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Solutions
Version 2, Eliminating the excess damping and achieving a linear
response curve.
For those who want it
all. For those who aren't afraid... or are to foolish to realize
they should be...
If you are good with soldering irons, don’t mind the risk of
messing with the guts of the gauge and or have too much time on your
hands but insufficient funds for a turbo this is the next step…
time to change the coil windings!
Warning: If you go forward with this I am assuming you have some
knowledge and comfort with soldering, working with simple resistor
circuits, multimeters etc. You should also be comfortable doing a
bit of improvising to create something to help wind the wires off
the coils and back on the coils. These aren’t 100% step by step. If
this stuff scares you just do Version 1.
I cannot stress enough the importance of labeling things (or even
better taking pictures) as you disassemble the gauge. Also, the
coils are wound from a near hair thin copper wire. It takes VERY
little force to snap this wire. You can splice it if needed (ask me
how I know) but who wants to do that.
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| Parts
To complete this mod you will need:
-Various 1/4 watt resistors. For the 100psi sender I needed
330ohm and 2.2k ohm 1/4 watt resistors. For the 80psi sender I needed 470
ohm and 220 ohm resistors. A 10 ohm resistor or variable resistor is
highly recommended for testing.
-100psi or 80psi oil sender with 10-180 ohm range. The
0-100psi sender can be used to give a 0-90 psi range (same as the
factory gauge). A 0-80psi sender can also be used with
different resistors. The gauge will still be set up to show 0,
30, 60 and 90psi. Sure the sender only goes to 80psi but the
gauge doesn't know that. My speedo goes to 150mph but I
suspect my Miata can't quite reach that speed.
A 12V power supply (Makita drill battery in my case) is
recommended but not required for testing.
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| The construction of
the windings and core.
The gauge motor is basically made from just a few parts. You have
the wire which is would around a two part white plastic former
/core. With all the wire removed the two halves of the plastic core
can be pulled apart. Inside you will find a black, round, magnetic
disc. The grease in question acts on part of this disc and the
plastic core. With all the wire removed removing the grease is a
simple mater of wiping it away with a rolled napkin.
The factory configuration consists of 4 sets of windings. When
looking at the motor you can see layers of wires wraped around the
plastic part Each time the wire changes its direction of wind that's
a new set. Each set is split in half running on either side of the
needle post.
It is important to get our bearings when working with the gauge
motor. Orient the motor so it faces you as if it were
installed in the dash. The shaft will face towards you. The OIL terminal
will be at top and the IGN terminal will be on the right. In this
orientation you see the coils are wrapped at 45 degrees from
vertical. The coils can be wrapped in two directions for each
orientation. For instance, a coil can go from 7:30 to
1:30 or 1:30 to 7:30. It is very important to note
the difference between 7:30 to 1:30 and 1:30 to 7:30. The difference
can cause the needle to move backwards… ask me how I know.
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(pic of the gauge
motor with out resistors)
(the windings and metal can aren’t shown in the
illustration) |
| Lets get pulling things apart
Remove the needle and gauge face from the gauge motor. Unsolder the
resistors. With a pair of pliers, carefully remove the three
terminals from the bottom of the gauge. They are a one way push in
deal. It takes a bit of force to remove them but they will come out.
Label them so you know where they go later on. Label their location
on the metal can and the plastic core.
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| Next remove the metal can. The can is heat
staked to the plastic core via some posts through the can bottom.
Cut off the 4 mushroom shaped posts. Next push the core out of the
can. I found it was easiest to push on the parts of the core where
the terminals used to be.
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| Congrats, now you have just the
windings and core of the gauge. It’s time to note a few parts.
Pressed into the plastic you will see three wiring terminals, GND,
OIL and IGN. GND and IGN both have one wire. The OIL terminal has
two. As part of this job you will have to either cut the wires off
these terminals or unsolder them. I chose to unsolder them.
Start by bending the coil wire terminals up away
from the coils. This gets them out of the way for unwinding
and allows you to get a soldering iron on them.
Also... remember the wire is very thin and easy to break so BE
CAREFUL with it!
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| To make winding easier I used to
small electric fans like you might find in a computer power supply.
Using some double sided adhesive I taped an empty solder spool to
one and the gauge core to the other. A few little cut up foam pads helped mount up
the awkwardly shaped core. Now I could wind the wire off or on the
core simply by spinning the part with my hand. A short bit of wire
to the gauge to make it easier to spin with one finger. I didn’t use
this twin fan idea the first time I unwound and rewound the gauge.
