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I Can't See All My Gauges!
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Thu Oct 08, 2009 7:06 pm    Post subject: Reply with quote

You have inspired me so I have bought a 944 gauge with econometer, I'm going to use the face from a standard 924 gauge, drilled for the econometer spindle and turn it into a boost gauge.

I have a mech boost gauge already fitted but it's cheap and nasty, also it doesn't operate smoothly also I miss having a clock!!

Rich
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Fri Oct 09, 2009 9:06 am    Post subject: Reply with quote

OK Rich, check this out.

FSD of econogauge is 0.8v

Map sensor to use is GM very common
"cheap as chips" at your local scrappy. get the plug and enough wiring too.







more info here http://www.aa1car.com/library/map_sensors.htm



use this as circuit design
(R2 is for calibration - C1 is Electrolytic Capacitor to give smooth gauge movement )
I was gona use wire wound resistor and pot!



Data sheet for 5v regulator 7805 you will find here (pin out data too)
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2148.pdf
get one in a TO 220 body, It may want a heat sink

I will re assemble tacho but have two wires coming out the back of it
(via a grommet) from the moving coil of the econogauge.
these wires will then go to a small ABS box under the dash where it will pick up a 12v supply from ignition circuit.
Three wires will then leave the box, go through the bulkhead via a grommet to the MAP sensor.
The MAP sensor will fix to the bulkhead in the engine bay with a vacuum hose going to one of the spare ports on my Audi 5000 throttle body.
A little fine tuning to the pot on the ABS box under the dash during a road test and were done!

what do you think?
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Fri Oct 09, 2009 11:17 am    Post subject: Reply with quote

Shurick wrote:
9XX Girl! wrote:
Shurick which side is your steering wheel? Mine is on the right side.

Sorry, missed that.


my steering wheel is on the right hand side.
I could not see the temp gauge. i guess the car was designed left hand drive.
i put the fuel/temp gauge on the right of the speedo and the tacho on the left of the speedo.
the stering wheel then obscured 5k rpm up to red line so i got me a 944 rev counter which is slightly different.
that solves the viability issue but gives me an econogauge (reads in Miles Per Gallon) that does not work.
this little mod will make the econogauge work and also, allow me to keep a check on vacuum leaks, - simplezzz
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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RC  



Joined: 25 Mar 2007
Posts: 2636
Location: Australia

PostPosted: Fri Oct 09, 2009 12:19 pm    Post subject: Reply with quote

Which way is FSD of the econometer? If it reads in reverse simply swap A & C on the MAP.

Quote:
I was gona use wire wound resistor and pot!

There`s no need. Total power dissipated through 15K is < 2mW. 7805 wont need a heatsink either.

A 10mFd may be too large, depends how much lag you want. Easy to experiment with a few different values without knowing details of the moving coil.

So you will get similar MPG reading at whatever KPa in first gear and also in top, regardless of RPM. An uncalibrated vacuum gauge rather than a MPG meter. If that`s all you want then its cool.
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Fri Oct 09, 2009 12:29 pm    Post subject: Reply with quote

Well prototype. it may need a tweak but i will let you know when it has been road tested. still need to colect some bits

i'm using wire wound cos ive got some kicking around, but it a simple vacuum gauge, yes
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Fri Oct 09, 2009 5:40 pm    Post subject: Reply with quote

I can't see the circuit here at work, but I was going to use a pressure sensor from CPC (they are local to me - check out their website, loads of components cheap!) They are PCB mount and go from 0-2 bar and use a 12V supply. Output is just a straight 0-12V I think. About £10 I think. My plan is to drop it all in the tacho case. That means there is 12V and ground handy already, just need a hole for the nipple on the pressure sender. A bit of clever electrics to get that down to a 0-1v and away we go.

Rich
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1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Fri Oct 09, 2009 8:49 pm    Post subject: Reply with quote

you will need some kind of regulator circuit cos the voltage on a car under normal operation conditions
does jump around a bit anywhere between 11v & 14v.
you don't want the gauge bouncing around when you put your turn signal on! Remember, FSD on the moving coil is only 0.8v

Also, the GM MAP sensor is for automotive use. i.e. oil / temp tolerant.
The pleasure gauge needs to give the same output stability over temperature changes.
Plus, you can get these sensors to deal with boost as well. Hence different part numbers and 1bar, 2 bar, and 3bar units.
the 924 engine does burn a little oil. thats how it was designed.
When i changed my throttle body it had oil residue in it.
BLOW-BY, via crank case breather into vac system.
the pistons only have 2 rings. I wouldn't personally worry about changing rings until i'm getting oil in the air cleaner, (which i am not) but it should be a consideration.
The gas pressure your gauge will measure will contain, oxygen, nitrogen, petrol vapor and a hole host of gas born hydro-carbons.
Plus under WOT conditions there is no Vacuum to keep this stuff out of the sensor. And i think your using boost.

