[Lightfoot]

I am trying to follow a diagnostic test procedure from a factory service manual.
At one of the steps, it asks if the reading at a specified terminal was "over 6,000 ohms".
Well, my digital ohmmeter happened to show the infinate resistance display. So, theoretically, is not 'infinate resistance' over 6,000 ohms? Or is there a difference between an open curcuit and extremely high resistance.
One could argue that the air between two peices of copper is over 6,000 ohms. I'm not trying to be a smart ass here, but this procedure has some bits in it that are a challenge to interpret correctly. And I've been pulling my hair out trying to get to the bottom of my no-start issue.
This reading is extra critical because it happens to be at a point where the procedure splits off to a different direction, depending if if the answer is yes or no.

Also wondering if I need to have my meter set to a different scale to 'see' something over 6,000 ohms. I tried switching it while I was checking but I don't know if I need a higher or lower scale.

I have a thread about what I'm working on in the Members Helping Members
section. Thought I'd stand a chance to reach more electronics wizards here.

[Greg Reid]

Sounds like they are looking for there to be no effective continuity there. Your meter could have a lot to do with that reading if it's not capable of ranging as high as the actual value.
What happens to the meter when you press the range button?
It should do something like give you the Ohm symbol, then a K with the symbol and then an M with the symbol. 'M' is for Megaohms meaning 'million' ohm. You may also see one with 'ns' which goes into the hundreds of million ohm range. You usually want the lowest range that will give you a reading.
Your meter may not be that sophisticated so if you don't see any of that you will probably be ok following your troubleshooting chart as a 'Yes' for reading over 6000 ohms.

[Goatracer1]

You need to use a scale where 6000 ohms falls in the middle of the scale. If you are on a scale that reads too low a value it will indeed read infinite. Without seeing your meter I can't tell which one to use. My meter has a 0-20,000 ohms scale would be correct for your problem. Do you have a scale that reads either 0-10K or 0-20K? either of those would be good.

[Shiny]

Can you clarify what you are measuring? It might not help me but someone with more knowledge could probably guide you if they knew more.

Unfortunately, high resistance could be needed to transmit a voltage signal so a true "open" could definitely be a problem in that scenario.

A measurement of ">6 Kohms" suggests a high resistance is needed, not an open. The procedure may be written poorly (ie a range of acceptable readings would have eliminated your uncertainty) but if it was intended to be "open", it probably would have said "open", "infinite", or > "XXX Mohms".

[Lightfoot]

Measuring a terminal that plugs into the ECM (at one of the C3I pins) to ground. I think it's job is to send a 5 volt signal to the ignition module.
It's the wire end that plugs into it, not the ECM itself.

My meter is a $15 digital multi-meter with a dial select switch. It has a 20K ohm setting and a 200K ohm setting. I think I had it on the 20K ohm setting when I made the test. I thought it was on 'basic' but I realized afterwords that I was mis-reading the dial.
Another question I forgot to ask was: If the reading is 1.50 when it's set on 20K, is that to be multiplied by 1000 or something higher than face value?

[Shiny]

Originally Posted by Lightfoot

Measuring a terminal that plugs into the ECM (at one of the C3I pins) to ground. I think it's job is to send a 5 volt signal to the ignition module.
It's the wire end that plugs into it, not the ECM itself.

Sorry, I don't know anything about this so don't know if a true open is a problem. Do you have a schematic or wiring diagram that says where this wire originates? There are several EEs on the forum that might be able to speculate on acceptable range of resistance if they knew what you were measuring.

Originally Posted by Lightfoot

My meter is a $15 digital multi-meter with a dial select switch. It has a 20K ohm setting and a 200K ohm setting. I think I had it on the 20K ohm setting when I made the test. I thought it was on 'basic' but I realized afterwords that I was mis-reading the dial.

As to the setting, you can rotate the dial between 20K and 200K while measuring the wire-to-ground resistance. This will not hurt anything. If the meter shows less than 200K on that higher-range setting then you should be fine. If the meter shows infinite or open on the 200K setting I'd still call it inconclusive.

Can you borrow a meter with a higher scale? Mine has a 20Mohm scale. If I measured higher than 20Mohm I'd interpret as "open" and be concerned.

Does the procedure you are following tell you what to do if the resistance is less than 6Kohms? This might help others understand what is on the other end of the wire.

I'm unfamiliar with "basic". Can you clarify what this is? How were you misreading the dial? Are there only two resistance ranges on the dial (20K or 200K)? Is it obvious which connectors to plug the test leads when measuring resistance?

Originally Posted by Lightfoot

Another question I forgot to ask was: If the reading is 1.50 when it's set on 20K, is that to be multiplied by 1000 or something higher than face value?