With the spool I just had the spool and core sitting on the table.
It took me about 4 hours to unwind then rewind the gauge. With the
fans acting as little turn tables I was able to unwind and wind the
coils in about 45 minutes including stopping to solder and change
direction. |
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| Unsolder or cut the wire connected
to the IGN terminal. I used a sharp solder tip to unwind the wire.
Start winding it onto the spool.
After unwinding coils 1-3, unsolder or cut the wires from the OIL
terminal. Unwind coil 4. You can now open up the two plastic parts
that support the coils. Inside you will see the needle stator and
the damping grease. I just used a bit of napkin to wipe the grease
away.
The little black part in the second photo is a magnet or ferrite
block that magnetically returns the needle to 0.
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Location of Grease |
| Now it’s
time to put it all back together with a new wind.
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| In the illustrations the arrow
will always point a long the wire to the spool of wire that is
waiting to be wound/ to the end of the wire. So for the first
set of winds it points to the end of the wire that will solder to
OIL. For the later coils it points at the end of the wire that
solders to IGN. Use your fingers to guide the wire onto the
core while you wind it. Keep a little bit of tension on the
wire so the coils aren't too loose. Again, be careful, the
wire is thin. Coil 4: Wrap 350 turns of wire around the cord in
the same orientation and direction as the inner most turns of wire.
For those who didn’t write down the orientation of that inner most
bit, hold the gauge in front of you with the shaft facing you, with
the oil terminal at 12:00 and the GND terminal at 3:00. Each time
the wire comes around it should start (rise from the table) the 7:30
position and finish (set/goes back towards the table) at the 1:30
position. Put half of those 350 turns on one side of the needle
shaft and the other half on the other side.
As there are a lot of turns, I recommend counting in blocks of
say 50 turns then putting a tick mark on paper. It also allows you
to be a bit more absent minded when winding. Also, watch out for
snagging the thin wire on the white plastic gauge former while
winding things up. You want all the wire inside of the former, not
hung up on the edges.
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1:30 Direction of wind, 350 turns |
| Next coil: Change the direction of
the wire so it rises at the 10:30 and sets at the 4:30. 550 turns,
one half on each side of the needle shaft. Any time I change
direction or solder the wire to a terminal I try to wrap it around
one of the plastic posts the same way Mazda did when they originally
wound the gauge. I don’t know if it’s needed but I feel better doing
it that way.
Part way through this you should run across what used to be the
bit of wire that was soldered to the OIL terminal. If you cut the
wire you will now need to join the two wire sections. If you didn’t
cut the wire you still need to keep the solder covered wire away
from the rests of the coils. I made one turn of wire around one of
the plastic post, put the soldered section of wire in the hole in
the white plastic post then made one more quick turn around the post
to hold it in place. After than, continue wrapping.
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Wrapping wire around the posts
4:30 Direction of wind, 550 turns
Locations for the soldered joint
in the wire |
| When you get to the end of the 530
turns, wrap a bit of the wire around the OIL terminal and solder
it in place. I very carefully used a knife to scrape
off a bit of the wire’s enamel so the solder could touch bare
wire. I’m not sure if this is needed. Congratulations, you have
created a new Coil B consisting of two sets of windings.
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| Lets get on
to the new outer most coil |
| Wrap the rest of the wire (~900 turns) in the
opposite direction (rise at 4:30, set at 10:30). Again, split the
winds between the two sides of the needle shaft. |
10:30 direction of wind, ~900 turns. |
| Time for new
resistors
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| We will change the values of R1 and
R2 and add a new resistor (R trim) going from the OIL terminal to
the GND terminal on the bottom of the gauge.
I have included resistor values for both the 0-100psi
and 0-80psi senders that should result in tick marks at 0, 30, 60,
and 90psi. However, those values assumed the sender has a linear output.
It assumes that 50 psi should result in a resistance of (180-10)*50%
= 85 ohm. Unfortunately what limited testing I could do with
the sender and my experience with the sender in my car suggests that
the VDO output is somewhat non-linear and reads a bit high (85 ohms
at 40-45psi rather than at 50).
R1: Do not change the 180 ohm resistor.
