EE question

When testing items like MAP sensor, I have on occasion wanted to estimate expected voltage drops across different resistances, or expected resistances for a voltage drop in the ECU sensor circuits. However, I don't know if the current is constant in these circuits, and i.e. there's the rub!

I know that at the extremes, limits, the current can not be constant, like 1 meg ohm versus 100 ohms, versus maybe .01 ohms. But what about 10 ohms versus 500 ohms? I would expect that there is a current limit in the ECU/PCM computers, but that is where my knowledge ends. Do any of the EEs here have any idea how to talking coming up with engineering estimates for this, and ideas as to what and how the ECU/PCM black boxes function. Wheatstone bridge comes to mind, I am not sure if the have a constant current control feeding them, or what? My guess is they are current limited. So once again, short of just collecting imperical block box and sensor data, I am clueless in Houston! LOL

Merry Christmas !!!!:sunshine:

Good questions, but I don't think anybody has cracked much of the Renix ECU.

I liked your idea of a group-purchased 2500 and asking 5-90 to manage it, that would help him in cracking the Renix.

I want to break one open just to poke around but I don't have a bad one and I haven't even had an XJ come in with a bad one.

I seem to remember a thread where an Eastern Europe guy had a bad ECU and opened it up to solder and/or replace something inside, but that is the only instance I can remember.

He was in Poland or Romania IIRC. He had one fuel injector that was not firing. Seems there was a transistor in the ECU that would sink the coil voltage across the fuel injector to ground, to make the FI open. There were 6 of them, one for each injector. One of them was bad. He found an equivalent transistor, and replaced it, and put the ECU back into service with no more problems.

I am hoping lawsoncl or RichP may drop by and answer my engineering fudge factor EE question in the first post.

If you join in, that will make 4 of us planning to possibly split the cost of an MT-2500 used off of EBay to park with 5-90 for research of the Renix ECU, and 5-90 would hold it on standby to loan back to us for a few weeks anytime we need it for our Renix jeeps.

In general, I think the ECU is just looking at the voltage from the sensors. The current flowing into the ECU from a sensor will likely be quite small, a function of the input impedance of the ECU circuit.

The MAP, and TPS are voltage output sensors. Their output is not low impedance, but probably around 500-1000 ohms. The CPS appears to be a Hall Effect voltage output sensor and as such a voltage output sensor as well. I haven't looked at the camshaft position sensor.

The TPS, CTS, MAT and MAP are all variable resistors. They do not produce a voltage output at all. The ECU sends a voltage to them, 5.0 volts DC, and then reads the voltage drop via a wheatstone bridge circuit inside the ECU. I am not an expert on wheatstone bridge circuits, but I suspect it is a current limited device.

The Renix TPS runs from 2200 to 8600 ohms. I have tested several. The MAT and CTS resistance from 100,700 to 185 ohms, -40 to 212 F.

The CPS or CKP as some call it, and the O2 sensor does produce a voltage output.

So back to my question in post one!

A variable resistor (ie pot) is a voltage divider. It is a voltage device as opposed to a current device. Take your pick, there are only two types of analog sensors, voltage and current.

Before you challenge my 40+ years of engineering technical expertise, I suggest you do your homework. A little knowledge is a dangerous thing.

old_man said:

A variable resistor (ie pot) is a voltage divider. It is a voltage device as opposed to a current device. Take your pick, there are only two types of analog sensors, voltage and current.

Before you challenge my 40+ years of engineering technical expertise, I suggest you do your homework. A little knowledge is a dangerous thing.

Click to expand...

You may have 40 years of EE experience, but you are not explaining yourself clearly! Your first original sentence, still makes no sense to me.

"The MAP, and TPS are voltage output sensors"

That sentence says to me that you are calling the sensors, like a MAP, TPS, CTS or MAT (which all simple, single variable resistors, with a single in and out connection), "voltage output devices", sense the sensor is a device, and therefore implying devices that put out a voltage, like a thermocouple, which they do not do. And I made straight A grades in all my english classes! So GRRRR! LOL. We can toss around credentials all night long if you wish, LOL. But my reply was not just to your post. It was to both posts, one by another.

And your second post has only added to the confusion so far. Neither answered my question? At least not in an understandable way to me,... YET!!!!!

Sorry, but I am old timer too, LOL! And just as cranky at times, LOL!:wave:

So you tossed out the term: "Voltage divider", which I Googled for the class (which includes me!):

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

That shows a dual resistor, not a single resistor. So are you talking about a single isolated "sensor", or the entire ECU-MAP "sensor" network? I am assuming after your second post that hopefully you mean the entire system, and not just an isolated sensor-resistor. Were you referring to my comment/question about wheatstone bridges, which I qualified in my first post, that my credentials are lacking in that area (EE), and I asked for insight, which I am still looking for!