Sorry again but hard to say without more info. Are there no units displayed? My meter has units displayed and on a 20K setting it displays "Kohms". If I have a reading of 1.5 on this scale, then the resistance is 1.5 Kohms, or 1500 ohms. Given your description of the meter, I doubt it is sensitive enough nor designed to measure 1.5 ohms on a 20K-ohm scale. What does it display when you short the two test leads? Mine will display 0.00 Kohms on a 20K scale. If I change the scale to 200 ohms, mine will display 0.9 ohms with leads shorted.

[Greg Reid]

If it's a supposed to have voltage on it then I wouldn't think an open reading would be a problem. They want to make sure that it's not under a certain reading because that would interfere with your five volts.
If I were in your shoes, I would consider that a 'move on to the next step'. What they are telling you is that anything over 6000 ohms is not going to hurt your 5 volt source. They don't want to tell you that you are looking for an infinite open because they understand that occasionally you may read back through another component like a capacitor. If you thought it was supposed to be open and that happened you may read 10,000 ohms and erroneously think that it had failed that step.

[Peter Serio]

Trying to use a $15 ohmmeter on this might be a big waste of time.

You can't diagnosis electrical problems that involve a car's computer using cheap tools. You might want to buy or rent a Fluke meter or similar. There are several Fluke multi-meters in the $200 (entry level) price range which would help you.

Also just to note when working on the GM car's computer or wiring into or out of that the shop manual says to use only a high-impedance multi-meter.

[Shiny]

Originally Posted by Greg Reid

If it's a supposed to have voltage on it then I wouldn't think an open reading would be a problem. They want to make sure that it's not under a certain reading because that would interfere with your five volts.
If I were in your shoes, I would consider that a 'move on to the next step'. What they are telling you is that anything over 6000 ohms is not going to hurt your 5 volt source. They don't want to tell you that you are looking for an infinite open because they understand that occasionally you may read back through another component like a capacitor. If you thought it was supposed to be open and that happened you may read 10,000 ohms and erroneously think that it had failed that step.

My apology if I mis-interpreted but understood "It's the wire end that plugs into it, not the ECM itself..." to mean he was measuring a pin on a wire or cable that plugged into the ECM and NOT the input to the ECM. I understand your point if he was measuring an input.

"Communication breakdown, it's always the same...."

[george kujanski]

firstly, the reason for measurements stating things like 6000 ohms or greater is because if the part is connected to an electronic circuit that is unpowered, the resistance measured is not deterministic, i.e. a known finite amount. Electronic devices act funny when measuring as above when not powered. The measurement stated above is mostly looking for a hard fault, such as a faulty or shorted output feeding your measuring point. I have often written specs such as the above.

Secondly, as mentioned above, if the value is actually say 6010 ohms as an example, the meter will read infinite on scales of 2 ohms, 2k ohms, etc. On the 20K scale you would get a reading of 6.01K. Scales above 20K like 200k will show 6.0K, etc.

Since you see infinite on the 20K range, i would go to the next range, i.e. 200K in this example and see if the reading is still infinite.

The meters i use will display appropriate units in each range such as Ohms, Kohms and Mohms. As in the example above, in the 20K range, the display will show Kohms and the reading will be 6.01.


As long as you are using a range where 6K is less than the max value for the range (20k), you can be confident in your reading.

george

[Greg Reid]

Originally Posted by Shiny

My apology if I mis-interpreted but understood "It's the wire end that plugs into it, not the ECM itself..." to mean he was measuring a pin on a wire or cable that plugged into the ECM and NOT the input to the ECM. I understand your point if he was measuring an input.

"Communication breakdown, it's always the same...."

Either way, my point would be he same. I think that what they are trying to establish is isolation for a circuit that will have 5 vdc present. I'm assuming that they want at least 6K ohms of isolation.
Point taken from Peter and George's posts also. This isn't like troubleshooting an inoperative brake light bulb...but for that particular step, I think that 6K of isolation from something is called for.

[Lightfoot]

Originally Posted by Shiny;

I'm unfamiliar with "basic". Can you clarify what this is? How were you misreading the dial? Are there only two resistance ranges on the dial (20K or 200K)? Is it obvious which connectors to plug the test leads when measuring resistance?

Sorry, By that I meant the lowest setting.
There are five settings for Ohms on this meter:
200 2000 20K 200K 2000K.
When I turn the selector switch, a decimal point moves to various positions, but there is no decimal point showing on the 2000 and 2000K setting.
An infinite resistance (open) is always a "1" on the far left of the screen.
Touching the leads together (zero resistance) shows three zeros.
No letters or symbols ever show. It has four total places for digits.
There are three jacks for the test leads to go and they are well labled.

When I was trying to do the step concerning 6000 ohms, I needed to have the ohmmeter between ECM harness terminal BC8 and ground.
Then, with the ignition switched ON, I needed to install a test light from 12 volts to ECM harness terminal BC7. So I had to have all four test leads from two devices contacting four points at the same time. This in itself was a balancing act, but I managed to do it again today. I even managed to switch to all the different scales while checking the reading and it always gave an open resistance display.

