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Old 12-29-2017, 06:21 PM   #1
Shtudwn
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66 C10 Engine Wire Harness w/ Gauges and HEI

Hello,

I'm in a quandary that maybe someone could provide some advice.

I have a 66SWB that was originally equipped with idiot light dash. I have since replaced that with a stock dash that has gauges. The truck now has a 350V8 and an HEI distributor.

I have been running on a hacked up engine harness and separate wires for the HEI and aftermarket Temp, Voltage, and Oil Pressure gauges but want to utilize my newly install dash with gauges.

I have purchased a Engine Wire Harness w/ Gauges and HEI for about $80 but am leery about opening the package and installing fearing that it will not get me what I need.

Question: Since my truck came with lights and not gauges, will this engine harness even work or will I need a different interior wire harness? If so, that might not be the direction i want to go because ultimately I want to go with an Autowire harness and completely redo the whole truck.

Ideas? Thoughts?

Thank you.

Dan
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Old 12-29-2017, 07:22 PM   #2
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

To make the stock gauge cluster fully functional in a truck that was originally equipped with a warning light cluster, you would ideally swap 4 of the wiring harnesses. Those being:

(1) The engine harness (that you purchased)
(2) The alternator / headlight harness
(3) The cab / under-dash harness
(4) The instrument cluster harness (you might already have this if it came with the cluster)

So that leaves you with the "alternator / headlight" and "cab / under-dash" harnesses that won't be compatible with the gauge cluster. American Autowire does make reproductions of these harnesses in their "Factory Fit" line of wiring that will work with the gauge cluster. They would be a direct plug-in swap and work with the gauge-style engine harness you have already purchased.

Note that the existing "alternator / headlight" and "cab / under-dash" harnesses can be modified to work with the gauge cluster. So that might be something you would want to consider (even temporarily) to get everything working.

American Autowire also makes a "Classic Update" line of wiring that is a complete kit. If you eventually plan on re-wiring the truck with one of these kits, you wouldn't need the separate engine harness. However, I don't think the Classic Update harnesses support the battery gauge (ammeter) in the gauge cluster ... you have to either leave it unused or swap it out for a voltmeter.
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Old 12-30-2017, 01:53 PM   #3
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Thank you. Great reply!

For reference here is your excellent list:

(1) The engine harness (that you purchased)
(2) The alternator / headlight harness
(3) The cab / under-dash harness

I wasn’t even aware that the Forward Lamp Harness (2) might be different but I now see, from your suggestion and my catalog, that it must be. Those are spendy by themselves at $95. Add to that the Engine Harness (1) at $80 and then there is the Dash Harness (3) which is $280. That’s $455! (if my quick math is good)

Let me jump to the question…..

Can someone give me a quick wire diagram for the Engine Harness w/ gauges and HEI? If so I will create something to get me by and when it comes time to rewire the whole truck I will make that decision at that time.

Thank you.
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Old 12-30-2017, 07:31 PM   #4
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

You're welcome.

The difference in the alternator / headlight harness is fairly minor. Basically, the brown wire that comes from the #4 terminal on the voltage regulator gets moved over to what's otherwise an empty cavity on the bulkhead connector. Then a black wire with a white stripe gets added for one leg of the battery gauge (ammeter) circuit. It goes into the bulkhead connector cavity where the brown wire was and runs to the buss bar on the horn relay.

There isn't a whole lot of difference between the warning light vs. gauge style engine harnesses either. The version for gauges doesn't have a dark blue wire for the "oil" light since the oil pressure gauge is mechanical. And the gauge version eliminates the light green wire that used to go to the "cold" light. A black wire takes the place of that light green wire and forms the other leg of the battery gauge circuit. It connects to the large "batt" stud on the starter solenoid. Finally, the dark green wire that used to go to the "hot" light gets re-used for the temp gauge sender and has a different connector to plug onto the gauge style sending unit.

