tracing a possible electrical short

[SteveJ]

23 minutes ago, RJK said:

SteveJ-AMAZING info here. very clear. thank you SO much. it's complicated stuff; I had to read your entire paragraph 2-3 times before I could visualize what you were describing, but now I think I can. In essence, it seems as though the white wire and white/red wire run thru a network, that includes the fuse box, ignition switch, and ammeter when they travel into the cabin. by disconnecting the molex connectors, and the connections in the engine bay, I am breaking apart that network. now, by testing at the connecting points, i can see where the network is grounding out(or close to it). very clearly put. I'll implement this procedure today, and report back.

I think you are understanding the testing concept now.

Now imagine a 3 inch bundle of wires, all of them are grey. One of those wires was not run correctly, and you have to find out which one it is. That really helps improve diagnostic skills. I find wiring diagrams and wiring harnesses with color codes to be SO much nicer.  

[Captain Obvious]

I'm finding the wiring documentation for the early cars to be lacking and questionable. And not having a 240 here to look at for reference, it's tough. I particularly dislike the way they depicted the fuse block. Took me some time to figure out what (I think) is going on at there. And not showing where the connectors exist on the wiring diagram doesn't help troubleshooting any. And neither does wire colors magically changing along the runs.

Anyway, might be old news for you 240 guys, but I came up with this:

 

Note that if you flip the fuse block over and look at the wires colors there, some of them are different than what's shown on the wiring diagrams. I'm assuming the colors shown on the FSM wiring diagram are on the HARNESS side of the fuse block connectors and then didn't bother to list the colors that exist actually inside the block. Most of them line up with the wiring diagram, but some of them do not.

Not having a 240 here for verification, I can't add much value. But I'll try to add a little.

[SteveJ]

That's why I made that drawing for the turn signals and included the connectors. It's really fun to diagnose the fan wiring.

I keep an old fuse box handy so I can locate the fuses easily when I'm responding to posts.

[RJK]

Ok, here's what I got from today....more baselines: starter motor sense wire is disconnected, as is alternator and fusible link. White and white/red wire connectors as PS footwell fender side are disconnected. White/red wire at center console, "pre fuse box", also disconnected. I am tying off ground pin of my DVM to an engine bay screw location.

 

1-tested both white wire, and white/red wire sections from engine bay to PS footwell location #1. No short; both read OL.

2-section of white/red wire from fender side to center console side: no short, OL.

3-where the 3rd section of white/red wire connects directly to fuse box, the right side of the 4th fuse down on right row, DOES show a short. See attached pics; this reads 2.7ohms. I see the junction in this wire; it looks to split off and go upwards(ammeter or ignition switch?).

4-left side of the same fuse does NOT show a short; OL on DVM.

 

What's next? Thanks again for the help!!!

[SteveJ]

With the measurement at the fuse box, you need to do things a little differently. While you took out the 4th fuse down, you are measuring on the common feed for fuses 3, 4, and 5. You need to pull all three fuses (circuit isolation).  Also make sure the headlight switch is in the OFF position. Also you may want to unplug the inspection lamp under the hood. Also test the fuse box with your doors closed. As @Captain Obvioussaid, you want to make sure you don't have a switch on, and that includes the switches for the dome light.

With the fuses pulled (and the switches off and inspection light disconnected) take measurements to ground at the points indicated by arrows in the photo.

Just for a reference, 2.6 ohms will drain your battery quickly, but it should not burn up your fusible link. That's a little less than 5A draw. That could be a couple of light bulbs you're reading.

As I said in a previous post, reconnect things in the engine bay one at a time and re-measure. If you do have a short, I'm more willing to believe its around the alternator. (When I have problems with my own cars, my first thought is, "What did I f*** with last?")

[Captain Obvious]

Steve, Backing up a little (in order to hopefully jump forward?). When you disconnect that four pin connector that goes out to the engine bay (the one with the R, RL, RY, GL wires)... The "short" goes away and the circuit under scrutiny goes OL. And on that connector, you have headlights, front marker lights, and the inspection lamp. All of those things are powered from the W/R wire feeding the fuse block. Either directly, or switched through the headlight switch.

Can we please please get a double dog sure that the headlight switch is off?

