I think there's a short in my new engine harness... how to track down...?

[Grec]

Just installed a new engine harness from Year One/M&H.

Very nice product, by-the-way, well, aside from my issue... :-)

The harness was for a 73' Challenger with a 340. My car now has a 440 so a couple modifications needed to be made:

1) Lengthing the water temp sensor wire
2) Lengthing the two wires that go to the distributor
3) Relocating where the oil sender wire came out (no cutting/splicing needed on this one)

All of the splices were done by carefuly stripping/twisting the wires, soldering and then a thin wrap of electrical tape. Nice and solid and very clean when taped back up.

Now here's the problem...

After I completley installed the new harness I went to attach the positive battery cable and got a spark. A pretty strong battery-terminal-pitting spark.

I never went ahead and made the full connection.

There is no dome light on or anything that should be drawing current. No key in ignition. There shouldn't be any draw and therefore shouldn't be any sparking. Right?

If I unplug the new harness at the bulkhead there is no spark when attaching the + cable. Also, if I leave the harness plugged into the bulkhead but unplug it in the main junction in the middle, I get no spark.

The problem is somewhere in the 2nd half of the harness where I only made 1 splice, so technically there's shouldn't be anything for that to short on. Right?

Any ideas on what might be going on or ways to test?

Will a short effect an ohm reading? I got 4-5 ohm reading on everything I tested, which I think was every connection.

[MEK-Dangerfield]

You are correct in that the engine harness should not draw current without the engine running. I don't like the sound of the 4-5 ohm readings you are getting. I know this is a pain, but I would disconnect each wire, then reconnect one at a time. After connecting each wire, measure it's resistance to ground. Hopefully this will clue us in on the problem area. I assume you are taking these measurements with the key in the "off" position?

  Mike

[Grec]

I took those measurements with the harness off the car, simply putting on probe to each end of each wire... the theory being that a bad connection would read as increased resistence...

You're thinking I sould do this while it's all connected?

[cudabob496]

Was the battery negative cable attached before you attached the positive? And is the engine and battery properly grounded?

[MEK-Dangerfield]

I took those measurements with the harness off the car, simply putting on probe to each end of each wire... the theory being that a bad connection would read as increased resistence...

You're thinking I sould do this while it's all connected?

I totally mis-read you. 


  If the harness is off, there should be NO resistance reading between wires(end to end). My suggestion was to connect each wire with the harness plugged in to the bulkhead to the connector/lead(or wherever it goes), one at a time, and see which one makes a spark when you try to connect the battery.


  Mike

[Grec]

Yeah, (-) was connected and the engine is grounded.

I'll give that a try, Mike.

Thanks.

[cudabob496]

4 to 5 ohm does not seem like an unreasonalbe resistance in the wires. I think my MSD 8 mm wires are about 10 ohm per foot, though they are not the same as strait wire I think. However, a wire with 4 to 5 ohm will conduct a lot of current to ground (make a nice short)

[Bullitt-]

I would not be concerned so much with resistance in a single wire but rather lack of resistance between two or more wires... with the harness off go through every combination of wires to see if you get any readings,  there should be no wire to wire continuity that I know of......
GUYS does this not make sense?

[Grec]

I can give that a shot, bullitt. However, there will be continuity from the blue wire to multiple other wires as this gets run into a junction that goes out to a few places... voltage regulator, ballast, choke warmer and alternator if I remember correctly.

Ahh... the joys of electrical. :-)

[Grec]

OK, I went through every connection and it looked good.

I placed the (-) probe from the ohmmeter, one at a time, to each individual spade connector from the bulkhead connection and then touched the (+) probe to every other connection and found no continuity on any wire that shouldn't have it.

Good news, I modified my new harness correctly and there doesn't seem to be a short anywhere.

Bad news, I'm still getting that spark when I try to attach the (+) battery cable.

[Super Blue 72]

Ok, he's my   to confused matters worse.....  

I'm not that familiar with E-body wiring harnesses, I just have some electrical hunches....

My opinion is that the resistance is negligible.  You test shows no continuity to each other which is very helpful.

First thing that comes to mind is that a big spark like that is caused buy a large current draw, which means something is GROUNDED.  Possibly a unplugged connecter touching metal or though pulling ang pushing of wires possibly the harness insulation has been cut by the frame.  I used to see a lot of this when I worked at Acura.  It might only be a tiny cut in the insulation leading to a grounded circuit.

You can either try to (IF IT IS A GROUND situation.) put one probe of an ammeter to the positive on the battery then the other to the positive battery cable and see the amperage draw.  With a big spark you should see some sort of draw.  Someone else maybe can mention here what is acceptable for a static current draw.

With the harness plugged in and the big spark present at the positive terminal you can then disconnect/move wires around in side the car where the problem is localized to see if the amperage draw decreases.  If you move things around and the amperage decreases it's sort of "blinds man bluff" because you don't know exactly where the ground may be and if it disappears it may not show up again until probably you're driving a long distance with the wife and kids in the car and you start smelling smoke 200 miles from home.    If you see the amperage draw decrease, focus on that branch of the harness.

Another way to check for ground is that you could use a multimeter (with the wiring plugged in a spark present at the positive terminal) with a probe on a good ground and the other probe on individual harness terminals at the potential area of focus and check for restance to ground.  If you get an excessively low ohms reading, that may be the branch of the harness to focus on.

Again, this is only if the harness has a grounded circuit.

Hope this helps!

[moper]

My idea's a variant of Super's... You ned to know how bad that draw is. Spark means a completed circuit, as was said. So, connec tthe pos side of the ammeter to the pos post of the battery, and connect the neg side of the ammeter to the pos cable (not connected to battery). It will read something. There are only a few items that will have power potential with the key off. (Your key is off, right??) That's the fender relay (starter relay), the alternator main wire (not the fields), and the starter solenoid on the starter. Nowhere else gets a direct connection to the battery. So, I would carefully disconnec tthe main feed to the alternaotr, and see if the draw goes away. If it does, the alt has a short. (fairly common failure). Same with the fender relay, and then the starter. One of them should be the culprit. If you find the draw, the issue is either the wiring, or the part. The wiring you can check for continuity easily enough. I'm more inclined to think it's a part problem, than a wiring problem. It wouldnt be the first time, but it's not that common.

[Grec]

You know what... my starter relay is new, too. I just realized I hadn't run her since I installed.

I'll give those suggestions a try.

Thanks, guys.

[Grec]

Looks like my less-than-year-old alternator is the culprit.

Disconnected the alt's main lead and the draw seems to have gone away.

Any idea what could have caused it to fail so soon? I'd hate to buy another one just to have it get toasted, too.

[ksierens]

Not sure, but for newer cars you must always disconnect the negative terminal first.  In fact, I have seen tags on rebuilt units that say that if you don't, you will void the warrentee.