Need some help with simple electrical issue!

I need some help dealing with what should be a simple electrical issue in my beloved 1996 Infiniti I30:

I think my problems all started over a year ago when I tried installing LED interior lights that a (reputable) guy on Maxima.org was selling.  It had been 5+ years since I bought the LEDs, and none of them worked, and when I went to reinstall the stock bulbs, I had trouble getting them to work again.  Since that time, the door light on my DS door hasn't worked, no matter how many new bulbs I try.  Same thing with one of my front map lights.  The main interior light has been weak and intermittent since I tried an LED in its place too.

Well today I decided I would dig into the problem and see if I could fix it.  I popped off the cover for my dead DS door light and checked for continuity with my multimeter, and it had continuity.  However when I checked to see if it had power, I got something like 0.03V.  I had my MM set to DC10A.  Pushing and releasing the door plunger didn't change the reading.  I then went over to my working PS door light.  I pressed the plunger down and removed the bulb.  The socket had continuity, but when I released the plunger, it didn't have any power going to it.  I then pressed the plunger back down and plugged the bulb back in.  It no longer works, and it's not burnt out!  I then went to check the front map lights.  The one on the DS hasn't worked in a while, but the PS one has.  I checked for continuity with the PS bulb lit up.  The DS had continuity, but when I checked the lit up PS bulb, it went out and won't come back on!  The PS has continuity as well.  It's almost like I'm blowing out fuses for the individual lights, but I know those don't exist.  I went around and checked all my fuses with my MM and all have continuity.  How in the world are the lights dying just by me removing them or touching them with my MM?  Why are they not getting power?

Also, I re-tried the procedure to have the body module check for error codes associated with the door locks, since my door locks have a mind of their own, and I often have to unlock the car (with my alarm fob) five times before all the doors unlock.  When I did this test a year ago, it said the PS front door lock actuator was bad.  Today, with most of the interior lights burnt out, I was unable to get the car into diagnostic mode.  I wonder if the diagnostic mode can only be accessed if all/most of the interior lights are working?  IIRC it uses the interior lights to tell you what the code is by blinking.

My electrical troubleshooting skills are pretty weak, but I'm trying to learn.  Does anyone have any ideas as to what I've done to my poor car?  Thanks in advance!!!

You had the multimeter set to 10 amps?  And you checked across a bulb socket with that?  POP.  You blew the fuse or the driver for that circuit, depending on how that thing is wired.

You don't check for continuity on +/-.

Just check your fuses and then check for voltage. 

You can use the continuity setting to check the fuses though. 

Checking for continuity happens on the resistance (ohms) setting. You can't use that setting on a powered circuit because the meter itself has very low resistance in that setting and will cause current through that circuit to go up until something bad happens (hopefully just a fuse popping). The 10A setting likewise has low resistance and can only be used in series with a known-functioning circuit (if you hook it up as a parallel path you still get the carnage) lest it also result in blown fuses or other damage from excess current flow.  When the meter is set to measure voltage, the meter itself has very high resistance and can be hooked to almost anything in almost any way without damaging anything. 

So basically, the only thing that is safe to measure with very little background electrical knowledge is voltage below about 30. Above 30 the capacity of human skin to save you from a lack of electrical knowledge becomes unreliable. Luckily most stuff on most cars is 12v or less. 

Also, love the car!! 

Dr. Hess said:

You had the multimeter set to 10 amps?  And you checked across a bulb socket with that?  POP.  You blew the fuse or the driver for that circuit, depending on how that thing is wired.

Yeah, I went back through my electrical book and some youtube videos this morning and realized my mistake!  I'm feeling pretty stupid right now since I think I made the problem worse.  I pulled two fuses for interior lights and neither was broken (visually).  Both had continuity with the MM (with red lead plugged into correct V hole).

I should clarify that I tested continuity correctly yesterday, the red lead was plugged into the V hole.  I only had the red lead in the A hole when testing for voltage (because I'm an idiot).

Slippery said:

You don't check for continuity on +/-.

Just check your fuses and then check for voltage. 

You can use the continuity setting to check the fuses though.

What do you mean by +/-?  I had my MM set to check continuity with the red lead plugged into the V hole and not the A hole.  I checked the fuse for interior lights and it's intact.  I tested the fuses both visually and with the MM set to check for continuity (it beeps).

