I am trying to hunt down a short in my ign system that's causing 20A fuses to blow as soon as I turn my ignition to the on or start position. I have tried using my multimeter but, apparently I'm not quite as good as I thought. I get a reading of 001 if I measure resistance between the B/W wire from the ign switch and the fusebox so that looks like it should but if I check between the B/W wire at the switch and ground I get 1485!! I also get the same reading if I check the opposite side of the fuse, the green wire. This leads me to a short to ground on the green wire side of the fusebox. Is this correct?

It looks like the green wire goes to the tach, speedo meter and a few other thing, any suggestions on how to narrow it down??

'71 240z 5-speed.

TIA!!

To narrow it down identify all loads (turn signal, meters, regulator, etc.) and disconnect them one at a time until the problem goes away. To make it easier replace the fuse with light bulb. Use a moderate size bulb like an 1156 or the brake light filament in an 1157. As long as a short is present the bulb will be bright but limit the current to a few amps and prevent the wiring from overheating. Once the short is removed the bulb will go out. The regular load current from a small load like the meters will not light the bulb. A larger load like the turn signals will light it about half brightness and the turn signals will also be dim. They might not flash as well.

Edited June 18, 201311 yr by beermanpete

That is a very good idea. I like it, very clever...

C

This is just so much fun!! I ended up pulling the dash to get to those damned wingnuts that hold the tach in (who uses wingnuts under a dash!?!) I also had to clean up some questionable wiring that the PO hacked in.

One question; the green wire that comes out of the fusebox (off of the IGN fuse) changes to a RL (red withe blue stripe) but in the fsm diagram, it still shows it as green, does anyone know if this is correct??

How far from the fuse did it change color? RL should be for gauge illumination.

According to the wiring diagram (72) the red/blue wire branches off from the green and goes to the meters to power the tach and the brake warning light. Where the branch occurs is not indicated.

Okay, I was looking from the top down and saw the gauge illumination first. I hate it when they duplicate wire colors like that.

  SteveJ said:

How far from the fuse did it change color? RL should be for gauge illumination.

On my harness, it changes at the fusebox connector right under the radio. The green wire is only ~6 inches long, it looks a lot longer on the diagram!!!

That sounds about right. Keep in mind that the wiring diagram is NOT even close to a scale drawing.

  SteveJ said:

That sounds about right. Keep in mind that the wiring diagram is NOT even close to a scale drawing.

Now you tell me!! (J/K!) At first I was hesitant about pulling the dash but, after seeing what the PO did, I would've been hard pressed to find ALL of the hacks that were perform! Between a really cheap alarm that I didn't even know was there, where he decided to tap into a "hot" wire for the non-existent radio and, the crimped wires for the antennae up and down switch (who uses the up and down switch??), I had some patching to do.

I still haven't found the corroded connection or hot wire grounding in the IGN circuit!! Hopefully cleaning all the connections under and behind the dash does some good!!

Edited June 19, 201311 yr by Da Flash
clarification

  beermanpete said:

To narrow it down identify all loads (turn signal, meters, regulator, etc.) and disconnect them one at a time until the problem goes away. To make it easier replace the fuse with light bulb. Use a moderate size bulb like an 1156 or the brake light filament in an 1157. As long as a short is present the bulb will be bright but limit the current to a few amps and prevent the wiring from overheating. Once the short is removed the bulb will go out. The regular load current from a small load like the meters will not light the bulb. A larger load like the turn signals will light it about half brightness and the turn signals will also be dim. They might not flash as well.

I have been trying this technique ( the light bulb instead of the fuse ) for a couple of days on and off and I always get a bright light. If I'm understanding this correctly, the IGN circuit sees 12vdc as soon as you turn the key to On/Start, so wouldn't it light the bulb with or without a short/bad connection?? Since I have the dash pulled back a bit, I have disconnected the gauges and anything else in the circuit but I still get the light as soon as I apply 12vdc to the circuit.

As long the bulb is connected in place of the fuse it is in series with the load and will only light if the load present draws enough current. To be clear, no wire goes to ground from the bulb. Both wires go to the fuse clips, one for each. Most of the loads are switched after the ignition by a second switch. For example, the turn signals only turn on with the turn signal switch. If the light bulb is on when all the switched loads are off then look at the what is not switched.

The loads not controlled by a second switch are:

Charging system

Ignition system

Meters

When the car is not running the regulator sees low system voltage and tries to turn on the alternator by passing current through the field winding. The field will draw about 4 amps which is more than enough to light the light bulb. Disconnect the regulator and see if the bulb goes out.

If the points happen to be closed when the engine is not running current will flow through the coil. That is another 3 amps trying to light the bulb. Disconnect the coil positive wire and see if the bulb goes out.

The meters should not draw very much current and it is not likely the bulb will light from this load unless the problem is in these circuits.

This is a process of elemination. You disconnect on thing at a time to see if the bulb goes out. Leave each one unhooked until you find the culprit by way of the bulb behaving as expceted. To add some resolution to the process, connect a volt meter to the load side of the bulb. A true short will hold the voltage at 0 volts. A normal load will allow the voltage to rise a few volts.