Dan2008

I have read through a lot of the threads in the FB forums and it looks like this is the answer to my needs but I have a few questions.

Since I have no OBD for input from various sending units (engine temp, oil pressure, etc), it looks like I will have to "roll my own" resistance-to-voltage convertors. Several threads discuss this notion and say to use a voltage divider made up of two resistors. From what I (think) I understand, I somehow need to convert the resistance from the sending unit into a range of 0v-5v. Is this correct?

That's fine, but I'm a software type and know almost nothing about circuits. Can anyone post a picture (not a schematic, I've seen those and don't know what I'm looking at) of such a component?

My next question concerns getting input from a pulse generator that replaces the speedometer cable. I think (there I go again, thinking about something I know nothing about) all I need is to know is the pulse rate per revolution then convert that into a human-readable output. Do I have to factor in gear ratios, tire diameters, etc? Can this type of input go directly into the FB or does it need some kind of conversion?

Thanks,

Dan

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Dan2008

Okay, I partially understand what is going on with the engine sensors now.

I removed the signal lead from the oil pressure gauge on another car (this one's a 1927 Ford with a 1967 Ford 289 hi-perf engine ) and measured voltage between the lead and gauge. With the engine turned off it was just about 10vdc (~9.99-10.03). With the engine running it was about 10.63.

Now, since this is a 12vdc system, I assume the ~10vdc when not running shows the minimum voltage (most resistance at the sensor) and the closer the pressure gets to the maximum (least resistance at the sensor), the closer it will be to sending 12vdc to the gauge.

Is this correct so far?

Here is the hurdle for me:

I think I have the voltage divider construction figured out. Simplistically speaking, you connect two resistors together in series and connect the signal lead between the resistors. The input on one resistor goes to the engine sensor (sending unit) and the output on the other goes to ground.

Am I still on track?

Now for the big question--how do I go about sizing the resistors?

From what I have read, it looks like I will need R1=700 ohms and R2=500 ohms. Here is my reasoning for that:

input voltage * (R2/(R1+R2))=output voltage
12*(500/(700+500))=5
500/1200=.416666666667
12*.416666666667=5

So, is this correct or do I need different size resistors here? Should they be 5 and 7 ohms? How about 5K and 7K ohms?

Having said (asked) all of that, am I correct in assuming the measured signal coming from the FB would be somewhere between 10vdc and 12vdc with ~10.63vdc being normal oil pressure (in this instance)?

My brain hurts...

Dan2008

Anyone???

greenman100

haha, easy now, it's been a day.

to put 4 years of electrical engineering education into a single post is not easy, but in short, we will need ot know the resistance and pinout of each individual sensor. Once you get that, we can move forward.

Bugbyte

You are on the right track, but as Greenman says, you should know the entire range of the resistance for each sensor. Then, proceed as you have posted.

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Dan2008

Well, the pinout is easy--they are all the old-fashioned single-wire sending units that screw into the intake manifold (temp sender) or engine block (oil pressure).

Finding the resistance range for the temp sender will be pretty easy but on the oil pressure I'll have to see if I can find that info as I really don't have a good method of applying known pressures to it.

Thanks,

Dan

greenman100

contact the sensor manufacturer.

Dan2008

Okay, I found the ranges for all of the senders.

The temp and oil pressure are both 240-33 ohms (low-high) and the fuel level is 0-90 ohms (empty-full).

Thanks,

Dan

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greenman100

http://www.mp3car.com/vbulletin/docu...-controls.html

go there, use solution C, come back if you have questions

Dan2008

That looks pretty easy but I do have one question. The text says R2 in two places but it looks like it should be R1 (see attached image).

Is R1 or R2 correct?

Attached Images

 

greenman100

yup, good catch

Dan2008

Dang, how 'bout that!!! Good instructions if I can understand them.

So, do I use the highest actual resistance reading or the highest rated resistance (90 ohms for fuel or 240 ohms for temp/oil) from the senders to size the other resistor as in the instructions?

I might be that the rated resistance is actually the highest since that is cold/no pressure. I suppose it is always good to check it just in case though.

Thanks for all the help! I'll be ordering my brain soon so I can start playing around with it (everyone always said that if I had a brain that I'd play with it) and creating my instrument cluster in the Fusion Control Centre Configurator.

Dan

greenman100

rated is what you want

Dan2008

Cool. I'm sure I'll be back needing help in the software forum when I get my stuff in.

RonS

Just a thought. Couldn't we use small hall effect current sensors? Put it inline with the sending unit. Measure the voltage and use that to get the reading. Then we could use 12 volts and not worry about voltage dividers. And if you already have gauges you could put the current sensors in. Then measure the voltage at different readings. Then you could setup your formulas to match the original gauge. It should work.

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