The gauge version of the cab / under-dash harness has a couple of wires re-located on the driver side bulkhead connector to work with the slightly different layout of the gauge style alternator / headlight harness. And has an added black wire with white stripe on that side to complete the connection back to the battery gauge. Over on the passenger side bulkhead connector (that plugs into the engine harness) it lacks the dark blue wire for the oil light. And it has a black wire (instead of light green) for the battery gauge (instead of the "cold" light). Finally, it doesn't have a connection between the dark green wire and ignition switch that was used in the warning light style harness to perform a "bulb check" on the "hot" light.


The change to HEI involves locating the special resistance wire in the engine harness, which typically has a cloth type insulation and runs down to the starter solenoid "R" terminal where it joins up with a yellow wire that runs up to the coil. Once located, you can remove that resistance wire (along with the yellow wire and connection to the R terminal) and replace it with a section of 12ga copper stranded automotive primary wire. You'll need a couple of Packard/Delphi 56 series terminals to crimp onto the wire (a male one for the bulkhead connector end and a female one for the HEI distributor end). And snap a Packard/Delphi #8917644 connector shell on over the female terminal (this plastic connector shell has a latch that will hold it firmly in place on the HEI distributor cap). No changes to the cab / under-dash harness are necessary for HEI since the rest of the run back to the ignition switch already uses regular copper wire.


As for wiring diagrams, I do have a few listed on my '66 Chevy truck gauge conversion page at: http://rmcavoy.freeshell.org/Truck_gauges.html They're about 3/4 of the way down the page under the heading "Factory Wiring Diagrams". And at the very bottom of the page, I have some links to some simplified diagrams I drew focusing on the differences between the warning light vs. gauge clusters. Those last diagrams are on my photobucket album and they're links, not 3rd party hosted, so they should still be accessible.
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Old 12-31-2017, 02:50 PM   #5
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Wow! Perfect!

When I removed my old Engine Harness there was only one wire connected to the starter. I believe it was the Purple Wire to the "s" terminal.

I really appreciate you "conversion" page. Just what I needed. Now I need to get a good source for wire and connectors. Any suggestions?
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Old 12-31-2017, 05:28 PM   #6
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

You're welcome again!

The old warning light style engine harness should have had two connections to the starter: (1) The purple start wire to the "S" terminal and (2) the ignition resistance wire & yellow wire to the coil that join up at the "R" terminal. So it sounds like someone might have already done some re-wiring of the ignition circuit. But that shouldn't be an issue since those wires get removed when you modify the harness for HEI anyway.

There are quite a few places to buy automotive wire and the Packard/Delphi 56 series terminals and connectors. Some sources include: www.terminalsandconnectors.com, www.waytekwire.com, www.wirebarn.com, and www.clipsandfasteners.com Electronics parts suppliers such as www.digikey.com and www.mouser.com carry the terminals as well. For in the engine compartment, I would recommend using wire with a high temp rated crosslinked polyurethane insulation such as type SXL, GXL, or TXL.
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Old 01-03-2018, 04:19 PM   #7
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Dude, this is great!

What size, 12, 14, 16, 18, Packard/Delphi 56 connectors?
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Old 01-03-2018, 05:51 PM   #8
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

You'll want to get the 56-series terminals that match the size of the wire you'll be using. 12ga is what's most commonly used for the circuit powering the HEI distributor. If you are planning on adding in the wiring for the factory battery gauge, that uses smaller 18 or 20ga wire (and, as shown on my website, I recommend adding some 4 amp inline fuses to both legs of this circuit to make it safer than the factory un-fused configuration). Note that there are some 56-series terminals designed to accept 2 wires together in the crimp cavity (they're usually listed with the word "double" along with the wire gauge) but I don't think you'll need any of those for this project.
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Old 01-13-2018, 07:46 PM   #9
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Hi Ray,

I think have everything correct as I used your description and page, as well as the Engine Harness I returned.

That being said, the harness I returned had a yellow wire from the 'R' Terminal on the starter up to the HEI. At the HEI connector it was doubled with a pink wire. this wire ran back to the bulkhead. I recreated that exactly and now I can only get 10.2 volts to the HEI when I crank the engine.

Any ideas why I am not getting 12 volts? Don't I need 12v?

