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Jonathan

ECU / electronics question

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I'm looking to finish off my little diagnostics multi function display thingy today - taking signal wire taps from the ECU.

I'm a bit puzzled as to how the ECU works out temperature from resistive sensors in the MAT and Engine coolant sensors.

Each sensor has 2 wires, 1 is called the signal, the other is ground.

What I need to do is take the voltage ref (pretty much what freescan does).

If I probe the signal wire I get 5 volts with ref to ground, if I then probe the ground wire I get perfect continutuy to the engine.

What I am a little confused about is how the ECU actually determines the 'voltage' across these sensors

The sensor is resistive, as it heats up it becomes less and less which means the volt drop across the thing is lower, but becuase there is no 'tap' with reference to earth how does the ECU know what voltage there is across it ?

I can grasp the 3 wire systems of potential dividers but I dont really know (without taking teh ECU apart) how the ECU works out volt drops using a 2 wire system...

I'm wondering now if the potential divider is actually built within the ECU - I would cut the signal wire and probe it but at the moment I havent got the ability to do that.

Any help would be appreciated, a lot actually hehe - (it could be I am being really thick here)

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There must be another resistor inside the ECU in series with the signal wire with the other end of this resistor going to a reference voltage, then effectively all it has to do is measure the volt drop across the temp sensor caused by the current changing as the temp sensor resistance rises and falls. As you say just puting a reference voltage at the end of a resistive sensor would acheive nothing as the voltage would never change until maybe it got to the point the sensor overloaded the reference voltage.

The circuit must be something like below, with the reading taken between chassis ground and the point between the sensor and the ECU internal resistor.

Chassis Gnd ------- sensor -------resistor------ref voltage

You should be able to see the change in voltage on the signal side of the sensor wrt to ground as the engine temp changes.

Martyn :)

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Yeah I think you're right, makes sense - I just choped one of the temp sensors on the ECU and put a terminal block in there to test it :)

I havent had time to do that yet, hopefully when I get my laptop to run freescan I can test that against my volt meter readings and fingers crossed !

Cheers

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Another possibility is that the ECU measures the current through the sensor as it keeps the voltage across the sensor constant. In order to do so the current needs to change if the resistance of the sensor changes due to a change in temperature.

Freek

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The signal is the center tap, there is an internal resistor to Vcc. Some sensors have the internal resistor to ground. Be careful that your external circuit does not load the signal.

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Yeah I'm hoping it wont ! :)

Gonna try it out tonight, basically I'm trying use the tap to supply a ladder circuit so I can use a sort of LED bar graph for the ECU readings inside the passenger compartment.

I like to have an idea of MAT/MAP/Engine coolant/RPM without running a laptop and freescan - I'm pretty sure it will work as it is a passive circuit I am using but I've said that before and regretted it !

If it works it'll be ace, I'd love to have an LED readout instead of looking at the dials and gauges - more for diagnostic and problem solving than anything esp since the boost will be going up this year.

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Can you not put in extra MAT and Engine coolant sensor just for your kit?

That way you won't have to touch the ones the ECU uses, and as you are

not taking from them, there's no risk of your extra circuit causing any signals.

Not Know...

:)

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I could but ideally I want a freescan away from the laptop - I want to know what the ECU is being fed.

In essence this is what is happening with the coolant dial for example - it does not show the ECU reading and I am getting errors.

The major things I like to keep an eye on are MAT and engine coolant.

I've just come in (I think I've been in the garage with the car running too long - feel really sick LOL, twat) and finished a test on the coolant - my VDO gauage on the dash is WAY out, yes OK it shows the engine hot but it's about 15 degrees off. I dunno why Lotus did this, the ECU reads the temperature around the inlet maifold jacket, the dashboard gauge is from the water pump - why not just take it from the ECU ? I'm sure there is a good reason which I'm just about to find out ! LOL

From Freescan, find :

50 = 3.67v

55 = 3.45v

60 = 3.25v

65 = 3.04v

70 = 2.81v

75 = 2.63v

80 = 2.41v

85 = 2.20v

That's now confirmed by having a DVM in line with the sensor wire so gas/on/cooking.

All I need to do now is calibrate my scale on the LED bar graph and I should have what I want - this is the thing I dont want an analouge gauage - what I wanted to do is have a colour scale from green to red, for the coolant (for example) I will have 20 LED segments Geen up until ~ 85degrees - Amber to 92 and Red over 92 as a quick indicator where the ECU is determinining the temp to be.

I can do this with boost as well using the green scale to 0-0.65 bar, amber 0.65-1bar and tipping red at absolute MAX boost - that way you dont have to pay too much attention to an analouge dial to see you're hitting max boost, you can see it out of the corner of your eye.

Nice thing is I can switch all these buggers off or dimm them down as well.

The LED RPM meter is pretty impressive if I do say so myself.

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Sorry - I have a working model at....work, it's on the prototype board there and I am just drawing up the circuit diagram to solder it up

As soon as I get it fitted to the car I'll post some pictures and all the info.

Keep in mind I work on this perhaps 30mins a day so prograss is slow...about 4 months so far LOL.

