Constant Hot After Harness Install

[YubaDirt]

Vehicle:
1974 Chevy K10 350/350 with AC

Questions:
1)Is the junction block and/or alternator supposed to have constant power while the key is off and out of the ignition? If not, what did I do wrong and how do I correct this problem?

2)What is the most professional source/method to draw power for the HEI?

3)Confirm the temp sending unit is a green wire? Is it a problem being a naked wire (no additional heat insulation) when used with headers?

Background:
I am completing the installation of a new engine; to which, the only external modifications are an Edelbrock Performer intake manifold, an Edelbrock #1406 Carb, headers and an MSD Pro Billet HEI distributer (the vehicle was originally equipped with points ignition).

I replaced the factory engine wiring harness because it was very rough. I ordered the harness from Brother's Truck. The harness was made by American Autowire. It came with a receipt only; no diagram or instructions. Brother's Truck refused help, claiming they were not techs. American Autowire refused to send any instructions or diagram because of liability and could not further assist without viewing the vehicle.

Between comparison to the factory harness and information from this website, I was able to complete an installation I am semi-confident with. However, using a test light, I discovered that the white wire to the alternator plug and all wires at the junction box are constantly hot.

What I did:

A) Both wire connections that exit the loom near the junction block were connected there.

B) The yellow colored coil wire that exits near the distributor was taped back against the loom to allow for later use since I will not use it with the HEI.

C) The red wire with the rubber hood was connected to the back of the alternator.

D) The white and brown wires were plugged in the side of the alternator.

E) Two fusible link wires and the positive battery cable were connected to the positive post on the starter solenoid.

F) The Purple wire was connected to the start post at the 3 o'clock position of the solenoid.

G) The yellow wire at the starter was also taped back against the loom to allow for later use since it will not be used in the absence of points ignition.

H) I connected the green wire to the temp sensor in the passenger side cylinder head.

I) I have left a wire unconnected that I assumed was intended for the idle stop solenoid. This wire also looks to be heat insulated.

Thanks for the help I've gleaned from the website so far and in advance for your advice here.

 

[chengny]

1. Is the junction block and/or alternator supposed to have constant power while the key is off and out of the ignition? If not, what did I do wrong and how do I correct this problem?


Yes, they are both supplied on unswitched leads directly from the battery positive (via the "B" terminal on the starter solenoid/fusible links). You did nothing wrong and so there's nothing to correct.

2. What is the most professional source/method to draw power for the HEI?

I suppose you mean, where is the best place to supply power to the coil - rather than draw power from it.

As you probably know, as original the positive post on the coil was supplied by two leads. One came from the "R" terminal on the solenoid (yellow) and the other came from the ignition switch (purple/black).

If you have changed the distributor over to an HEI, you no longer need the yellow power supply (it was used to provide additional power during periods of engine cranking - the HEI distributor doesn't need it). If you can locate it, I would just use the original ignition key switched power supply (the purple and black lead). It should run closely along (or in the same group) as the oil pressure sensing lead and the coolant temp sensing lead.


3. Confirm the temp sending unit is a green wire? Is it a problem being a naked wire (no additional heat insulation) when used with headers?

Yep, I am showing the coolant sensing lead (whether 6/8 cylinder or a switch/sender) as being dark green.

No problem with just having the normal factory insulation. The temperature of the wire used to connect the sensor to the gauge (i.e. the sensing leg), has no effect on the resistance - and consequently will not skew your dash indication.

Just be sure and use good electrical practices when routing this wire. To prevent damage from excessive heat, it would be prudent to provide extra insulation (increased clearance is better) in the areas where it passes close to the header piping.

 

[chengny]

However, using a test light, I discovered that the white wire to the alternator plug and all wires at the junction box are constantly hot.

Again, that is correct and according to design. Also, in addition to the white wire (that connects to the alternator) always being hot - so too, should the red one that goes to the alternator. These both should see battery voltage at all times.

It goes like this:

The big main cable - that is connected to the battery positive - is the root power supply for the entire electrical system. At the "B" terminal on the solenoid, two lines branch off and feed the ignition switch (via the firewall connector) and the junction block/generator. On the outlet side of the JB, the circuit splits up even further. So you soon have unswitched battery power running all over the place. I cropped out the salient section of the 1974 wiring diagram and tried to color code the various circuits. I am color blind so "don't judge me" if this key is wrong:

Red = unswitched battery power

Yellow = decommissioned auxiliary coil feed wire from solenoid "R" terminal

Purple = switched coil power supply (from ignition switch)

Green = sensing leg for engine coolant temperature indication/warning

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[chengny]

Going down your check list:

What I did:

A) Both wire connections that exit the loom near the junction block were connected there. Sounds right - if they are big red wires with ring terminals, that's where they belong. The white excitation wire to the alternator should split off from one of the reds close to the JB (via a fuse holder). The other red should have 3 branch leads coming off of it - after the fusible link.

B) The yellow colored coil wire that exits near the distributor was taped back against the loom to allow for later use since I will not use it with the HEI. As discussed - that's correct.

C) The red wire with the rubber hood was connected to the back of the alternator. Yep, that's right.

D) The white and brown wires were plugged in the side of the alternator. Again, they are correctly connected. The white is for excitation of the alternator field and the brown is for a voltmeter (or low voltage warning light as the case may be).

E) Two fusible link wires and the positive battery cable were connected to the positive post on the starter solenoid. That's right.

F) The Purple wire was connected to the start post at the 3 o'clock position of the solenoid. This wire was not discussed above but you have it connected correctly. That is the power from the start contacts in the ignition switch and - after passing through the neutral start switch (or clutch safety switch if equipped with a manual transmission) - it is supposed to go to the "S" terminal on the solenoid.

G) The yellow wire at the starter was also taped back against the loom to allow for later use since it will not be used in the absence of points ignition. Check.

H) I connected the green wire to the temp sensor in the passenger side cylinder head. Usually the sensor is mounted in the threaded boss on the LH cylinder head and the green wire runs down the LH valve cover. A more representative indication of engine coolant temperature is obtained when the sensor is mounted at the front of the LH cylinder head - rather than near the back as is the case with the RH head. This is due to the fact that the coolant enters the heads at the rear and then flows forward as it moves toward the water outlet (thermostat housing).

As it flows through the passages in the heads it carries away the heat of combustion and it's temperature increases as a function of time spent in the cylinder head. If you position the sensor in a spot that is located immediately after the coolant has just entered the back of the head...you will get an artificially low indication of it's actual temperature. This is only as compared to if the sensor is located near the thermostat - where it exits the head. IOW, your dash gauge will be reading about 20 degrees lower than if it were in the normal front spot. No big deal - as long as you are aware of this.

I) I have left a wire unconnected that I assumed was intended for the idle stop solenoid. This wire also looks to be heat insulated.

There are only two wires to the engine proper that we haven't discussed. One is the sensing leg for oil pressure (dark blue ) and the other is a (tan) wire for and connects to an idle control device.

 

[YubaDirt]

Chengny,

Thank you for the comprehensive response. You've answered exactly what I needed and then some. I very much appreciate the time you put into answering me. I have run across your past posts during previous research for other issues and they have also helped immensely.

The temp sensor location I indicated above was a typo. I meant to write "driver's side". So now I've got some piece of mind moving forward to the next portion of this project. I'm going to dive in to the ignition switch tomorrow to address the last (that I know of) of the wiring issues.

Thanks!

 

[chengny]

Good luck. I'm going to be AFK for a while but you've got this covered.