Also for the benefit of the class (which includes me), here is the Wheatstone bridge (a 4 resistor network, versus 2 resistors in a divider?) which I mentioned as the only thing I was vaguely familiar with as being used for measurement of this type:

http://www.citycollegiate.com/wheatstone_bridge.htm

So you have my attention, but please explain in more detail, and please more clearly what you are saying, and I might learn something!!!!:worship:

Oh, and just to kick the rock again, I think I know what you meant to say, but this statement was not complete it seems, and was also therefore, technically from an English point of view, also wrong and confusing!

"A variable resistor (ie pot) is a voltage divider"

"A variable resistor" is singular as in one resistor, the functional word being "A", and the web site I linked is showing a "voltage divider" as having 2 resistors in the schematic! LOL, Sorry, devil made me do it, LOL.

I found a page on Hall effect effect and variable reluctance CPS/ CKP sensors:

http://www.proformance-engines.com/may_tech_tips/Crankshaft_Position_Sensor.pdf

Both of which I would like to understand much better as well, though they were not what I asked about in the Thread opening question.

So teach me some EE sir! The student is awake and paying attention!!!

Oldman, would I be right in assuming that RL in the circuit below (1) would be the variable resistor / sensor, like the TPS?

(1) http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

then the other two resistors would typically be in the ECU?

I am reading this now:

http://www.allaboutcircuits.com/vol_1/chpt_6/1.html

FWIW, I just re-read my original question, in post #1, and it is poorly worded to say the least. The confusion started there, no doubt! I think I was in a rush to post it and get out the door, and failed to proof read it.... So back to the topic, sensing circuits, or "Voltage dividers"!!! Lead on old_man, this looks very interesting!!!

I see now, how a Potentiometer, like a TPS can be called and considered a "voltage divider":

http://www.allaboutcircuits.com/vol_1/chpt_6/1.html

Picture is worth a thousand words! LOL. Helps to know what a voltage divider is, how it is defined! I never realised that a POT (potentiometer) was a voltage divider, because I was never properly shown what a "voltage divider" was to begin with. I always thought of them as a simple variable resistor, and never "knowingly" used them as a voltage divider. Therefore it was one of the holes in my knowledge I was looking to fill!

But a CTS, and an MAT is not a voltage divider by itself, it only has two connections. Their resistance varies as a function of temperature. So I need to go back and look at the MAP sensor. I forget, is it 2 wires or three? Is it a POT, or a simple two wire variable resistor in the circuit?

My Renix manual shows the MAP sensor to be a POT, 3 wires, and therefore a voltage divider, so now I understand what you were saying, at least partly. So now I see a level of complexity I had never noticed, recognized, or understood before. Now, is a voltage divider also somehow used in the ECU as well, or is there something else going on inside the ECU?

FWIW...most complex control units that use voltage sensor inputs will sample the voltage input and either amplify it to either enable or disable (trigger) a circuit to drive an electromechanical device, or use the voltage level to control the frequency of a pre-determined waveform or pulse width of a predetermined waveform that will drive an electromechanical device. (fuel injector for example). So in a nutshell, that is what goes on inside an ECU from a purely most primitive level.

(from my 01 FSM)

The MAP sensor is a linear sensor; meaning as
pressure changes, voltage changes proportionately.
The range of voltage output from the sensor is usually
between 4.6 volts at sea level to as low as 0.3
volts at 26 in. of Hg. Barometric pressure is the pressure
exerted by the atmosphere upon an object. At
sea level on a standard day, no storm, barometric
pressure is approximately 29.92 in Hg. For every 100
feet of altitude, barometric pressure drops 0.10 in.
Hg. If a storm goes through, it can change barometric
pressure from what should be present for that
altitude. You should know what the average pressure
and corresponding barometric pressure is for your​

area.

joe_peters said:

Good questions, but I don't think anybody has cracked much of the Renix ECU.

I liked your idea of a group-purchased 2500 and asking 5-90 to manage it, that would help him in cracking the Renix.

I want to break one open just to poke around but I don't have a bad one and I haven't even had an XJ come in with a bad one.

I seem to remember a thread where an Eastern Europe guy had a bad ECU and opened it up to solder and/or replace something inside, but that is the only instance I can remember.

Click to expand...

