Multimeter questions

[lolcantturn]

After being into RC cars and building/messing with their PCB's from about 11-14 years old, and owning my problematic E30 for about 3 months, I've never actually owned a multimeter..
now that I have one, my guessing game is over and I've figured quite a few stuff out.
That being said, I'm having kind of a problem getting a resistance reading. I tried to read the resistance of one of my injectors, and I kinda got a fluctuating reading, then it went to 0. I tested out a resister on an old PCB I had laying around and it didn't go to 0, it actually read something, and then I tested another resister on that same PCB and it had a fluctuating reading, and then went to 0ohms.
What gives?

[trackjunkie21]

clean the surface you are trying to get a reading from, if there is alot of debris on it, then it will OL

[lolcantturn]

clean the surface you are trying to get a reading from, if there is alot of debris on it, then it will OL

clean as a whistle

[Car Addict]

The obvious question did you isolate the injector from the circuit?

also was it 0 or was it OL

[lolcantturn]

The obvious question did you isolate the injector from the circuit?

also was it 0 or was it OL

Well yes, of course! haha. It was 0.

[ttrousdell]

0 ohms in the injector lol. dont sound right

[Rigmaster]

Any chance you have it on the diode test setting? I think mine does this when it's on that setting.


???

[DaveSmed]

Use that setting very judiciously in automotive applications. Ohming out an injector is fine. Ohming out connectors and wires is illegal.

That setting should only be used for checking components that are completely disconnected, and safe to check like that. Checking solid state stuff with an ohmmeter is bad habit as well for what you will be doing. (I.E. don't play with the DME with that set to ohms)

Voltage drop is the way to go for that stuff.

[lolcantturn]

Use that setting very judiciously in automotive applications. Ohming out an injector is fine. Ohming out connectors and wires is illegal.

That setting should only be used for checking components that are completely disconnected, and safe to check like that. Checking solid state stuff with an ohmmeter is bad habit as well for what you will be doing. (I.E. don't play with the DME with that set to ohms)

Voltage drop is the way to go for that stuff.

good info indeed.
solid state= something that's always hot, right?

[DaveSmed]

Nope, electronics. Don't ohm stuff like headunits, check control, the DME, the ABS module, the OBC, etc. Stuff like that. Remember, when set to ohms, the meter is actually outputting voltage. If you wouldn't jump something to voltage, don't ohm it out.

[DaveSmed]

The best way to check stuff is to use the voltage setting for 99% of your diagnostics.

Ohms is used for checking separate components, like injectors, lightbulbs, sensors, etc. Usually, you have a good idea when it is an individual component that shit the bed (gee, why doesn't this one injector work when the other five do on a batch fire setup...) so you kinda look for that going in. Even most sensors can be checked by voltage output, some requiring it for meaningful readings.

The other thing with checking voltage is to leave the circuit connected.

Always think of this- you can take a one foot length of phone wire, and a one foot length of 0 gauge wire and ohm them out. Both are almost zero ohms. You can connect both to your car battery's positive terminal and measure to ground and see 12.6v on both. BUT try to power your starter with them... Only when connected, taking voltage drop measurements can you really get a feel for where the problem in your circuit is. In that case, your other two tests with your meter lied to you and you would have incorrectly looked elsewhere for a problem.

[trackjunkie21]

Use that setting very judiciously in automotive applications. Ohming out an injector is fine. Ohming out connectors and wires is illegal.

That setting should only be used for checking components that are completely disconnected, and safe to check like that. Checking solid state stuff with an ohmmeter is bad habit as well for what you will be doing. (I.E. don't play with the DME with that set to ohms)

Voltage drop is the way to go for that stuff.

Kinda got me interested now, mind explaining why it's illegal?

[lolcantturn]

Nope, electronics. Don't ohm stuff like headunits, check control, the DME, the ABS module, the OBC, etc. Stuff like that. Remember, when set to ohms, the meter is actually outputting voltage. If you wouldn't jump something to voltage, don't ohm it out.

The best way to check stuff is to use the voltage setting for 99% of your diagnostics.

Ohms is used for checking separate components, like injectors, lightbulbs, sensors, etc. Usually, you have a good idea when it is an individual component that shit the bed (gee, why doesn't this one injector work when the other five do on a batch fire setup...) so you kinda look for that going in. Even most sensors can be checked by voltage output, some requiring it for meaningful readings.

The other thing with checking voltage is to leave the circuit connected.

Always think of this- you can take a one foot length of phone wire, and a one foot length of 0 gauge wire and ohm them out. Both are almost zero ohms. You can connect both to your car battery's positive terminal and measure to ground and see 12.6v on both. BUT try to power your starter with them... Only when connected, taking voltage drop measurements can you really get a feel for where the problem in your circuit is. In that case, your other two tests with your meter lied to you and you would have incorrectly looked elsewhere for a problem.

:bow:
Great information. Really appreciate it. I've slowly been understanding the BMW Electronics troubleshooting diagrams so it'll all come in handy soon.

Both my 2nd and 1st injectors aren't reading anything..maybe they're both fucked? haha


My coolant temp sensor (brand new) reads to spec and doesn't fluctuate, so I guess my injectors must really be done. If the resistance reads 0ohms, then they're finished right?

[DaveSmed]

Kinda got me interested now, mind explaining why it's illegal?

Simply being a wiseass of course, but ohming out a wire or connector never nets any useful information. To properly measure ohms in a circuit, it must first be isolated. So say my power windows don't work, and I think its that door jamb connector.

I disconnect the window motor, unplug the switch at the console, and ohm it out. I could be seeing anything, you will get an ohm reading through the water in the plug even, its a measurement without quantity when checking circuits. That information will not help you determine why the window doesn't work. Its the same failing as the phone wire-0ga wire to power the starter example I used above.

Now, instead I unplug the window motor, and check voltage at the motor connector while hitting the button. I see full battery voltage because although the connector is corroded, the multimeter is sensitive enough to see the voltage through that crappy connection. Multimeters are extremely high impedance and provide almost no load to a circuit.

A plain old test light would have probably even caught that, test lights do load a circuit enough in many cases to pick up on that. When you plug the window motor in, you are using it to load the circuit, providing the most accurate, real life measurement of the circuit's performance. If you have voltage across the window motor connector, with the motor plugged in, the diagnosis is pretty much complete then and its time for me to shop for a new window motor.

[DaveSmed]

Actually, I have a good demonstration on many E30s. Anybody have a working aux fan that doesn't come on with the A/C button? (If it spins and the fuse isn't popped, it probably works, even if it never comes on)


lolcantturn, your car is actually broken at the moment? Motronic doesn't handle shorted injectors too well. Unplug the ones you are getting screwy readings from and see what happens. (two might be pushing it to start the car though, the M20 makes a shitty 4 cyl)