Vigo said:

Checking for continuity happens on the resistance (ohms) setting. You can't use that setting on a powered circuit because the meter itself has very low resistance in that setting and will cause current through that circuit to go up until something bad happens (hopefully just a fuse popping). The 10A setting likewise has low resistance and can only be used in series with a known-functioning circuit (if you hook it up as a parallel path you still get the carnage) lest it also result in blown fuses or other damage from excess current flow.  When the meter is set to measure voltage, the meter itself has very high resistance and can be hooked to almost anything in almost any way without damaging anything. 

 

So basically, the only thing that is safe to measure with very little background electrical knowledge is voltage below about 30. Above 30 the capacity of human skin to save you from a lack of electrical knowledge becomes unreliable. Luckily most stuff on most cars is 12v or less. 

 

Also, love the car!! 

Thanks, I love the car too! :)

Yup, I think I've got ohms/continuity understood pretty well.  MM red lead was plugged into normal V hole (not the A hole) and I set MM to make a noise if there is continuity, and it beeped when touched to all fuses.

I'm now back working on the car and readings are all over the place.  I'm worried I fried something when I had the MM set up to test amps instead of voltage, but no fuses are blown, and I'm not sure what to check next.  Good god I need some help!

Edit- The reason I had the red lead of the MM plugged into the amp hole instead of the volt hole was because I was previously testing for a parasitic drain, which I believe is tested using the amp setting, and I was too dumb to switch the red lead back to the volt hole.  I'm an idiot and make problems worse :(

Does the car have a BCM or body computer? Maybe that’s messed up? It just seems so unlikely in a car like that. 

dculberson said:

Does the car have a BCM or body computer? Maybe that’s messed up? It just seems so unlikely in a car like that. 

It does, and I'm starting to suspect that it may be tied to the problem with the door locks.  I'm not sure if the BCM controls interior lights or not though.  Since I've done all the diagnostic work I can related to the lights, I'm currently working on figuring out what's going on with the door locks.  As of now it seems like both DS and PS front door lock actuators may be bad, but I suppose that could also mean the BCM is going bad.  Sometimes the DS front door remains locked and all other doors unlock, and sometimes the PS front remains locked while all others unlock.

What are you checking for continuity? Continuity in a multimeter refers to a continuous loop from the red lead of the meter to the black lead.  It checks for that loop by running the power from the internal battery of the multimeter to the red lead, through whatever you are measuring, and back to the black lead.  That function is only useful for circuits you completely understand that do not have their own power.  If you are using that setting on a circuit that is supposed to be powered up and lighting up a bulb the least bad thing you will get is useless information from your meter, and the bad things range on up to burning out the meter itself or blowing up the internal battery in the meter.  Just don't ever use that setting on something that has power. 

On a car with power you check for voltage at the socket with the 0-30V setting and the red lead in the v/ohm hole.  If you hook the black lead to a good chassis ground, like the seat bolts, and use the red on voltage settings to probe one side of the bulb will have 12v or so and the other nothing useful with a digital meter.  Once you have confirmed a positive voltage shift the black probe to the other bulb contact to verify the bulb ground path. 

As indicated before, don't use the Amp hole or settings for diagnostic tests except on circuits you can completely isolate. 

If I had to guess the direct short you created with the meter on amp setting fried either a power supply wire into the door or a ground grouping inside the door that all your malfunctioning parts share.

oldopelguy said:

What are you checking for continuity? Continuity in a multimeter refers to a continuous loop from the red lead of the meter to the black lead.  It checks for that loop by running the power from the internal battery of the multimeter to the red lead, through whatever you are measuring, and back to the black lead.  That function is only useful for circuits you completely understand that do not have their own power.  If you are using that setting on a circuit that is supposed to be powered up and lighting up a bulb the least bad thing you will get is useless information from your meter, and the bad things range on up to burning out the meter itself or blowing up the internal battery in the meter.  Just don't ever use that setting on something that has power. 