Old_man started another thread yesterday about his new project, poking around inside a Renix ECU to solve a low voltage problem in the ECU while cranking with a battery that drops in volts to 9 volts while cranking.

http://www.naxja.org/forum/showthread.php?p=245178424#post245178424

"I'm an electrical engineer. I have run across a deficiency in the Renix computer. I went with very large battery cables and this give me better cranking speed with the starter.
You would think that better cranking speed would be good. Well it is but it then shows up another problem. I let my XJ sit for about 6 weeks without starting it. I hop in and it cranks nicely but doesn't start. I look at the battery voltage and it is around 9v. The bigger cables allow the starter to still crank nicely at 9 volts, but 9 volts isn't enough for the Renix computer to wake up and provide spark.

The problem lies in the use of a 3 terminal regulator to generate the 5v used by the processor and sensors. The old regulators needed a fair amount of voltage above 5v to regulate correctly. I am switching the old school 3 terminal regulator with a pin compatible low drop out regulator based on newer technology. This will allow the computer to work even when the battery voltage drops.

I will be doing a writeup and posting the part numbers in a day or two. I still need to disect the circuitry to see if there is a low voltage trigger circuit as well. "

_____________________________

Now if I can just get my poorly worded, original question answered in words I can understand? LOL

Interesting read on sensors sensor circuits:

http://www.analog.com/library/analogDialogue/archives/42-03/CH03-H8703.pdf

McQue said:

FWIW...most complex control units that use voltage sensor inputs will sample the voltage input and either amplify it to either enable or disable (trigger) a circuit to drive an electromechanical device, or use the voltage level to control the frequency of a pre-determined waveform or pulse width of a predetermined waveform that will drive an electromechanical device. (fuel injector for example). So in a nutshell, that is what goes on inside an ECU from a purely most primitive level.

(from my 01 FSM)

The MAP sensor is a linear sensor; meaning as
pressure changes, voltage changes proportionately.
The range of voltage output from the sensor is usually
between 4.6 volts at sea level to as low as 0.3
volts at 26 in. of Hg. Barometric pressure is the pressure
exerted by the atmosphere upon an object. At
sea level on a standard day, no storm, barometric
pressure is approximately 29.92 in Hg. For every 100
feet of altitude, barometric pressure drops 0.10 in.
Hg. If a storm goes through, it can change barometric
pressure from what should be present for that
altitude. You should know what the average pressure
and corresponding barometric pressure is for your​

area.

Click to expand...

Interesting that the 01 MAP voltage signal output is linear versus air pressure. I somehow doubt the Renix MAP is linear, but I guess it could be. So if the output is linear, does that mean the current through the MAP is constant? Is that a requirement for it to be linear? I need to go back and study the voltage divider stuff I found last night.

With a constant DC voltage source, current flow thru a resistor will decrease if the resistance value increases, and vice versa. (Ohms Law) right?

McQue said:

With a constant DC voltage source, current flow thru a resistor will decrease if the resistance value increases, and vice versa. (Ohms Law) right?

Click to expand...

Yes, but, Old_man has, if I understand him, said that for a MAP and TPS, it is not a simple, single resistor, but something so simple that I missed it in my random walks through pieces of EE over the years, it is called a voltage divider (not sure I understand it completely yet), but some things are starting to make more sense to me than ever did before. The TPS has 3 wires, not just 2, same for the MAP sensor. They do work the same way the CTS and MAT work, as they have 2 wires, and look to the ECU like a simple resistor. But the MAP and TPS act as voltage dividers. What I still don't have a clear picture of is what is going on inside the ECU. Old_man may have been suggesting that parts in the ECU in combination with the CTS or MAT make a voltage divider, but I am not sure. I think the clues in the links I posted earlier.

Problem is we do not know if the voltage or current from the ECU is constant under load. I would think they try to be constant voltage, but as you know, with enough overload you can drag a 12 volt battery down to 9 volts cranking an engine, so there are limits. I am not sure, but I think they are constant voltage, 5V, or at least that is the design target constant voltage.

But here is the deal I think with a voltage divider like the TPS. The resistance across the entire TPS (or MAP) is constant because R1+R2 is always equal to R3, so R3 is one constant. R3 is resistance across the entire range Pin 1 to pin 3 of the voltage divider style sensor (TPS or MAP) in this case. R1 and R2, pin1 to pin2 and pin3 to pin2, respectively, would be R1 and R2, and both would vary with the sensor location, Idle to WOT. But the sum of R1 and R2 would always equal R3. So the ECU has a known input voltage to pin 1 (presumably a constant 5 V?), and the voltage out from pin 2 goes to the ECU to be measured, and pin 3 would be ground, almost 0.0 volts presumably? R3 resistance is constant.

