, if you're saying this bouncing around of the needle happens at a
> given rpm and engine temp, then I'd be inclined to suspect the sending
unit
> rather than the gauge itself. If I remember correctly, it is located near
> the base of the oil filter... not very easy to get at, but if you've got
> small hands, not a big job to replace.
>
> sorry if we initially mis-interpreted your query.
>
> Pete
> --
> Outgoing mail is certified Virus Free.
> Checked by AVG anti-virus system (http://www.grisoft.com).
> Version: 6.0.760 / Virus Database: 509 - Release Date: 10/9/2004
>
> "Mike" > wrote in message
> news:kxEcd.745719$M95.195850@pd7tw1no...
> > I appreciate the responses. I have seen similer oil pressure gauge
> activity
> > in one other Miata so I just assumed this is a common problem. But
nobody
> > has answered my question!! Where is the problem - gauge, sender,
something
> > else?? Thanks for all you who have told me where the needle should sit
> under
> > various conditions.
> > To add more detail, occasionally the gauge seems to be pinned, or sits
> > fairly steady somewhere left of the middle. It seems whatever it does on
> > startup continues until the next startup, and then it may work the same
or
> > different.
> >
> > Mike
> > "Mike" > wrote in message
> > news:%Ifcd.117802$a41.58196@pd7tw2no...
> > > My recently acquired 1990 MX5 oil gauge seems to have a mind of it's
> own!
> > > Sometimes it's pinned right over, most times it bounces around from
full
> > > scale or some where in the middle it bounces up and down about 25%
above
> > and
> > > below the centre mark. I'm not really worried but..... is it the meter
> or
> > > the sending unit or what? Big job or an hour for a weekend mechanic?
> > > Anything else I should know about this "problem"? Thanks in advance.
> > >
> > > Mike
> > > PS: If replying directly, remember to remove "nospam" from my email
> > address.
> > >
> > >
> >
> >
>
>
>

Now here is an answer for me. Thanks a lot - just exactly what I needed to
know. I'll put the DVM on it one of these days when and if it ever gets
warmer. My baby is going into storage in another couple of weeks so I will
probably do it in the spring. I'll save/print your answer for future
reference. Thanks bunches

Mike

"KWS" > wrote in message
news:%pGcd.277169$3l3.209171@attbi_s03...
> The oil pressure meter is a fairly simple gage that is electrically in
> series with the sending unit. Current through the gage is determined by
the
> resistance of the sending unit, which changes with pressure.
>
> According to the shop manual, the sensor resistance should be between 110
> and 130 ohms when the engine is not running and 13 to 55 ohms when it is.
> From this, I would assume that 13 ohms is probably the value when the
meter
> is around full scale (maximum pressure).
>
> So I would suggest doing a simple resistance measurement first, assuming
you
> have an ohmmeter and access to the sensor. From the shop manual, it
appears
> to be on the driver's right side of the engine, behind the alternator,
> slightly above and just behind the oil filter. The information isn't all
> that clear, but it is someplace in this area. It will have a single wire
> attached to it, probably with a connector, as Mazda does this right.
>
> Any cheap ohmmeter will do; they sell them at Harbor Freight for about
$5.
> If that's not it, then it's the wiring or the gage in the dash. A way of
> checking this out is to put about a 47 or 27 ohm resistor (get one at
Radio
> Shack - go for 1/2Watt or greater) between the single wire that goes to
the
> sensor and ground. The meter should go someplace towards the middle and
not
> jump around.
>
> In any event, it looks like a simple job to replace the sensor.
>
> Good luck!
>
> Ken
>
>
>
> "Pete Breemhaar" > wrote in message
> ...
> > Mike, if you're saying this bouncing around of the needle happens at a
> > given rpm and engine temp, then I'd be inclined to suspect the sending
> unit
> > rather than the gauge itself. If I remember correctly, it is located
near
> > the base of the oil filter... not very easy to get at, but if you've
got
> > small hands, not a big job to replace.
> >
> > sorry if we initially mis-interpreted your query.
> >
> > Pete
> > --
> > Outgoing mail is certified Virus Free.
> > Checked by AVG anti-virus system (http://www.grisoft.com).
> > Version: 6.0.760 / Virus Database: 509 - Release Date: 10/9/2004
> >
> > "Mike" > wrote in message
> > news:kxEcd.745719$M95.195850@pd7tw1no...
> > > I appreciate the responses. I have seen similer oil pressure gauge
> > activity
> > > in one other Miata so I just assumed this is a common problem. But
> nobody
> > > has answered my question!! Where is the problem - gauge, sender,
> something
> > > else?? Thanks for all you who have told me where the needle should sit
> > under
> > > various conditions.
> > > To add more detail, occasionally the gauge seems to be pinned, or sits
> > > fairly steady somewhere left of the middle. It seems whatever it does
on
> > > startup continues until the next startup, and then it may work the
same
> or
> > > different.
> > >
> > > Mike
> > > "Mike" > wrote in message
> > > news:%Ifcd.117802$a41.58196@pd7tw2no...
> > > > My recently acquired 1990 MX5 oil gauge seems to have a mind of it's
> > own!
> > > > Sometimes it's pinned right over, most times it bounces around from
> full
> > > > scale or some where in the middle it bounces up and down about 25%
> above
> > > and
> > > > below the centre mark. I'm not really worried but..... is it the
meter
> > or
> > > > the sending unit or what? Big job or an hour for a weekend mechanic?
> > > > Anything else I should know about this "problem"? Thanks in
advance.
> > > >
> > > > Mike
> > > > PS: If replying directly, remember to remove "nospam" from my email
> > > address.
> > > >
> > > >
> > >
> > >
> >
> >
> >
>
>