On a car with power you check for voltage at the socket with the 0-30V setting and the red lead in the v/ohm hole.  If you hook the black lead to a good chassis ground, like the seat bolts, and use the red on voltage settings to probe one side of the bulb will have 12v or so and the other nothing useful with a digital meter.  Once you have confirmed a positive voltage shift the black probe to the other bulb contact to verify the bulb ground path. 

As indicated before, don't use the Amp hole or settings for diagnostic tests except on circuits you can completely isolate. 

If I had to guess the direct short you created with the meter on amp setting fried either a power supply wire into the door or a ground grouping inside the door that all your malfunctioning parts share.

Thanks for posting, that's more or less is what my buddy told me over the phone.  I learned my lesson about using the amp hole on the MM!  I had it in there because I had just finished testing for a parasitic drain and I didn't think to switch it back to V/ohm.  I clearly can't learn electrical stuff from a book, so I guess I'll just have to keep making mistakes until it sinks in! :)  As far as I can tell the MM is still working, so I've got that going for me....

I re-did my tests using correct methodology that my friend walked me through over the phone.  Thanks Brian, you're the best!  Here are the results:

And here's some info about the body control module (BCM) from the FSM:

Brian seems to think I've got a bad ground somewhere related to the PS door light.  He recommended I find trace it to the ground and/or the BCM.  What I don't understand is how I had three working lights yesterday (out of five) if I have a bad ground.  If there are no blown fuses right now, and lights still aren't working in any location, I can't help but think something fried inside the BCM.  Brian also wondered if one of the door switches/plungers is going bad and not letting enough current get to the bulbs.  We know everything is getting voltage, but we don't know what's going on with the current.  Am I understanding this correctly?

The BCM controls my locks, interior lights, and the sleep mode for the car.  I've got no interior lights, locks with a mind of their own, and a HUGE parasitic drain.  If the BCM is faulty and not going into sleep mode, I believe that could cause a large drain, no?  My lack of understanding is pushing me to believe the BCM is the cause of all my problems, but I know it's probably not that simple.  Does anyone know what is typically involved with replacing a BCM on a 90s Japanese car?  Does it need to be programed to match my car/key?

More to the point, since I'm unable to make sense of wiring diagrams, how can I figure out where the ground is for that PS door light?  I'm pretty sure I should start there, but I'm not sure how.

Thanks to everyone who has responded!  I'm pretty dumb when it comes to this stuff, but I'm determined to learn!

EDIT:
I went back for like the 4th time and did find fuse #26 to be blown.  Maybe I missed it before since it's up so high in the box?  Interestingly enough, I found a 10A fuse in there, even though it's supposed to be 7.5A.  Hopefully that extra current that got through didn't mess up whatever that fuse is protecting :(  I JUST went out and replaced all of the interior bulbs, and I now have map and overhead lights back!!!!!!!!!!!!!!!!!!!!!!  But still, no door lights.  Back to the drawing board....

Trying to measure resistance to ground for something that has voltage on it is going to give you useless information and can potentially damage your meter. Resistance is just like continuity,  it uses the battery in the meter to send voltage through the circuit from one test lead to the other. If you hook that up to an external voltage (external to the meter) you are playing with fire.

Measure voltage to ground first.

Anything you find voltage on don't measure again with any part of your meter except for voltage.  Any other setting gives you garbage readings and can damage your meter. 

Once you have determined that there's no voltage at a point you can check that point for resistance or continuity to ground. 

BCMs almost universally work by grounding a small control signal to energize or trigger a relay that then handles the load in question.  For dome and door lights generally the door pin switches supply a ground for the lighting and the BCM controlled relay is either in series with those switches on the ground side of the lights or on the supply side interrupting power to the lights. 

Have you pulled the plug off of the BCM to check to see if you have corroded prongs or evidence of excessive voltage? 

It sounds like you either have a bad ground, or the BCM controls the ground side. If the second is the case then it is probably something wrong with the door switch. Can you make the light work with the manual override (If the car has one)? If so then it is probably the door switch.

oldopelguy said:

Trying to measure resistance to ground for something that has voltage on it is going to give you useless information and can potentially damage your meter. Resistance is just like continuity,  it uses the battery in the meter to send voltage through the circuit from one test lead to the other. If you hook that up to an external voltage (external to the meter) you are playing with fire.

Measure voltage to ground first.