When i worked for Jaguar many moons ago. We had this problem.
The ECU for the XJS V12 had a vacuum sensor inside it.
There was a vacuum hose that ran from the inlet manifolds through the bulkhead to the ECU. Fine when it was new!

Now common practice now is to put these sensors in the engine bay and run the wiring to the ECU. Remember the sensor is comparing ambient external pressure with manifold pleasure not cab pressure with manifold pleasure. They also take temp into account.
Plus, don't want to build your gauge to find its not oil proof,

But let me know your design ideas. I'm at prototype build stage now.
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Fri Oct 09, 2009 9:24 pm    Post subject: Reply with quote

RENAULT MAP SENSORS GM TYPE BARGAINS

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=180409109011&ssPageName=STRK:MEWNX:IT
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924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Fri Oct 09, 2009 10:10 pm    Post subject: Reply with quote

The S1 engine I helped rebuild had 3 rings:- 2 compression and an oil scraper...

http://cpc.farnell.com/honeywell-s-c/24pcdfa6a/pressure-sensor-2-30psia/dp/SN35976

Sensor is chemical proof, the vac line I'll be using is on the DTIC unit in the cabin already. It's an absolute sender so will read from 0-2 bar with reasonable temp stability.

The voltage needed is 10V for the sender so a 10V regulator will drive the thing. Same circuit as yours from there though. I'll also include a unidirectional suppression diode to clamp the voltage coming in to 12V (Assuming I can find one the right value!) that way any noisy voltage spikes won't damage anything.

An idea from Megasquirt is to fit a fuel filter in line with the vac line, this give a degree of damping to the pressure to the sender as well as preventing any filth getting into it.

That's if the postie decides to delivery anything...
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Sun Oct 11, 2009 4:09 am    Post subject: Reply with quote

Rich H wrote:


That's if the postie decides to delivery anything...


I know the feeling. A post strike is all we need. Anyway, looks like you got it all covered. Hope we're gona get some Pics when you are done.

So if you are gona use the 924 gauge plate in the 944 gauge, how are you gona label graduations for your vacuum gauge? Also, 944 gauge “0 rpm” mark is 15 degrees anticlockwise of the 924s. Your numbers will be wonky on the rev counter? Or… are you gona have the vacuum gauge to the right of central and “0 rpm” mark at the 3 o’clock position?
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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Rich H  



Joined: 10 Jun 2007
Posts: 2665
Location: Preston, Lancs, UK

PostPosted: Sun Oct 11, 2009 5:24 am    Post subject: Reply with quote

I have a cunning plan with a photocopier...

My plan is either:

To drill the 924 gauge face for the mpg gauge spindle then photocopy and print off a boost gauge face onto satin paper and glue in place (might look sh!t)

Or just ptint off a whole gauge face with a boost gauge on it.

Or use a bit of a multi function gauge dial, the temp gauge, trim the edges and stick that in place

The Rev counter needle just pushes on so I'll just pull it off and recalibrate it.

Thinking about it the off center gauge might work... I'll have to see what it all looks like.

It#'s going to take some fiddling about any way

Yes I'll take pics!
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec
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9XX Girl!  



Joined: 20 Sep 2009
Posts: 1617
Location: Cornish Riviera SW England

PostPosted: Tue Oct 13, 2009 8:29 am    Post subject: Reply with quote

Tach mod is now complete

The two wires for the moving coil of the MPH Econogauge have been removed and taped up. I then soldered two new wires (coloured blue) on to the MPG gauge.
The top one is soldered to the 6mm female crimp that connects to the brown ground wire that goes onto the casing.
The bottom one goes through a hole in the the casing.



this shows the tach being closed. I crimped it shut with pliers and used a modified cloths peg to prevent the front of the trim from squashing.



