relay bleed through cadi trunk pop dmm

I realize that (-) triggered circuits usually show a (+) and same the same thing with (+) triggered circuits resting at or close to ground - hence the "relay bleed" part, but just having the guts to put the opposite polarity to that wire scares me sometimes (there are times when I'll test a set of lock wires that don't really show any significant change on my meter using the lock switch, but when I probe the questionable circuit with voltage or ground it activates the locks). I like the idea about putting a resistor inline on the 1 amp fuse as well. Our shop is one that doesn't allow test lights of any kind, not even "computer safe test probes". I will look for something like dualsport said - "pulse detection mode". Any recommendations for a good ~$200 meter? Thanks guyz!

masterodisaster wrote: .... but just having the guts to put the opposite polarity to that wire scares me sometimes...

Maybe I'm missing something (though I know what you mean....) but polarity is irrelevant from a testing POV - keeping in mind that a test lamp is a resistor that limits current (ie ~80 Ohm for a 2W bulb). Or use back to back LEDs for lower currents.... It doesn't matter if you bridge "down to" GND or "up to" +V. This is not to be confused with switching - ie, whilst it is ok to parallel-connect grounds (ground switching), it is not ok to parallel different control voltages - especially if they are different. But they are shorted together whereas a test light, or voltmeter, has impedance. (But not an ammeter.) I never liked test lights - instead preferring my DMMs etc. But there are times when a test light can do what a DMM can't. And test lights can function as handy non-destructive triggers - though I get the impression that's not always intentional lol. Not long after starting on this forum, I was determined to get/make a test light. I reckon I still will. One day. Then I won't have to be so careful testing hot leads (eg - is it +12V through the dome light, or raw +12V??). Your shop sounds a bit primitive. If they do not allow lights, I presume they have logic probes, state analysers, or CROs. Else sophisticated sample & holds etc. But a $2 LED or light is probably cheaper etc. (Is there a valid reason - or is it another case of misunderstood technicalities, or "a little knowledge...."?)

Well from what I'm told "test lights" pull too much current through the light and can "damage vehicle computers", so this is why we aren't allowed to use them. I've been doing car audio a long time and have never damaged a computer yet (knock on wood)... I used to use a type of test light that used 2 leds (one red (+) and one green (-) and it worked great, but was told it could still damage a vehicle computer, so I only use it on "older" cars. Anyway we have to pay for our own tools and I can't afford high priced oscilloscopes and analyzers on installer salary. I don't know what you mean by "misunderstood technicalities" or "little knowledge" - maybe "I ain't none too smart", but that is why I'm asking for opinions. For me at least - polarity matters when you have to trigger a circuit and you don't know which polarity it requires to do so. Giddyup!

Master - I wasn't thinking of you, nor criticising. You are smart - you are here! (And I ain't too smart on test lights - it's a relatively recent awakening for me. But the experienced on this site have a few good stories and uses...) The over-current draw is a valid reason for not using test lights and that's where logic probes come in. But most CPU outputs should be robust enough. Some Delco literature from the late 1980s said something like test lamps under 2.5W or 3.4W or similar were ok for any testing. (Except f.ex things like the oxygen sensor/s that you don't even test with a DMM - definitely not on resistance scale anyhow!) But that was then and circuits have changed. Though I'd reckon low power LEDs should be okay - most things will handle a few mA. But that defeats some of the advantages of a bulb type tester (I think?).       My "misunderstood technicalities" & "little knowledge" was directed at all too common misunderstandings that often lead to the wrong or opposite solution. (Why do I think if The President's "Internet Shutdown" button in case of a terrorist attack? But that was a April Fool's joke.)    But incorrectly used DMMs can blow ECUs, as can static discharges and a host of other things. And in a market that tends to de-ruggedise equipment in favor of revenue, playing safe is good. But I'll leave the lite discussion to the experts. Good luck! (I'll be reading their replies too!) And if I remember where I recently saw the cheap mini-LCD CRO for some crazy $50 or $150 - and it seems ok - I'll let you know.... (eBay??)

The rule about no test lights is ridiculous; its a "My way or the Highway" attitude. If someone comes up with a great idea, do the powers that be listen? I have a DMM but I will only use it in testing when I think I'm hitting on data wiring. Even with BCM inputs I can assure you that using an incandescent bulb tester is fine because you're looking at trigger wires and they will either change polarity or go from open to closed circuit. That tester you mentioned, somewhere under my 4 Weller Pyropens I have one, wonderful for finding tach quickly. Think it was a Mac 120 or Snap-On equivalent. I guarantee I will find door trigger contacts or door lock triggers/motor wires faster with a test bulb than with a DMM. Anyway if I screw up I've got $6000 of diagnostic gear to clear it.

As an addenda, I'd rather state don't pull airbag connectors or control units with the ignition on!

Yeah howie - I agree that it is closed minded to say "NO TESTLIGHTS PERIOD", but this is a very large chain of stores that has some serious amateur installers working for them. They don't really care about how fast you are, just that you aren't causing vehicle damage that costs them money. I'm still unsure how test lights do damage, but I'm taking their word that it does happen - so the dude abides. I'm looking at the new Fluke 287 DMM that has "event recording" on the display - looks pretty sweet. Now I just need to scrape up $400 or so. oldspark - no sweat - I bought one of those cheap LCD mini CRO's" - it's pretty cool, but I'd still like a real one. The instructions are all in Chinese (did find a few English instructions on the web), and it just doesn't feel like the real thing. search "arm dso portable mini" on Ebay - I picked mine up for $85 shipped. CRO=Cathode Ray Oscilloscope/Oscillograph for all you noobs. Thanks guys - I'm really diggin this forum.

Fluke 287 seems a bit of overkill for simple pulse detection, but if some future use can use its capabilities, it's good to have- If you're so inclined, you can put something like this for just pennies and your time. It won't draw down your test signal as it's high impedance, and detects the presence of any brief negative going pulse triggers by stretching out the yellow LED on time. It does need a connection to power, which can be vehicle supplied or you could use a battery. In that case you just connect the ground to the car's ground, and just supply the probe signal input. Note if you're looking for a single shot positive going pulse the circuit would have to be modified first.

dualsport - you are THE man. Fluke 287 is definitely overkill. Might I ask where you got this diagram? What do the red, green and yellow LEDs indicate? I'll have to give this diagram to my electrical engineer buddy. Thanks!

The logic probe just uses some CMOS gates which are very high impedance inputs to drive the indicator LEDs. The 2.2M resistor will put a slight current on the input, but it's minimal enough not to worry about. The purpose of that resistor is to feed back and toggle when nothing's connected to the input, which is handy as an indication that your signal is an open circuit vs. an actual high or low.

You could just remove the 2.2M resistor if you don't need to try detecting an open circuit. Add a 10M pulldown resistor on the input in that case. Only thing If you do that is the red LED will be on by default even if you're probing an open circuit, so you won't be able to tell if it's a real ground you're connected to or not.

The green LED lights up when the input is high, and the red lights up when it's low.  The Vcc voltage you use for the circuit determines the transition threshold; it's half the voltage.  You might try using a 9V battery for power if you want to avoid having to connect up 12V separately to it, should still be okay for 12V signals.

The yellow LED indicates the presence of a pulse on the input; if there's a signal that's constantly toggling up and down, it'll light up continuously.

If a single negative going transition occurs, it'll light up for a few seconds before going off again.  It could be modified by using the last gate in the chip to invert the incoming signal if you need to detect a single positive going transition.