relay bleed through cadi trunk pop dmm

I am just curious when testing suspected circuits using a multimeter and you have a wire resting at a positive voltage (+12.6V) how can you determine if it is a negative trigger without potentially causing damage to the vehicle computer. It is only an issue due to the multimeter my installer was using wasn't displaying the quick pulse of voltage properly. Are there certain specifications I should look for on a DMM that would be best for the automotive installer? I personally have $200 Snap On that works great, but my installer has a $100 Fluke meter and it is "slow to react to voltage changes"... I want him to have one that will have a higher "sample rate" if you know what I mean. In any case this Cadillac Deville we did today had a trunk pop listed as a (-) trigger, but it ended up being a (+) trigger and for some reason and I had a tough time determining it's trigger type with his meter (the wire stated it was located under the back seat by the Fuse panel and battery, and I was scared to feed (+12V)---> into this wire for fear of blowing up the ECU). I'm just curious how most of you guys handle this type of situation - I usually use my best judgement and feed a HIGHLY suspected circuit the polarity stated in the "Directechs info" (with a 1 amp fuse inline just in case). I know this is bad, and want to resolve my reluctance to feed a suspected, but not 100% sure circuits with voltage or ground to test their "activation" (could be light flash, door locks, trunk pop etc.)... Thanks for all the input! Keep up the good work!

You could try an analog meter, or a LED. It could be a good use for a test lamp, but experienced users should be able to advise & explain. (howie ll comes to mind.) Or a logic analyser (with a 5V liniter/divider if required). Or maybe a home brew "sample & hold" circuit (diode, capacitor). Or some -ve triggered circuit... The only way a typical digital meter will sense it is if it samples at the right time. Extending the sample time won't help. You need a DMM that has a peak-sample&hold function.    Analogs (ie, meter movements) may move fast enough to indicate a pulse. And there is always the good old CRO.... Maybe others know of some suitable cheap instruments....

The Fluke, if setup correctly, should read virtually any signal in a car.  You MUST disable autoranging for pulses.  What happens is the range exceeds the first autorange step and causes the meter to freeze just long enough that the pulse is missed.  If you disable autoranging and only look at xx. or xx.x vdc.  This should allow you to see the pulse.

Also, if you must "guess" at a signal I prefer to put a ~220 ohm resistor in line with a 1A fuse.  This will theoretically limit current to around 50mA. 

Kevin Pierson

Ditto what K said.  You can not buy a better meter.  The autoranging feature of the meter is why it is having trouble.

Some Cadillacs have 12 volts on the arm that the trunk latches to.  I see a lot of guys touching their meter lead to that piece of the car to check for voltage at an amp.  When the trunk lid touches the arm it is grounded through the lid and the auto close motor pulls it down.  Just an FYI, I know it is not related to your problem.

Howie agrees with KP and Mr. I ref fluke meters but would still use a "fast and dirty test light" because the bulb protects. Yes I recently triggered a DTC engine light on an MX8 probing the inst. panel but not being a VW it cleared after 10 secs.

howie ll wrote: Howie agrees with KP and Mr. I ref fluke meters but would still use a "fast and dirty test light" because the bulb protects. Yes I recently triggered a DTC engine light on an MX8 probing the inst. panel but not being a VW it cleared after 10 secs.

... 

M.E.C.P & First-Class

Go slow and drink lots of water...Procrastinators' Unite...Tomorrow!

Oh all right it was a dig at bloody VAG "catch the aftermarket out" software. I'm debating how to raise the £4000 ($6000) for a decent diagnostic unit.

masterodisaster wrote: I am just curious when testing suspected circuits using a multimeter and you have a wire resting at a positive voltage (+12.6V) how can you determine if it is a negative trigger without potentially causing damage to the vehicle computer. It is only an issue due to the multimeter my installer was using wasn't displaying the quick pulse of voltage properly. Are there certain specifications I should look for on a DMM that would be best for the automotive installer? I personally have $200 Snap On that works great, but my installer has a $100 Fluke meter and it is "slow to react to voltage changes"... I want him to have one that will have a higher "sample rate" if you know what I mean. In any case this Cadillac Deville we did today had a trunk pop listed as a (-) trigger, but it ended up being a (+) trigger and for some reason and I had a tough time determining it's trigger type with his meter

If the wire rests at 12.6V it probably isn't a + trigger- it generally needs to sit at the opposite state of whatever triggers it. A DMM should be used prior to test lights or other devices that draw any appreciable current, when testing unknown signals. As far as detecting fast pulses, the meter would need to have some peak detection or logic mode to pick it up reliably. The sampling rates of DMMs aren't fast enough to pick up very short pulses in normal measurement mode, even if set to fixed range. If the meter doesn't have any suitable pulse detection modes, you could look for one that has the feature, and they're available on a lot of meters much cheaper even than the Fluke. Or you could put together a CMOS logic probe that will easily detect pulses, without loading the signal, which is necessary in case the signal is sensitive to injected current. Analog meters and test lights have much lower input impedances so they require more care with unknown circuits.

Frankly if I was testing that wire and knew it's function, I'd use my trusty snap-On test light, lets face it you will know it's at rest polarity, pos or neg, then use the trunk release, that will tell you whether you have a trigger wire or a motor wirer and if it's polarity if a trigger or relay interrupt (5 wire) if it's a motor wire, simpler and much less time than it took me to write this.