Finished Tach
I made sure i installed the wire with an appropriate grommet.
Ground for the MPG gauge is shared with the Tach on pin 4
The flying lead is MPG gauge +0.8v FSD



next stage is to build the PCB for changing the MAP sensor output to the MPG gauge input signal.
_________________
924 (84) N/A 2 ltr - 5 Speed - BLACK - Project (looking for 200BHP, any ideas!)
924 (81) N/A 2 ltr - 3 Speed Auto - SURINAM RED (Metallic) - Near Original Spec (sticking with originality)
BOTH ON THE ROAD, BOTH USED EVERY-OTHER DAY
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RC  



Joined: 25 Mar 2007
Posts: 2636
Location: Australia

PostPosted: Tue Oct 13, 2009 2:11 pm    Post subject: Reply with quote

9XX Girl, whats the resistance of that econo meter moving coil? Guessing <1K, a few hundred ohms perhaps? Feel sure that its a low impedance and will not be deflected by the higher impedance MAP unit, at least not via a 15K/5K voltage divider.

That MAP sensor appears to have an integral amp to output a linear 0-5V. Disregard my previous comment about reversing the supply. Any idea of its rated max output current? May just be able to use a lower value voltage divider, 250 ohms will give you 20mA.

IMHO the best option is to use a basic voltage follower to drive your meter. A simple op amp will be easier and cheaper than a discreet transistor design with no regard for the base voltage offset compensation. The op amp also gives you the advantage of using either the non-inverting or inverting input depending on which direction (higher or lower MPG) the meter deflects. Also easy to fit a trimpot in the feedback to change the gain from 1, which along with the input voltage divider pot should allow you to adjust the min-max range of the meter.

Rich, sure that the sensor you linked to is a resistive wheatstone bridge device. They are quite accurate and fast response but have a very limited output swing. They are generally intended to have their output processed by an instrument amplifier. The 10V is a baseline value which makes it easy to compare & calculate output voltage per pressure unit (PSI)
Quote:
30 psia = 11mV/psi typ.

So with 10V across the wheatstone bridge (a dual voltage divider) the output at 1 bar will be only 162mV, half that with a 5V supply. Thats why it has to be amplified. A variable current is often preferred to a variable voltage and the signal processed by an op amp based instrument amp. A dedicated instrument amp IC is technically superior. Personally I like the AD623 since it is suitable for a single supply and well priced. I have used it on my fuel pressure gauge and logging interface that uses a similar type pressure sensor.
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Scorpio  



Joined: 05 Jul 2007
Posts: 1957
Location: Brisbane, Australia

PostPosted: Tue Oct 13, 2009 2:37 pm    Post subject: Reply with quote

RC wrote:
9XX Girl, whats the resistance of that econo meter moving coil? Guessing <1K, a few hundred ohms perhaps? Feel sure that its a low impedance and will not be deflected by the higher impedance MAP unit, at least not via a 15K/5K voltage divider.

That MAP sensor appears to have an integral amp to output a linear 0-5V. Disregard my previous comment about reversing the supply. Any idea of its rated max output current? May just be able to use a lower value voltage divider, 250 ohms will give you 20mA.

IMHO the best option is to use a basic voltage follower to drive your meter. A simple op amp will be easier and cheaper than a discreet transistor design with no regard for the base voltage offset compensation. The op amp also gives you the advantage of using either the non-inverting or inverting input depending on which direction (higher or lower MPG) the meter deflects. Also easy to fit a trimpot in the feedback to change the gain from 1, which along with the input voltage divider pot should allow you to adjust the min-max range of the meter.

Rich, sure that the sensor you linked to is a resistive wheatstone bridge device. They are quite accurate and fast response but have a very limited output swing. They are generally intended to have their output processed by an instrument amplifier. The 10V is a baseline value which makes it easy to compare & calculate output voltage per pressure unit (PSI)
Quote:
30 psia = 11mV/psi typ.

So with 10V across the wheatstone bridge (a dual voltage divider) the output at 1 bar will be only 162mV, half that with a 5V supply. Thats why it has to be amplified. A variable current is often preferred to a variable voltage and the signal processed by an op amp based instrument amp. A dedicated instrument amp IC is technically superior. Personally I like the AD623 since it is suitable for a single supply and well priced. I have used it on my fuel pressure gauge and logging interface that uses a similar type pressure sensor.


RC ---you are so geek right about now!
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1979 NA
MS1..EFI..
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BILSTEIN SHOCKS
GT BASED CUSTOM BODYKIT

Brisbane , Australia
Think mean think fast
all youll see is
my Porsches Arse!!!
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RC  



Joined: 25 Mar 2007
Posts: 2636
Location: Australia

PostPosted: Tue Oct 13, 2009 2:47 pm    Post subject: Reply with quote

So in other words Scorpio, most of that went way over your head.

Does it make you feel intellectually inferior knowing that a chick could understand?
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