Using a multimeter
Discussion
Short story: Can I use one to tell me if a terminal is putting out a negative signal?
If so i'll put a pic up of it.
Longer story for Stevie
I need to know if some ECU pin outs are switched to negative or not.
I usually use a Powerprobe, but someone borrowed it and it doesn't seem to register ground/earth anymore. I need to do this tomorrow and can't pick one up locally. Power probes tell you whether it is + or - when you poke it.
If so i'll put a pic up of it.
Longer story for Stevie
I need to know if some ECU pin outs are switched to negative or not.
I usually use a Powerprobe, but someone borrowed it and it doesn't seem to register ground/earth anymore. I need to do this tomorrow and can't pick one up locally. Power probes tell you whether it is + or - when you poke it.
Edited by 227bhp on Friday 9th March 00:12
Penelope Stopit said:
Yes you can, with your meter set to volts measure from battery positive to the ECU Pins to check for negatives and measure from battery negative to the ECU pins to check for positives
Thanks, doing that does it check by not actually sending any current, but just measuring it? What I mean is, I could do it by using a simple piece of wire and a light bulb, put wire to + then to light bulb, then when wire from light bulb finds a - it will light up, the problem with that is it's then sending + through something delicate which is sending out a - and damage it.227bhp said:
Penelope Stopit said:
Yes you can, with your meter set to volts measure from battery positive to the ECU Pins to check for negatives and measure from battery negative to the ECU pins to check for positives
Thanks, doing that does it check by not actually sending any current, but just measuring it? What I mean is, I could do it by using a simple piece of wire and a light bulb, put wire to + then to light bulb, then when wire from light bulb finds a - it will light up, the problem with that is it's then sending + through something delicate which is sending out a - and damage it.If or when you have time will you please give me your opinion on this https://www.pistonheads.com/gassing/topic.asp?h=0&...
Thank you in advance
Edited by Penelope Stopit on Friday 9th March 10:17
Penelope Stopit said:
227bhp said:
Penelope Stopit said:
Yes you can, with your meter set to volts measure from battery positive to the ECU Pins to check for negatives and measure from battery negative to the ECU pins to check for positives
Thanks, doing that does it check by not actually sending any current, but just measuring it? What I mean is, I could do it by using a simple piece of wire and a light bulb, put wire to + then to light bulb, then when wire from light bulb finds a - it will light up, the problem with that is it's then sending + through something delicate which is sending out a - and damage it.Surprised a Powerprobe sends current through though? It's a Sealey:
http://www.uktools.com/sealey-ppx-auto-probe-plus-...
The neg side has broken, but pos still works and i've used that to check them ok? You put it on and press the rocker switch to switch on a supply or don't press it and it checks for + or - instead.
It will do it but remember to connect the test prods correctly. If the black one is on earth/ground & the red one on the pin under test then a +ve output will show as a positive voltage (or maybe just the voltage without a sign in front) & a -ve output will have a - sign in front of the value. If you get the leads swapped you'll get swapped results.
227bhp said:
Ok thanks, will do.
Surprised a Powerprobe sends current through though? It's a Sealey:
http://www.uktools.com/sealey-ppx-auto-probe-plus-...
The neg side has broken, but pos still works and i've used that to check them ok? You put it on and press the rocker switch to switch on a supply or don't press it and it checks for + or - instead.
The fact that the Powerprobe has short circuit protection built into it tells me that it does draw current through it, if the polarity lights in the Powerprobe don't draw current through the circuit they are checking then the test should be ok, the Powerprobe polarity testing side of it may use an electronic voltage sensing circuit that doesn't draw current but does control the polarity lights, similar to a relay doing the work but done with electronics, there is only one way to find out if a Powerprobe draws no current when polarity testing and that is by connecting an Ammeter in series with it when carrying out a test - I only use test lamps and Multimeters so am unable to give you a 100% yes/no for Powerprobe polarity testing - The Powerprobe current protection circuit makes it look like a no goer but.....as aboveSurprised a Powerprobe sends current through though? It's a Sealey:
http://www.uktools.com/sealey-ppx-auto-probe-plus-...
The neg side has broken, but pos still works and i've used that to check them ok? You put it on and press the rocker switch to switch on a supply or don't press it and it checks for + or - instead.