The last circuit I built uses a few trimmer pots to raise and lower the max/min input voltages - so instead of measuringfrom 0-5v (which on the temp scale would be infintly cold to VERY hot)

I can trim it to 1.15v to 4.05v (which is about 40c to 105c)

Gonna do a lambda sensor as well (like a swing-o-meter) for the A:F ratio

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Circuit diagrams would be nice, too... :)

Edited by sanj

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As it happens I am doing something very similar for The Mutant, except I am using a 24 x 2 line alphanumeric LCD display.

This is an early prototype running.....

IMG_0796750x562.jpg

It currently shows real time AFR, battery voltage and external temperature as digits, plus full screen warnings and alerts for stuff like low oil pressure, high coolant temp etc.

The PCB for The Mutant has 5x analogue (0-12V) & 11 digital inputs

Analogue

-----------

oil pressure

coolant temperature

fuel level

external temperature

spare

Digital

-------

Alternator

Handbrake

Brake fluid level

Screen Washer fluid level

Tacho

Door switch

Vehicle speed sensor

Lights

Coolant level

2 x spare

It also has an RS232 interface which connects to the VEMS ECU and gives AFR and battery voltage.

The code could easily be changed to display pretty much anything you want as digits or a bar graph, so if there is any interest for this I could get some extra PCBs made up ?????

Hilly

Edited by hilly

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LOL do you know how long it took to do my circuit ! Anyways I'd be interested in seeing what you've done, the led segment is just the only way I could think of doing it with my limited knowledge in electronics, plus it was fairly cost effective, well, under

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My unit is based around a PIC microprocessor and not a lot else, there is a bunch of signal input conditioning and a 5V regulator, total parts cost (excluding display) is probably about

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I'd be interested. What are you using for sensors, add-ons or the factory bits?

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Depends on what parameters are being looked at.

The air fuel ratio and battery voltage are taken from my VEMS ECU via an RS232 interface.

The external temperature is via a dedicated semiconductor sensor.

The fuel level, oil pressure and coolant temperature is piggybacked from the dash gauges.

Brake fluid level, alternator status, handbrake, tacho, door switch, VSS, and lights are the OEM sensors/switches.

Screen wash and coolant levels will require additional sensors.

Hilly

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Or the signals can be nicked from the ECU (hoping the ecu doesn't mind).

The main reason why I say is the dash gauges (in my case the speedo and the temperature) are so far out I cannot even rely on them

My engine temp generally sits about 82-85 degrees, my dash gauge says 65-70 - thats not a problem for most people and I admit I am a little bastard about detail but the ideas was is that I want to know what info the ECU was getting.

A little LCD display with

MAT 23 (press button)

ENG 83

PSI 20

Would be ideal, can you record max readings ? I can on the LED gauge by using the LM3914 (I think thats what it's called) chips DOT function and a huge hysterises circuit on the end so it take hours for the circuit to reset - thats useful for reading max MAT and boost.

TBH Hilly I will probably stick with the LED thing I have done, only because it's taken me so long it'd be a shame to bin it :)

It'd be cool to see yours though, mebbe move over to that later !

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Making it store max reading since the ignition was last turned on would be a breeze,.

It would need a switch input to swap between real time and max, but you could assign the digital (or analogue for that matter) inputs to do what ever you wanted.

You could even get it to display a warning if any of the inputs went above definable limits............

Hilly

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Thats handy then :(

(interest swing o-meter back to hilly's idea)

Sounds like a great alternative to driving around with the laptop for sure !

Next question, can I have the LCD in red ? :)

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Hi Hilly,

how's the progress on this ?

I would be interested in building one up if you have some PCB's produced??

Regards

Steve

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Hi Hilly,

how's the progress on this ?

I would be interested in building one up if you have some PCB's produced??

Regards

Steve

Blimey, how spooky.......

I haven't done anything on this for months and the day I start getting everything ready to get a PCB made someone asks about it !

I have just got the PCB fab started and it will be 3 weeks before the bare boards come back, I then need to assemble and test it so you are looking at 4 weeks before I know if it works OK, not that I foresee any issues as the prototype works fine.

The bare PCBs are

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Well I've made my LED one - not fitted (been working on the other car.

It definatly works on the bench powersupply and is extremely accurate.

Funny enough i was looking at it for the 1st time today and wondering how to go about fitting the LEDs

Modifying the original gauges would be cool but might not be a good thing if I re-sale.....dunno yet.

The above would be ideal but I cannot program PIC chips and I'm not about to start !

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Hilly, still be interested in this when you're done as a small multifunction display.

In the mean time I've just fitted a dual (in/out) temp sensor to the intake ear and the silicone hose between the chargecooler and the plenum.

Whats interesting is the difference is the real air temperature and the one the MAT records.

Radiated heat from the engine into the part the MAT sensor screws into and also warming of the MAT sensors exposed body causes it to read higher than 'real' temperatures.

All confirmed by laser thermometer which is also calibrated.

I wonder if this has any bearing on power / efficiency ?

I've always wanted to build a chargecooler heat shield from treated thin section ply wood, I even have the template for it but never got around to it (theres an idea...) and I am convinced this will do something for the car.

As a guide the real air temp is about 20% lower than the ECU MAT, they're both within 3" of each other and it's just that one is screwed into a rather good heat conductor (aluminium).

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