[jhein]

As part of my project, I decided to get a new engine harness. Nothing wrong with the old one functionally, just wanted a new harness for appearance sake.

Yesterday after making carb adjustments I noticed a stumble while test driving it. Felt like maybe fuel starvation. I thought maybe it was a poorly vented gas tank or other fuel system problem. No rush of air when I removed the fuel cap and the stumble continued. Couldn't find any vacuum leaks but the carb wasn't very tight so I snugged that up thinking that may be it. Before I tightened up the carb the idle had become very erratic. After that it seemed back to normal. I went to back out of the shop for a test drive but as soon as I tried to back out it stalled. Then it stalled again. Then I saw smoke from the engine compartment. I popped the hood and at first it looked like it was coming from the shaker, so I thought maybe fuel vapor? Then I looked closer and could see it was coming from the engine harness at the firewall. The tape had melted off in two branches of the harness about 5" long sections each. I immediately disconnected the battery. Fortunately nothing caught fire. Also very fortunate that I was here at home.

So it seems like it's either a defective harness or bad work. The harness was just a simple swap. The only mod was for the HEI. I think I recall it says you need a 12 gauge wire for that. I guess if they used wrong gauge wire that could do it. But the guy who actually did the install is the exact same guy who modded my other harness for HEI so he supposedly knows the job. The harness is from American Autowire. Anyway, what a mess and disappointment.

[Formulas]

packard 56 terminal connectors at the firewall, work with factory correct diagram, and rebuild it

[Schurkey]

...looking for the actual cause of the trouble that started this mess.

Undersized wire?

Poorly-crimped connections?

Incorrect routing of wires within the harness? For example, did they connect a blower-fan resistor wire to the electric choke, and a choke wire to the coil, and the coil wire to the blower fan resistor?

[jhein]

Originally Posted by Schurkey

...looking for the actual cause of the trouble that started this mess.

Undersized wire?

Poorly-crimped connections?

Incorrect routing of wires within the harness? For example, did they connect a blower-fan resistor wire to the electric choke, and a choke wire to the coil, and the coil wire to the blower fan resistor?

That's the thing. What happened? I talked to my guy and we'll have an autopsy. I just hope the problem/damage is isolated to the engine harness and not any other electrics. The good news is that I don't need an fuel system upgrades (yet). LOL.

[25stevem]

It would seem from your description that the near full on fire condition that you had was related to the powering of the ignition system.

If it’s not due to unintended different’s in the gauge ( more on this later in my reply) then the only other logical reason would be due to connector conditions, especially crimp on types.

Connectors must be crimped properly.
The wire needs to be stick out passed where the metal portion ends that gets crimped down.

Poor crimp connections will get hot when they can not pass the amount of current they are being called upon to handle.
Flat type spade connectors of each gender need to engage each other very firm, so the Female end when slipped on needs to be tight enough to be a bit of a pain to get on.

Any connector(s) in a harness that needs to go to ground must have clean bare steel to bolt to.

A very upsetting issue that I have come across since the pandemic is off shore made wire that is advertised as let’s say for example 16 gauge, but when I counted the strands on the inside was only 18 gauge.

Your new engine compartment harness could have unknowingly be made with such.

If your calling on a given gauge wire to carry 80% or more of its rated current then this reduction in gauge is a big issue!

On a side note let me ask this, do you have both banks of the motor grounded well?

If you have electronics grounded to the firewall then you need to be reading very low resistance from the firewall back to the ground terminal of the battery, and all too many times this is not the case.

[jhein]

I was so disgusted yesterday I had to just walk away from it. This morning I went out and checked it out. All the connections and grounds were good except the ground on the back of the alternator was loose. I could wiggle it with my fingers. The power wire to the alternator looks good. As for as the melting it seems like it was isolated to the electric tape and doesn't look like the wires themselves melted, from what I can see.

So here's what I think happened. It left the shop with the alternator ground tight enough to work but eventually loosened up until it became symptomatic.

I haven't reconnected the battery but, I'm thinking that it may be good enough for me to try and see if it runs and if so it may be good enough to get it back into the shop and have the harness replaced. What do you think?

[NeighborsComplaint]

I would hate to be in your shoes on this. Wire getting hot enough to melt even the tape is in indicative of high resistance, a short to ground, or a device drawing more amps than the wire was intended to carry. In any case, it causes the wire to carry more amperage than it was intended to. I can't see how a faulty ground connection would cause that. You would just have an open circuit and no current flow.

[West Coast GTO]

Wire size in relation to amp draw is specific. So, if the wire size is small and the amp load is large, then the wire heats until it melts OR the fuse saves the circuit. This is how incandescent lights work.
I would be more concerned about the fusing protecting the wire and, if a fuse is provided for that circuit, why it did not blow.
One thing I am not aware of is do new wiring harnesses have or need fusible links.

[jhein]

Originally Posted by NeighborsComplaint

I would hate to be in your shoes on this. Wire getting hot enough to melt even the tape is in indicative of high resistance, a short to ground, or a device drawing more amps than the wire was intended to carry. In any case, it causes the wire to carry more amperage than it was intended to. I can't see how a faulty ground connection would cause that. You would just have an open circuit and no current flow.

Originally Posted by West Coast GTO

Wire size in relation to amp draw is specific. So, if the wire size is small and the amp load is large, then the wire heats until it melts OR the fuse saves the circuit. This is how incandescent lights work.
I would be more concerned about the fusing protecting the wire and, if a fuse is provided for that circuit, why it did not blow.
One thing I am not aware of is do new wiring harnesses have or need fusible links.