The benefit in using a Multimeter is that you can see what voltage there is at a pin, any tester with a polarity light will only show a light and it is often the case that a fault in a circuit is being caused by a voltage drop, for instance - You have a car that has a fault in that the fuel pump relay isn't clicking in - You have tested at the fuel pump relay and now suspect there is a fault at the ECU negative output that clicks in the relay - You connect a powerprobe or similar tester to the fuel pump relay control pin at the ECU and the negative polarity light lights up - You are now baffled as to why the relay isn't clicking in - You pay an auto electrician to come and have a look at the fault - The auto electrician connects his multimeter to the ECU fuel pump control pin and tells you that there is 7.5 volts at the pin that should have battery voltage at it - You curse yourself because you own a Multimeter
Edited by Penelope Stopit on Friday 9th March 10:47
Penelope Stopit said:
The fact that the Powerprobe has short circuit protection built into it tells me that it does draw current through it, if the polarity lights in the Powerprobe don't draw current through the circuit they are checking then the test should be ok, the Powerprobe polarity testing side of it may use an electronic voltage sensing circuit that doesn't draw current but does control the polarity lights, similar to a relay doing the work but done with electronics, there is only one way to find out if a Powerprobe draws no current when polarity testing and that is by connecting an Ammeter in series with it when carrying out a test - I only use test lamps and Multimeters so am unable to give you a 100% yes/no for Powerprobe polarity testing - The Powerprobe current protection circuit makes it look like a no goer but.....as above
The benefit in using a Multimeter is that you can see what voltage there is at a pin, any tester with a polarity light will only show a light and it is often the case that a fault in a circuit is being caused by a voltage drop, for instance - You have a car that has a fault in that the fuel pump relay isn't clicking in - You have tested at the fuel pump relay and now suspect there is a fault at the ECU negative output that clicks in the relay - You connect a powerprobe or similar tester to the fuel pump relay control pin at the ECU and the negative polarity light lights up - You are now baffled as to why the relay isn't clicking in - You pay an auto electrician to come and have a look at the fault - The auto electrician connects his multimeter to the ECU fuel pump control pin and tells you that there is 7.5 volts at the pin that should have battery voltage at it - You curse yourself because you own a Multimeter
Thanks for taking the time for the exhaustive reply! My multimeter is like my PC - never gets used to its full potential. The benefit in using a Multimeter is that you can see what voltage there is at a pin, any tester with a polarity light will only show a light and it is often the case that a fault in a circuit is being caused by a voltage drop, for instance - You have a car that has a fault in that the fuel pump relay isn't clicking in - You have tested at the fuel pump relay and now suspect there is a fault at the ECU negative output that clicks in the relay - You connect a powerprobe or similar tester to the fuel pump relay control pin at the ECU and the negative polarity light lights up - You are now baffled as to why the relay isn't clicking in - You pay an auto electrician to come and have a look at the fault - The auto electrician connects his multimeter to the ECU fuel pump control pin and tells you that there is 7.5 volts at the pin that should have battery voltage at it - You curse yourself because you own a Multimeter
Edited by Penelope Stopit on Friday 9th March 10:47
Well i'm afraid this isn't working 
It doesn't differentiate between what i'm testing (what I want to know is + or -), just the supplied voltage.
If I put the red + probe to a + supply and test a variety of connections with the - probe it tells me the voltage, but only tells me that i've used + as a supply. Alternatively if I reverse that it tells me i'm using a - as a supply. Pic of Multimeter below.
Maybe i'm doing something wrong?
Pic of MM below.
It doesn't differentiate between what i'm testing (what I want to know is + or -), just the supplied voltage.
If I put the red + probe to a + supply and test a variety of connections with the - probe it tells me the voltage, but only tells me that i've used + as a supply. Alternatively if I reverse that it tells me i'm using a - as a supply. Pic of Multimeter below.
Maybe i'm doing something wrong?
Pic of MM below.
I am not 100% sure I am following you so this may or may not help depending on what you want, apologies if it doesn't.
I am thinking you are trying to see if an output from an ECU is being switched "down to earth" by the ECU.
If that is the case read on, if not try explaining it again.
1. Disconnect the wire from the ECU as that isn't helping.
2. Switch the MM to the 3rd setting with the weird (Greek letter Omega) symbol for measuring resistance.
3. Connect the black lead to battery earth.
4. Connect the red lead to the terminal you want to test.
If the terminal is "down to earth" the meter will read some small reading typically in single figures/decimal points indicating a "short circuit".
If the terminal is not "down to earth" it will read OL/OC/+/^ or something else indicating on "open circuit" (read instructions if it helps) there are other ways, but this is the easiest.
I am thinking you are trying to see if an output from an ECU is being switched "down to earth" by the ECU.
If that is the case read on, if not try explaining it again.
1. Disconnect the wire from the ECU as that isn't helping.
2. Switch the MM to the 3rd setting with the weird (Greek letter Omega) symbol for measuring resistance.
3. Connect the black lead to battery earth.
4. Connect the red lead to the terminal you want to test.
If the terminal is "down to earth" the meter will read some small reading typically in single figures/decimal points indicating a "short circuit".
If the terminal is not "down to earth" it will read OL/OC/+/^ or something else indicating on "open circuit" (read instructions if it helps) there are other ways, but this is the easiest.
E-bmw said:
I am not 100% sure I am following you so this may or may not help depending on what you want, apologies if it doesn't.
I am thinking you are trying to see if an output from an ECU is being switched "down to earth" by the ECU.
If that is the case read on, if not try explaining it again.
1. Disconnect the wire from the ECU as that isn't helping.
2. Switch the MM to the 3rd setting with the weird (Greek letter Omega) symbol for measuring resistance.
3. Connect the black lead to battery earth.
4. Connect the red lead to the terminal you want to test.
If the terminal is "down to earth" the meter will read some small reading typically in single figures/decimal points indicating a "short circuit".