R2 for 100psi sender: Use a combination of resistors to create a
77 ohm
resistor between IGN and OIL (replaces the 100 ohm resistor). I used
the 100 ohm and a 330 ohm resistor in parallel (1/4 watt resistor is fine for
the 330 ohm resistor)
R2 for 80psi sender: Use a combination of resistors to create an
87 ohm resistor. I used a 470 ohm resistor connected in
series with a 330ohm resistor to create a 690 ohm resistor (1/4 watt
is fine for these two resistors). That resistor was then
connected in parallel with the original 100 ohm resistor.
R trim (100psi only): 2.2k ohm between OIL and GND.
If you want to experiment with your own resistor settings:
I experimented with different values for R2 and R-trim before
selecting the values above. If you would like more low end
sensitivity reduce R2 and increase R trim. I looked at R2
values between 50 and 150 ohms and R trim values between 300 ohms
and infinity (factory trim, no resistor). A lower value for R2
will increase the range of needle motion but the response will be
more sensitive at the low end. A smaller value for R2 make the
gauge more linear but reduces total needle sweep.
If you are using a variable resistor for bench testing you can
adjust the resistor until the needle points to the H mark.
Then measure the resistance of the variable resistor (remember to
unplug it before measuring). Now you now what resistance will
give you the travel you want. Figure out the resistance of
your sender at say 90 PSI (I wanted H to mean 90). For the 80
and 100 PSI senders that would be 201 and 163 ohms respectively.
Now figure out what resistor in parallel will give you the measured
value. In my case I measured about 152 when the needle
indicated H. (2200 ohms^-1 + 163 ohms^-1)^-1 = ~152 ohms. At
2200 ohms this resistor as virtually no effect on the 0 PSI position
(2200 in parallel with 10 = 9.95)
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| Bench
testing and Putting the Needle in the Right Spot
By now you have rewound the coil and
should have the correct resistors on the gauge. All that's
left is to position the needle and check our work.
In the car: I prefer to test the gauge before
putting it back in the car but you don't have to. Don't put
the needle back on the gauge. Install the gauge back in the
cluster and connect all the cluster wires. Turn the car to ON
but don't start the motor. Lightly install the needle so it
points at 0. Start the car and drive it around to see if you
like the new gauge. If you do press the needle down the rest
of the way and be happy.
On the Bench: This is my preferred method. We are going to simulate the gauge on the car. Very lightly put
the needle on the shaft pointing at about the 9:00 position.
Based on my experience, when the gauge is not powered and the needle
is above
the hard stop it seems to want to settle at about the 9:00
position.
Connect ~12V to the IGN
terminal. Connect GND to ground. I used a 12V cordless drill battery
as the power supply. Finally connect a 10 ohm resistor across OIL
and GRD. A variable resistor works nicely since it helps check the
full gauge range. You could also just use the gauge if it's
out of the car.
When you connect all this up you should see the needle move. When
it stabilizes pull it off the shaft and reposition it so it points
at the Low tick mark.
If you are using a variable resistor you can simply dial up the
resistance until the needle points at each of the tick marks. I got
a very nice 15 turn 0-2K variable resistor from Radio Shack. Measure
the resistance of the variable resistor and convert that back into
oil pressure. I used the following formula to convert oil pressure
to resistance R=10+(oil pressure/100)*170. Now you know what
each tick mark means! So even if you choose something other
than 0, 30, 60, 90 you will still know what each tick mark actually
means.
At this point you can also decide if you would like to increase
or decrease the range of motion or change the response of the gauge
via changes to R2 and R-trim.
I know it’s a number of steps but you
end up with an oil pressure gauge that works!
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| Why the update?
When I originally modified my gauge I noticed a delay
when starting. When I would turn on the car it might take a
few seconds before the gauge showed any oil pressure. As soon
as it started to move it worked perfectly. It also always
seemed to take longer to wake up on a cold morning. I was
never really able to figure out why but I made some changes to my
winding to try to improve the issue. This resulted in a
few more coils that needed to be wound and the use of the trim
resistor.
Well it turns out the delay wasn't electrical at
all. A small bit of tape residue had found it's way onto my
needle's stop post. The delay was just the time it took to get
the needle to un-stick itself! With that in mind I decided to
simplify the procedures. The gauge now has just three coils
(The first version had 5). The new versions no longer need the
trim resistor (but I was too lazy to edit the pictures). I
basically just reverted back to the design I was using before I
tried to deal with the start up delay problem.
So that's why I've updated the page.
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