All very good points. Here's one aspect I'm struggling to understand. Why was it working fine for quite a while and once it started acting up it progressed really fast? The harness is from American Autowire and was brand new. If it had small gauge wires that would overheat, why wouldn't it have happened from the start? OTOH, if it was due to inadequate gauge wiring, then the wires would overheat but the total amperage carried would still not be excessive and blow a fuse, is that thinking correct?

["QUICK-SILVER"]

That white looking wire with spots "looks like" resistance wire for points.

Resistance wire can get hot enuff to melt tape without burning/melting its insulation.

Where does that wire go? Like HEI BAT terminal.

Does it have a splice in it behind the wiper/washer hoses?

If spliced, does the other wire go to the starter solenoid "R" terminal?

Clay

[jhein]

Originally Posted by jhein

...The power wire to the alternator looks good...

Correction, that should say power wire to the distributor, not alternator.

[jhein]

Originally Posted by "QUICK-SILVER"

That white looking wire with spots "looks like" resistance wire for points.

Resistance wire can get hot enuff to melt tape without burning/melting its insulation.

Where does that wire go? Like HEI BAT terminal.

Does it have a splice in it behind the wiper/washer hoses?

If spliced, does the other wire go to the starter solenoid "R" terminal?

Clay

Yeah, that wire caught my eye. I don't know if that's the power wire that goes to the HEI, but it could be. I'll have to check.

And if it is that may be the problem. The DUI dist instructions say to wire it directly from the ignition switch with a 12 gauge wire and it specifically says to eliminate any "resistance wire". So maybe we're onto something here.

[jhein]

From DUI:

[78w72]

i ordered a engine harness for a 72 firebird from american auto wire, even after specifically saying im using a HEI they sent the harness with a smaller ign wire used in 72. luckily me & my dad noticed it before installing, called AAW & they paid to ship it back & fixed the ign wire.

take a closer look at what wire got hot when you do the autopsy, if it melted the tape chances are it melted the wire covering or will show signs of overheating. the hei wire should be a big pink'ish color 12awg. maybe your guy just assumed the new harness was correct for the HEI vs when he modded the other one?

is the separate ground wire to the alt a 1970 thing? all other mid to late 70's 2nd gens ive seen dont have a separate ground to the alt, just the 2 wire connector & the short wire to the battery for charging.

[jhein]

Originally Posted by 78w72

i ordered a engine harness for a 72 firebird from american auto wire, even after specifically saying im using a HEI they sent the harness with a smaller ign wire used in 72. luckily me & my dad noticed it before installing, called AAW & they paid to ship it back & fixed the ign wire.

take a closer look at what wire got hot when you do the autopsy, if it melted the tape chances are it melted the wire covering or will show signs of overheating. the hei wire should be a big pink'ish color 12awg. maybe your guy just assumed the new harness was correct for the HEI vs when he modded the other one?

is the separate ground wire to the alt a 1970 thing? all other mid to late 70's 2nd gens ive seen dont have a separate ground to the alt, just the 2 wire connector & the short wire to the battery for charging.

When I ordered my harness from AAW, they said if I got the HEI modded harness I would also need to replace my dash harness. So I just went with the original style harness thinking my guy would mod it just like the last one. I don't recall seeing that resistance wire in the old harness so I'll have to check that too.

But, now I think that may be the issue. They used the existing resistance wire in the harness rather than running a new wire straight from the ignition switch.

[78w72]

Originally Posted by jhein

When I ordered my harness from AAW, they said if I got the HEI modded harness I would also need to replace my dash harness. So I just went with the original style harness thinking my guy would mod it just like the last one. I don't recall seeing that resistance wire in the old harness so I'll have to check that too.

But, now I think that may be the issue. They used the existing resistance wire in the harness rather than running a new wire straight from the ignition switch.

strange, they never mentioned changing the dash harness when i bought mine... & im not sure why that would be needed as there is no connection from the 12v ign/coil wire to the internal dash wiring. the tach wire is completely separate from the dash or engine bay harness.

hope you get it figured out & back up & running.

[jhein]

OK so they did use the resistance wire in the AAW harness for power to the distributor. Picture below. I don't know what the yellow wire is for but it appears to be part of the AAW harness.

So where's the best place to tap into the ignition source?

[Scott65]

The best overall, long term solution is to replace the resistance wire at the bulkhead.

[78w72]

if you artent in a hurry id contact AAW to see if they will replace that wire like it should have been. if not, im pretty sure you can replace it yourself by pulling the pin from the block & soldering on a new 12 awg wire. then it still taps into the original fuse block source for power.

[jhein]

Originally Posted by 78w72

if you artent in a hurry id contact AAW to see if they will replace that wire like it should have been. if not, im pretty sure you can replace it yourself by pulling the pin from the block & soldering on a new 12 awg wire. then it still taps into the original fuse block source for power.

So that's another point. I tried to order the harness from the AAW website with the HEI mod. The site wouldn't let me. So I called and like I said, the guy told me that's because if you get the HEI mod, you have to get a new dash harness too. So then I said OK, I'll just have it modded.

But here's the thing, it was a made-to-order harness. That being the case, why didn't they just make it with the proper wire? Makes no sense. Now I'm going through all this mess for no flippin good reason.

So now I'm pissed at AAW and I'm pissed at my guy too because he should have known this too. What a totally avoidably mess.