If the terminal is not "down to earth" it will read OL/OC/+/^ or something else indicating on "open circuit" (read instructions if it helps) there are other ways, but this is the easiest.
This will be a non damaging test? I'll give it a go.I am thinking you are trying to see if an output from an ECU is being switched "down to earth" by the ECU.
If that is the case read on, if not try explaining it again.
1. Disconnect the wire from the ECU as that isn't helping.
2. Switch the MM to the 3rd setting with the weird (Greek letter Omega) symbol for measuring resistance.
3. Connect the black lead to battery earth.
4. Connect the red lead to the terminal you want to test.
If the terminal is "down to earth" the meter will read some small reading typically in single figures/decimal points indicating a "short circuit".
If the terminal is not "down to earth" it will read OL/OC/+/^ or something else indicating on "open circuit" (read instructions if it helps) there are other ways, but this is the easiest.
I'm trying to find which pin triggers the fuel pump relay, it will be an earth (I am told) and that is what my new ECU throws out.
I've managed to track down every pin out and successfully wire it, but this one has eluded me, for some reason (see below) the manufacturer has changed it from the other models. The only other thing I can think of is the new ECU has a fault and it isn't putting out the correct signal, maybe I can check for that later.
In addition to that and to further complicate matters this is what happens with original ECU connected:
Cold engine:
Ign on, no fuel pump
Engine turning, fuel pump runs.
Engine stalls, fuel pump stops.
It then changes state (for hot running fuel evaporation issues):
Warm engine:
ign on, pump runs for (say) 15 secs then cuts.
Engine starts, fuel pump runs.
I haven't tested to see if pump stops when engine stalls hot, but I suspect so.
So something in the original ECU is taking into consideration the engine turning (via cam/crank sensor) and also temperature via probably the water temp sens.
I'll have to speak to the ECU supplier, but it seems that we could be creating a potentially dangerous situation whereby if the car is in an accident and it's wired for: Ign on = pump on it could keep pumping fuel when engine has stalled (in an accident).
What I think is that the newer models had ABS, this has a motion sensor which is also used to kill the pump in an accident.
Krikkit said:
Or use the 4th setting on the wheel- it'll buzz if the two probes are electrically connected.
That's checking for continuity, I'm not sure that is of use to me.I would be wary of using the resistance measurement mode on an I/O pin unless you know what sort of circuit is driving it. An ECU external pin ought to be buffered and tolerant of having voltages applied to it, but I wouldn't assume it was.
Measuring the voltage would be safer if you don't know what the pin expects.
I would put the multimeter on DC voltage mode with a scale that can display 12V.
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
If you see an intermediate voltage in both cases or a voltage much smaller than +/- 12v then the pin is not being driven high or low.
Measuring the voltage would be safer if you don't know what the pin expects.
I would put the multimeter on DC voltage mode with a scale that can display 12V.
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
If you see an intermediate voltage in both cases or a voltage much smaller than +/- 12v then the pin is not being driven high or low.
GreenV8S said:
I would be wary of using the resistance measurement mode on an I/O pin unless you know what sort of circuit is driving it. An ECU external pin ought to be buffered and tolerant of having voltages applied to it, but I wouldn't assume it was.
Measuring the voltage would be safer if you don't know what the pin expects.
I would put the multimeter on DC voltage mode with a scale that can display 12V.
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
If you see an intermediate voltage in both cases or a voltage much smaller than +/- 12v then the pin is not being driven high or low.
if I have understood him right (still not 100% certain) he is trying to check an ECU output that switches a terminal to ground.Measuring the voltage would be safer if you don't know what the pin expects.
I would put the multimeter on DC voltage mode with a scale that can display 12V.
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
If you see an intermediate voltage in both cases or a voltage much smaller than +/- 12v then the pin is not being driven high or low.
For this to do anything it has to be putting an earth on a powered circuit to operate a peripheral, so by definition, it has to be able to "see voltage" on that terminal.
GreenV8S said:
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
Sorry, I don't mean to sound arsey but?Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
That isn't in the slightest confusing!
In both you say "if the meter shows roughly plus or minus 12 volts".
In both it will show plus or minus 12 volts.
E-bmw said:
GreenV8S said:
Connect one lead of the meter to the pin you want to check.
Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
Sorry, I don't mean to sound arsey but?Touch the other lead on vehicle ground. If the meter shows roughly +/-12V, the pin is being driven high.
Touch the other lead on battery +ve. If the meter shows roughly +/-12v, the pin is being driven low.
That isn't in the slightest confusing!
In both you say "if the meter shows roughly plus or minus 12 volts".
In both it will show plus or minus 12 volts.
Put one lead on the pin, the other lead on +12v. If the meter shows 12v, the pin earth/"low".
It'll be +12v or -12v depending on which way round the leads are held.
Red to +12v, the meter will show +12v.
Black to +12v, the meter will show -12v.
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