Now this is where my knowledge breaks down. I still need to thoroughly read and study the links I posted on the voltage divider myself. I suspect old_man was also hinting that the ECU uses a voltage divider internally somehow, or partly inside the ECU, perhaps with, or for, the single resistor sensors like the CTS and MAT? In other words the CTS or MAT would be R1, and R2 would be inside the ECU!!!!!!!!!!!!!!!!

So back to your question. If the TPS has a constant voltage feed to R3, and R3 is constant, then I would presume that "I", the current, is also constant across the pin1 to pin3 contacts of the TPS or MAT (R3)? That makes things much simpler.

That may be what old_man was trying to tell me. But the teacher seems to be on strick for now?:worship:

If R2 is inside the ECU for the CTS and MAT, then we have a little different puzzle to solve, as R1 is not constant, and therefore R3 would not be constant, but R2 would be constant in such a circuit!!!! That is as far as I have gotten.


I have a 50 page report due in about 48 hours, so I need to switch to that for now.

Ecomike said:

Interesting that the 01 MAP voltage signal output is linear versus air pressure. I somehow doubt the Renix MAP is linear, but I guess it could be. So if the output is linear, does that mean the current through the MAP is constant? Is that a requirement for it to be linear? I need to go back and study the voltage divider stuff I found last night.

Click to expand...

By linear they mean that the voltage output varies linearly with respect to pressure. So if the pressure doubles, the output voltage doubles.

Quote: Originally Posted by Ecomike
Interesting that the 01 MAP voltage signal output is linear versus air pressure. I somehow doubt the Renix MAP is linear, but I guess it could be. So if the output is linear, does that mean the current through the MAP is constant? Is that a requirement for it to be linear? I need to go back and study the voltage divider stuff I found last night.

Old_Man: "By linear they mean that the voltage output varies linearly with respect to pressure. So if the pressure doubles, the output voltage doubles."

At first look, your comment makes sense, and my question / comment sounds silly, but now I recall, I was thinking of the entire current through the MAP (across R3) as a voltage divider, not about the voltage divider signal from the center tap wire coming from between R1 and R2. I take it the so called voltage output is the lead from where R1 and R2 meet up in the MAP sensor? Am I wrong in thinking that a constant voltage across R3 (R3=R1+R2) from the ECU, would imply a constant current through R3? I realize anything going on from the R1 to R2 junction wire leading to the ECU, would need to be high impedance in the ECU to avoid changing the current while the ECU reads the R1-R2 tap voltage, but / or am I oversimplifying things going on the ECU side of the circuit that make constant current across R3 not possible?

joe_peters said:

Good questions, but I don't think anybody has cracked much of the Renix ECU.

I liked your idea of a group-purchased 2500 and asking 5-90 to manage it, that would help him in cracking the Renix.

I want to break one open just to poke around but I don't have a bad one and I haven't even had an XJ come in with a bad one.

I seem to remember a thread where an Eastern Europe guy had a bad ECU and opened it up to solder and/or replace something inside, but that is the only instance I can remember.

Click to expand...

Mr_W in Czechoslovakia, he had a blown injector drive and sorted out a replacement.

Joe, aren't you some variety of EE? I've got a collection of RENIX ECUs, and I could come up with a sacrificial lamp or two for you if you are willing (I have one with a blown driver from South Africa, and a few "Worked When Pulled" units as well. Let me know.)

I've noted that the RENIX ECU is a fairly hardy unit - probably because Bendix built the thing, and Bendix-built electronics are usually built by Bendix/King. For those of you who haven't held G/A pilot's licenses, Bendix/King is a name you'll see on a lot of avionics...

I've tracked the last known US patentholders of RENIX to ACTIA Corp. (Elkhart, IN,) but I need to start trying to prise loose information from them again. Are there any RENIX junkies up around Elkhart/Chicagoland that would be willing to make a short trip to get our collective foot in the door?

Why do you want to know what the current is from the sensor? In all likelihood, the ECU doesn't care what the current is, it only cares about the voltage. It would be very unusual if it was otherwise; however, the French were involved so most bets are off. The current that the ECU sinks will vary as the sensor's output voltage varies, because the ECU's input impedance is constant.

For 5/90, King was bought by Bendix who was then bought by Allied Signal who was then bought by Honeywell. We're simply lucky that the Renix ECU is so incredibly robust considering who Bendix's partner was.

Yeah, I'm an old EE too. Most of the young (under 50) EEs don't even know a diode when they see one.