Oil Pressure Gauge to ECU sensor

[Jethrow]

Hi all,

Hoping you guys can help out a roadster builder who is installing a LS3 into his cobr@ replica, seeing as you have the LS power over here.

I am running the GMPP stand alone ECU that can be connected to the standard LS3 oil pressure sensor. There is a pin out from the ECU wiring bulkhead connector that could then go to the F5R gauge. The query is to whether this will scale right with the factory five gauges?

I guess real question is, is the signal output from the GM sensor the same signal as what the gauge is expecting? I do know the output is 32*sensor voltage - 16 = psig.

The GM also give some conflicting advice about whether or not the ECU will fail to function without the oil pressure signal. Some places it says to make sure its connected, other sections say the ECU does not require it. Do you guys have any feedback on this?

Many thanks

[00SS_M6LS1]

I've read concerns about this, but never had a conclusive answer. From my
experience, the oil pressure sensor does not disable the engine running
capability.

Don't quote me just yet; I'll check it out, and respond.

As for the guage, you can always use a resistor to calibrate the needle position
once you get a point of reference. Best is to use a trusted mechanical
gauge to calibrate/check the electrical signal.

[Shane]

The GM oil pressure sender is only good for the signal to the ECU. You can not use it to operate the gauge...and you also can't use the FFR sender to send a signal to the ECU. The dash gauge works off of 12v. The ECU sends a 5v signal to the GM sensor....they are not compatable.

[Jethrow]

Thanks for the responses guys.

I look forward to some confirmation as to whethr the ECU is happy not to receive the oil press signal meaning I can just use the F5 sensor in its place.

[Shane]

I have not had a problem in all of the GTM's I've had thru the shop. I don't know of any of them that had the oil pressure sender to the ECU hooked up.

[00SS_M6LS1]

Hmmm...Voltage divider. Why wont this work to bring the voltage into range?

Low power, 1/4 watt (maybe 1/2 watt) resistors?

---+ 12V
-
-
-
----xxx(var.)xxx +5V
-
-
v (ground)

Just to clarify, are we discussing the Autometer 4327?

[Jethrow]

Actually I have the F5R vintage look gauges, I think they are from speedhut? But your logic seems reasonable.

And thanks for help Steve - it does make me feel happier to delete the LS3 sensor.

[00SS_M6LS1]

If that's the case I'll try it with my gauge, and draw up a diagram with
the resistor values. Give me about an hour, or two.

[00SS_M6LS1]

Remove obsolete circuit. See below.

[Jethrow]

Thanks.
I do have to admit this is a bit over my head, but it looks impressive.
I had a look at the vintage gauges, however I do not see any part numbers or any marks on them at all.

[00SS_M6LS1]

Remove obsolete circuit

[00SS_M6LS1]

Tim,

Do you own, or have access to an Ohm Meter (or multi-meter)?

I`m not getting much in the way of technical info on your gauges.

All we really need to know is the DC resistance of the input terminal to
ground.

PM your home e-mail/phone number so we can firm this up quick.

If anyone else has the same gauges, feel free to post the resistance reading.
We'll need them to determine the proper resistor values. I will also post
the results here for anyone else wanting to convert their gauges.

[Jethrow]

Hi,
I put my multimeter on the input and ground to the gauge but it gave no reading, ie open circuit. Not sure if this was the right answer.

PS - For work I will be out of country for 2 weeks (off to Mali!) so I will be radio silent till then.

Thanks again.

[kgtm]

Attached is about what a gauge looks like, even from point S to GND, it takes about 2 volt and 20 ma for full scale, also it is none linear while the sender output is linear with .5V offset and 1V for each 32 psi.

For normal operations it take about 80 to 100 ma to set the gauge to full scale.

so not easy to do if you want accuracy over all range and only if the sender can drive 20 ma load without any issues which my guess is was not design to do.

I have made devices for my car and is does this conversion also for me, but so busy right now have not done much last 6 months.

Mostafa

Capture.jpg

[00SS_M6LS1]

Thanks for that little schematic.

I'm measuring 85.5 ohms on my Autometer oil gauge.

The question marks on the diagram don't make me feel too warm, and fuzzy,
but once Tim returns we can have him retest the resistance of his gauge.

With that info, the resistor calibration should be fairly close.

I will also check the response of the meter movement vs. voltage input
to get a sense of linearity.

[kgtm]

I think the point here is the GTM oil and water gauges are not high impedance, and it take lot of current to drive one per designed ( 80 to 100 ma), the least current ( highest impedance) is GND to S, even this will draw 20 ma, also if use this way the reaction is very slow, since only one coil is biased.

the coil resistance is about 300 ohm, the 20 ohm in often not even there so zero, the the 300? will depend on the gauge and sender type, this may be 120 to few hundred.

Mostafa

[00SS_M6LS1]

Mostafa,

Check out the YouTube video I linked a few posts prior.

The meter movement is fairly quick even with the voltage divider present.

This is because we are simply presenting a "ratio" of sorts. The gauge
will receive the proper operating voltage, and therefore the current through the
gauge can be tailored by the values used in the divider circuit.

