Keep blowing the "IGN" fuse!!!

[Da Flash]

I've been trying to track down the source of my "IGN" fuse blowing within seconds of turning the key to "ON". From my probing around with a multimeter, I've discovered that the green wire side of the fuss holder is getting continuity with ground (i.e. the dash metal frame. I've looked for a wiring schematic that has the green wire on the fuse box transitioning to a red/blue wire after the 6 wire connection but, all the diagrams that I've found don't show the wire changing colors!! From my probing around, I've discovered that this wire goes to the hazard switch but, cannot figure out where it would be coming in contact with the dash frame. I have the dash out of the car, for ease of checking things out.

For what it's worth, the engine runs fine but, I don't have gauge lights, gauges or a tach.

1. does anyone have a schematic that has the green "IGN" wire changing to red/blue at the conntector??

2. Are there any places where this wire could come in contact with the dash metal frame??

This electrical stuff come be a major PITA!!

[Mark Maras]

The short could be anywhere in that circuit, from the bulb sockets back to the power source including the ignition switch. Have you connected your ohm meter and wiggled the affected wire harness to see if the short circuit goes away? We will need to know what year you are working on to reference the wiring diagram.

Mark in Portland

[Da Flash]

The short could be anywhere in that circuit, from the bulb sockets back to the power source including the ignition switch. Have you connected your ohm meter and wiggled the affected wire harness to see if the short circuit goes away? We will need to know what year you are working on to reference the wiring diagram.

Mark in Portland

Thanks for the reply, I have wiggled the harness with no noticeable difference on the meter. My car is a '71 240z (8/71 manufacture date) and the fusebox has short pigtails.

Edited February 20, 2014 by Da Flash
clarification

[Zed Head]

atlanticz has a few odd wiring diagrams. Something might show itself.

Z Tech Tips Electrical (AtlanticZ.ca)

[Da Flash]

atlanticz has a few odd wiring diagrams. Something might show itself.

Z Tech Tips Electrical (AtlanticZ.ca)

Thanks, the schematics weren't much help but, the "how to for multimeters" was educational!! I openly admit that electrical has never been my forte. I was under the impression the continuity and resistance were one and the same but, after reading I see the difference. Just to be clear, I am using continuity on my meter and I get a beep and a reading ~30 between the green wire at the fusebox and ground when I have the tach, oil/temp and or fuel/amp gauges (I don't have the clock installed)connected to the harness. The continuity goes away as soon as I disconnect all 3 gauges. I was under the impression that the gauge lights were the culprit but, they can be installed without the grounding issue.

1. Do I seem to be on the right track??

2. What does the number 30 represent when doing a continuity test??

[SteveJ]

Use resistance to test. Don't trust the beeping.

Let's think about why fuses blow. They blow because they have too much current flowing through them for the timeframe they are designed for. The fuse blows to protect the wiring from burning up.

Why was there too much current? Refer to Ohm's Law. I (current) = V (voltage) divided by R (resistance). Voltage is roughly constant at around 12 volts, give or take a couple of volts. For a 10A fuse, if the resistance on the circuit drops below 1.2 Ohms, you are getting to the limits of the fuse. (This is why I look at resistance and not continuity.) Beware of autoranging meters, too. All of the sudden the scale could change from Ohms to milliOhms.

So, what causes the low resistance? Of course, the obvious is a dead short. Corrosion in bulb sockets can bridge the gap between positive and negative. Also, a relay coil's insulation could break down, bypassing windings and dropping the resistance. The ignition coil could fail in a similar manner. Abrasion in insulation could allow a small area to come into contact with the chassis ground.

So, let's work on the diagnostics. (By the way, I spent about two hours today tracking down a short in control wiring for some electrical gear. Even with experience, it can take time.) Can you post pictures of the fuse and wiring in question? I'm tired right now and can't picture it in my mind. I especially want to see where the fuse is. Are you talking about the one in the fusebox?

Look for any wiring that may not be factory. Look for splices, electrical tape, etc. Post pictures of those anomalies, and give a description of where you find them in the harness. Maybe Saturday, I can finally look at the wiring diagrams and give you some other points for testing.

I hope this helps.

[Da Flash]

Thanks for the reply Steve, the fuse that keeps blowing is the third one down on the left in the main fuse box located under the ashtray, it's labeled "Ignition". I have attached a few pics, it's the green wire being pointed to and it transitions to a red/blue wire after the connection. The other pic is of the back of the dash with the oil/temp, fuel/amp and tach disconnected. I just checked the resistance from the fuseholder to ground (the metal dash frame) and I get readings of .060 with either oil/temp or amp/fuel connected and .456 with the tach connected. I get .030 with all of them hooked up, this is with the meter set to 2K.

[SteveJ]

Excellent information. We can work with this. I'll take a look at the wiring diagrams and see if I can give you some more to look for. That is unless BeerManPete doesn't beat me to it.

[beermanpete]

The IGN fuse feeds the meters (R/Y), the charging system (B/W), the choke warning lamp ®, the turn signals (G) (through the hazard switch) , and the brake warning lamp (R/Y). Try disconnecting all of these and and test each for shorts/resistance as well as the wire from the fuse which should be an open circuit when all the loads are removed.

[SteveJ]

See, he beat me to it.

[Da Flash]

Thanks for the reply Pete, I'm assuming that you mean to check the resistance at the harness connector for each device and the fusebox, correct? I'm under the impression that with the dash removed from the car, all of the devices would be irrelevant except for the meters.

Please correct me if I'm wrong.

Edited February 23, 2014 by Da Flash

[beermanpete]

I forgot you have the dash out but, yes, that is what I meant. Separate the various circuit branches and test each one separately.