Anything you find voltage on don't measure again with any part of your meter except for voltage.  Any other setting gives you garbage readings and can damage your meter. 

Once you have determined that there's no voltage at a point you can check that point for resistance or continuity to ground. 

BCMs almost universally work by grounding a small control signal to energize or trigger a relay that then handles the load in question.  For dome and door lights generally the door pin switches supply a ground for the lighting and the BCM controlled relay is either in series with those switches on the ground side of the lights or on the supply side interrupting power to the lights. 

Thank you for the explanation, I'll go re-check the resistence for the door lights tonight the correct way and report back.

grover said:

Have you pulled the plug off of the BCM to check to see if you have corroded prongs or evidence of excessive voltage? 

Nope, but I did take apart the dash and have access to the BCM, so I'll check that tonight.  Thanks for the tip!

pjbgravely said:

It sounds like you either have a bad ground, or the BCM controls the ground side. If the second is the case then it is probably something wrong with the door switch. Can you make the light work with the manual override (If the car has one)? If so then it is probably the door switch.

 

Now that my dome light works as intended (turns on when a door opens) I believe that means my door switches/plungers are working as intended, right?  I'm not sure what you mean by a manual override though, can you explain?

Alright, I unplugged the battery and waited for the remaining charge to drop off the line.  I waited until it got down to 1.5 mV before doing the resistence test.  And the results sure are different than when I did the test with power on the wires!

I only retested the door light's resistence since the others all work now.  I'm sure these new numbers mean something but I don't know how to interpret them.  Can anyone assist?

95maxrider said:

Now that my dome light works as intended (turns on when a door opens) I believe that means my door switches/plungers are working as intended, right?  I'm not sure what you mean by a manual override though, can you explain?

In the old days, your headlight switch on the dash, when turned all the way counterclockwise turned on the dome light. Most cars have the headlight switch on a stalk now.

If the dome light works, what is now the problem if any?

pjbgravely said:

95maxrider said:

Now that my dome light works as intended (turns on when a door opens) I believe that means my door switches/plungers are working as intended, right?  I'm not sure what you mean by a manual override though, can you explain?

In the old days, your headlight switch on the dash, when turned all the way counterclockwise turned on the dome light. Most cars have the headlight switch on a stalk now.

If the dome light works, what is now the problem if any?

Ahh, gotcha.  The problem now is just with the lights on the doors, all overhead lights work again. 

Does the door light suppose to turn on at the same time as the dome light? If so, then you know the control is working.  I would then look for a bad ground or a bad hot at the hinge.

pjbgravely said:

Does the door light suppose to turn on at the same time as the dome light? If so, then you know the control is working.  I would then look for a bad ground or a bad hot at the hinge.

Yes, the door lights should come on with the dome light when the doors open, so I suppose that means both door switches/plungers work, and the controller for them works as well.  Which means something wrong on that specific circuit.  if these wires are all tied up in a loom, is there a test I can do to help narrow down the problem area?  Anything I should be looking for?

I should also mention that when I tried to install LEDs (that were supposed to work in my car) for the trunk and keyring lights (many years ago), neither worked, and the stock bulbs no longer worked in those locations either after trying the LEDs (same story as the LED bulb in the DS door).  I've read people mention a "proper" way to install LED bulbs without actually saying what that proper way is.  I assume disconnecting the battery before installing the LEDs?  Because I sure didn't know to do that back then.  Aside from fuses, what could I wreck by plugging in LEDs while power is still on the circuit?  Could that affect the BCM?

Does anyone have any more thoughts on what my newer resistence measurements mean?  Am I still looking for a messed up ground or burned out wire, or is there something else I should be looking for?

If all the issues are within the door, I would start by looking closely at the wiring from the body to the door. Twenty years of opening and closing has likely chafed something.  Might be a bitch to get at but also the problem may show up just messing around with it. If while moving it around  things suddenly start working again you know it is in that bundle.

The top prong as 5.1 volts either looks like the BCM is controlling through the ground side and is dimming the bulb. If that is not the case then then you have a bad  ground.  Make sure your measurements are done with the bulb in the circuit. Perhaps draw the circuit out and show where you are getting the measurements. You will probably understand more when you do this.

Getting a wiring diagram is the best way to diagnose electrical problems.