I was able to get close in the video; although not perfect, with more
calcuations, and the oil sender factored (using a potentiometer), I'm
pretty certain this will work.

If not absolute values, at least a very close representation of pressure
in the oil system.

I'll try to get a more complete circuit built, and tested by the start of
next week, and post another video.

[kgtm]

It does not change what I have said, same picture, from gnd to S, it takes about 20+ ma ( 85 ohm) and about 2 volts for full scale! he does no say much about resistors size and current through them.

Again not sure if one can draw 20 ma from the sender without affecting the sender, generally amps may be good to 5 ma or so. also scaling is an issue if you want it to be accurate to full scale.

Mostafa

[00SS_M6LS1]

Remove obsolete circuit

[kgtm]

What are you connection to gauge?

12v to I
Gnd to GND
and variable resistors to S

or just
I open
GND to GND
and variable resistors to S

Mostafa

[00SS_M6LS1]

Invalid circuit. See below.

[kgtm]

The issue with diagram is the PSI goes up as resistor value goes lower( lower voltage at S term), while with the LS3 sender, the voltage is .5 at 0 psi and goes up 1V per 32 psi after that and is no resistive it is true amplifier, so reverse.

Below may work, but gauge has very low bias and may be prone to vibration errors, but the needle will move in right direction and could be calibrated to be correct at least at one spot.

even this the sender may be able to drive, since it will need about 15 ma for full scale.

Mostafa

Capture1.PNG

[00SS_M6LS1]

Ahh, that's much different than the initial diagram that was posted.

Then my first video would be more appropriate for connection.

Now we know the gauge will function at 6VDC, and below so maybe your
last diagram will work fine. I can try that layout, and post the video.

What if we were to use a diode on the sender input lead (I) to drop the
the offset? I'm thinking that will work...but then, It's almost 3:00 AM
here, and my brain is just a bit tired.

Germanium diode is about 0.3-0.4 voltage drop if I recall...? Or an Si
diode is 0.6 VDC drop. That will eliminate the offset, and then the resistor
can be used to calibrate the new range.

I agree, if nothing else, the circuit should be calibrated/biased for needle
deflection to read in the normal operating pressure range for best
accuracy.

[00SS_M6LS1]

Good news, the diode works! It's pretty close right now; just needs
a bit of tweaking to calibrate the scale.

I have a video uploading right now, but until then here are the numbers:

0.700 VDC = 0 PSI

2.000 VDC = 60 PSI

2.500 VDC = 95 PSI

Current limiting resistor = 150 Ohm
Diode in series with input signal from oil sender (drops 0.600 VDC) to eliminate the offset.

Total current (at max deflection) = 11.6 mAmps
Total power = 58 mWatts

Approximate cost to convert = $2.00

I'll post the video, and schematic in the next reply.

[00SS_M6LS1]

Gauge conversion video:

GTM LS3 Meter Calibration - YouTube

[kgtm]

First schematic was mostly how the gauge looks like inside.

Really not sure if you need the diode, if you notice the gauge does not sit at zero, it needs some current to get to zero any way so you can use the .5v offset, and cal for 32 to 64 psi (1.5V to 2.5V from sender).

Now the real issue, tap the gauge and you will see it moves and dose not come back by itself, so it sort of work, but not sure how it will do in car and all type of vibrations.

This gauge in normal connection consumes about 50 to 100 ma and I think this is mostly done to keep the needle in place on the road, and to help with ground noise.

This may be great for some though, good work.

Thanks,
Mostafa

[00SS_M6LS1]

Mostafa,

I'm not experiencing any sort of needle deflection with my Autometer,
Pro Comp gauge (PN#4327). I flicked the side in every direction quite
forcefully without any movement whatsoever.

It's very possible that the internal mechanism varies in tolerance which
explains the difference in current limiting resistors, and the need for mine
to use a diode.

If my guage uses 120 ohm as per your schematic, the needle deflects
beyond 100 PSI @ 2.5 volts input.

At 0.5 VDC and 120 ohms, the needle is positioned slightly above 0 PSI

The resistance is very sensitive to the range; with only 20-30 ohms
difference, the scaling changes about 15 PSI.

Without the diode, I'm not able to get the needle to swing between 0 PSI
and 100 PSI using 0 - 2.5 VDC.

In any case, we've setup some options for those wanting to use their
gauge with the LS3 Sender. I think by utilizing a variable potentiometer
in the circuit, these setups can be tailored for variances in the gauges,
and vehicle electrical systems (voltage, oil sender units, etc.).

Thanks again for providing the drawings!

[kgtm]

I have the same Gauge, on mine it goes from zero to about 100 with 120 ohm and no diode, the difference may be slight change in number of turn on the coil, again is not accurate at all point, could be set below 50 about right and above 50 you get up to %30 off, but may be does not matter.

You can also connect to just I and S(GND), and a about 60 ohm in series, this has better correct range to 3.5V or so for 100 PSI, the only issue with this may be more current draw from the sender and it may no provide it.

Mostafa