So I'm satisfied that I had an accurate result. I also have decided that 'open resistance' should not qualify as a Yes to the over 6000 ohms question.

As far as the quality of my multi-meter, I know it's cheap but I'm also working with 1989 OBDI technology here.

I haven't quite found the actual cause of my crank-no-start problem but I feel I have narrowed it down quite a bit.

Greg, I have been continuing through the test sequences, whether I have a definitive Yes or No answer on some steps. No worries about damaging equipment, I just want accuracy and confidence in my findings.

To everyone, I appreciate all your input.

["QUICK-SILVER"]

Originally Posted by Lightfoot

Or is there a difference between an open curcuit and extremely high resistance.

Infinite and open both give the same end result... No current can pass through.

Open: No connection there for current to go through.

Infinite: Circuit is there but resistance is so high that no current can pass through.

Your infinite reading is well past 6000 ohms for this test.

On direct ignition cars I've seen modules and computers trashed from bad plug wires. 50,000 volt spark will find it's way to ground if it can't get to the spark plug. That ground is usually the coil's (-) terminal that goes directly in the module. That's enough to nuke a module and if just part of it finds its way to the computer... It' can get toasted.

This is why it's so important to use a spark plug or spark tester to check for spark on these cars, instead of arching plug wires like you would an HEI or points distributor.

Clay

[Shiny]

Originally Posted by Greg Reid

Either way, my point would be he same. I think that what they are trying to establish is isolation for a circuit that will have 5 vdc present. I'm assuming that they want at least 6K ohms of isolation.
Point taken from Peter and George's posts also. This isn't like troubleshooting an inoperative brake light bulb...but for that particular step, I think that 6K of isolation from something is called for.

Understood and agree. My point was simply that maxing out a 200Kohm meter scale does not distinguish between very high resistance and an open connection. To me, the point of the check was to verify integrity of the circuit. The strategy/goal may be verifying ability to maintain a 5V signal and ">6Kohms" makes sense for that. ">6Kohms" will not, however, assure detection of a catastrophic failure that causes an open circuit.

Net: If I measured >2Mohms, I'd suspect an open and look harder in that direction.

[Authentic-242]

It is important to note that in order to measure resistance, the meter must supply a known electrical signal (probe 1) to the device being measured which it then compares to the return signal (probe 2). These signals have a + or - polarity to them. For a simple single resistor, reversing the probe polarity to the leads of the resistor will result in the same measurement. However, circuits that contain diodes and transistors may give different results when the leads are reversed. In simple terms, diodes and transistors act as electrical valves, they can be open or closed. If the ohmmeter probe polarity biases the diode closed it will act as a near short, if polarity is reversed it will act as a much higher resistance. Bottom line, always reverse the probe leads on an unknown circuit to compare resistance measurements and don't try to measure resistance on a powered circuit.

[Bill Hanlon]

The guy that wrote the spec didn't do his job. Instead of "over 6000 ohms" he should have written "over 6000 ohms and less than 100,000 ohms" (or whatever the max resistance should be).

[Greg Reid]

Originally Posted by Bill Hanlon

The guy that wrote the spec didn't do his job. Instead of "over 6000 ohms" he should have written "over 6000 ohms and less than 100,000 ohms" (or whatever the max resistance should be).

Agreed to a point.
Perhaps infinite is ok....perhaps there is no maximum. Just anything over 6000 ohms. In that case he did his job. If there actually is a maximum number then yes, you are correct that he didn't do his job by not listing it. The only point I'm trying to make is this... Infinite is over 6000 ohms and AS WRITTEN...it should be a 'yes'.
If the troubleshooting procedure is incorrect, well then you are pretty screwed from the perspective of using their troubleshooting guide.
Generally speaking, if I'm following a manufacturer's troubleshooting guide, I suspect that they knew what they were doing so I follow it as written.
As for polarity, true that you can get different readings dependent upon how you place the probes. In that case, I would expect a decent procedure to guide you as to what probe goes on what pin.

[Ben M.]

Originally Posted by Greg Reid

Perhaps infinite is ok....perhaps there is no maximum. Just anything over 6000 ohms. In that case he did his job. If there actually is a maximum number then yes, you are correct that he didn't do his job by not listing it. The only point I'm trying to make is this... Infinite is over 6000 ohms and AS WRITTEN...it should be a 'yes'.

100% ditto Greg. I've been over this with technicians before and sometimes they want a maximum, even if "anything over 6kOhms" is what you as the engineer wrote down in the procedure you gave them. From a 5V computer perspective, a 6kOhm resistance between 2 points may as well be infinite if you are trying to get anything other than a DC voltage. Lots of capacitors and coils on these older ECMs that can make the readings strange if you're used to looking for open\short circuits only.

I suspect the test was written for an analog meter and while using a digital meter is still perfectly fine, you have to understand the limitations of the test procedure as it was written for the tooling of the time. Yet another thing I've come across in updating engineering procedures of things from the 1980s and earlier.