[RJK]

thanks guys! i checked that all my switches were off at the steering column, however, i'll do a triple check today. i'll also activate said switches with the doors closed, and watch my readings. the PS door and hood have been open on all of these readings. i would love nothing more than to find out that the readings are due to user error!

 

as to a short at the alternator, the two alternator wires are separated in the wiring loom, and i can visibly see that the wires have integrity and cannot physically short TO anything.

[SteveJ]

9 hours ago, Captain Obvious said:

Steve, Backing up a little (in order to hopefully jump forward?). When you disconnect that four pin connector that goes out to the engine bay (the one with the R, RL, RY, GL wires)... The "short" goes away and the circuit under scrutiny goes OL. And on that connector, you have headlights, front marker lights, and the inspection lamp. All of those things are powered from the W/R wire feeding the fuse block. Either directly, or switched through the headlight switch.

Can we please please get a double dog sure that the headlight switch is off?

I imagine that if @RJKisn't following my directions VERY carefully, he will be measuring the impedance of the loads on several branches. That is what I'm seeing with the readings so far.

So, to RJK, as you take your readings, keep this in mind: I=V/R. In other words, current is voltage divided by resistance. We can also put the equation as R=V/I.

This is how to think of it. You have about 12.6 volts as a lower threshold for a good battery. You want 40 amps or less flowing through the fusible link, and the 40A value is only if the battery is down on charge. (By the way, use a battery charger to help a weak battery, not your alternator.) so the resistance readings on the main wires (white or white/red) that would be giving you problems would be 12.6/40 or about 0.32 Ohms (or less).

You seem to be getting higher values than that. It could mean that you have disconnected the short, or you burned off the short (not a good thing). I've been giving you guidance to verify that there is not a high impedance connection to ground (the remaining connection after you burned off a short) that cannot be accounted for by typical loads such as parking lights, headlights, dome light, inspection light, etc.

[Captain Obvious]

1 hour ago, SteveJ said:

keep this in mind: I=V/R.

We talked about it before (I think), but you have to be careful how you apply that formula to incandescent lamps due to the dramatic change in resistance as the filament heats up.

For example, I just took a quick measurement off a little 3W inspection lamp bulb and got about six or seven Ohms cold. But for a 3W bulb like that, the resistance SHOULD be 48 Ohms. The trick is that it IS about 48 Ohms WHEN HOT, but at room temp, it's under ten.

A couple of those in parallel like the dome light (because the door is open) and maybe the engine inspection lamp, or the map light in the console, or the ....

Hope I'm adding value and not muddying the waters.    

[SteveJ]

8 minutes ago, Captain Obvious said:

We talked about it before (I think), but you have to be careful how you apply that formula to incandescent lamps due to the dramatic change in resistance as the filament heats up.

For example, I just took a quick measurement off a little 3W inspection lamp bulb and got about six or seven Ohms cold. But for a 3W bulb like that, the resistance SHOULD be 48 Ohms. The trick is that it IS about 48 Ohms WHEN HOT, but at room temp, it's under ten.

A couple of those in parallel like the dome light (because the door is open) and maybe the engine inspection lamp, or the map light in the console, or the ....

Hope I'm adding value and not muddying the waters.    

I think you are muddying the waters this time, Capt. RJK had the insulation burn off his fusible link without the link blowing (assuming that it was fusible link wire used) after installing the Nissan Frontier alternator. Now he is trying to verify whether or not he has a short somewhere. Since he doesn't seem to have a car battery available, he is using an ohmmeter. I have been giving advice on isolating circuits to verify there isn't a shorted branch that could have taken out the fusible link. In that case the I=V/R equation is very relevant because I'm trying to get him to focus on low resistance readings. The reading he got at the fusebox is probably a light. If not the parking lights, maybe the engine bay inspection light. It's on the same fuse.

After confirming there isn't a dead short in the cabin, I asked him to connect the engine bay connections one at a time and measure. I'm thinking that the likely culprit was miswiring on the alternator with likely no other damage on the wiring.

 

[Patcon]

He mentioned the PS door was open,  wouldn't that supply a path to ground?

[SteveJ]

7 minutes ago, Patcon said:

He mentioned the PS door was open,  wouldn't that supply a path to ground?

Yes, but if I'm not mistaken, that fuse was pulled in the photo in post #28.

The bottom line is that the readings may indicate a battery drain, but so far, I don't recall seeing a reading that would indicate a dead short.