Thank you.
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Old 01-13-2018, 08:22 PM   #10
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

I have seen a few replacement harnesses configured for HEI set up like the one you returned. Having the yellow wire running from the starter solenoid "R" terminal joined up to the pink wire that feeds power to the HEI is kind of redundant in most cases. But I think there are some applications where the original ignition switches only powered the coil in the "run" position and relied on that connection from the starter solenoid "R" terminal to power the coil while cranking. So the companies making the wiring harnesses probably figure it's easier just to put that yellow wire in there to make sure the HEI will have power regardless of the type of ignition switch.

Have you tried measuring the voltage right at the battery while cranking the engine? Starters draw a lot of current so it's perfectly normal for the battery voltage to drop down to around 10V or so while cranking the engine. That should still be plenty of voltage to operate the HEI though.

But just to make sure you don't have any issues with the ignition switch, wiring, etc, you might want to check the voltage drop between the positive battery terminal and the HEI power feed terminal. You shouldn't be seeing more than 0.1V (100mV) drop if all the wiring, connections, etc. are okay.
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Old 01-13-2018, 08:36 PM   #11
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Thank you Ray.

Could there be an issue with having that redundant yellow wire hooked to the starter? Are most of the HEI connections just out of the bulkhead at the firewall and don't go down to the starter?

So the yellow wire down to the starter is a connection for a standard coil system while the pink wire is for the HEI
Could the connection to the starter be causing an extra draw? Should I just disconnect the yellow wire?

Thank you for all your help.
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Old 01-13-2018, 08:46 PM   #12
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

You're welcome!

No, having the redundant connection to the starter solenoid "R" terminal won't hurt anything or cause any extra current draw. It's basically just providing 2 paths for battery voltage to reach the HEI while cranking the engine.

As long as the ignition switch is powering the HEI through the pink wire in both the "run" and "start" positions, having the yellow wire from the starter solenoid "R" terminal really isn't necessary. But having it there won't hurt anything either.
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Old 01-13-2018, 09:00 PM   #13
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Interesting.

I tried to start again and it will crank, sound like it will run but only while cranking.

I connected the HEI directly to the battery and WHAM! runs perfect!

Let me ask.... should the pink wire on the dash harness provide 12volts from the ignition, no matter whether the dash harness is one with lights or gauges? Even if it wasnt, shouldn't the yellow wire make up for any issues since it is run from the 'R' terminal on the starter to the HEI?

Dan
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Old 01-13-2018, 09:22 PM   #14
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Yes, the pink wire in the dash harness should be providing battery voltage to the ignition in both the "start" and "run" positions. And the connection / routing of that pink wire should be the same for both warning light & gauge style harnesses.

The yellow wire down to the solenoid "R" terminal will only provide power to the ignition when the solenoid is engaged to crank the engine. It switches back off once you stop cranking the engine.

Since your engine only wants to run while the starter is engaged (or with a jumper wire directly from the battery to HEI), it sounds like there is an issue with the pink wire not providing power to the ignition. That could be due to a bad connection or defective contact inside the ignition switch.

Try connecting a test light or voltmeter to the pink wire at the back of the ignition switch. It should have battery voltage when the key is turned to the "on / run" position. If it does, try testing again on the pink wire that goes into the firewall bulkhead connector. Then again on the engine side of that connector. Working your way along the circuit, testing for voltage at the various points should allow you to pinpoint the trouble spot.
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Old 01-13-2018, 10:54 PM   #15
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

sJust to illustrate what Ray is talking about, here is a couple of diagrams showing the wiring on the 66.

The cab wiring with the key switch and the pink wire.


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The engine side with the connector at the top right edge.

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I noticed that the purple solenoid wire is only 16 gauge on this diagram. Is that correct?
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Old 01-13-2018, 11:11 PM   #16
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Thanks everyone.

Guess who, when building the new engine harness, put the HEI wire on the wrong end of the connector? Yep! Seems to make a difference! Lol
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Old 01-14-2018, 04:07 PM   #17
Shtudwn
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Just when I thought all was good. I have a drain... or so I think since my battery was drained this AM. I will charge and check.
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Old 01-14-2018, 07:09 PM   #18
ray_mcavoy
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Glad to hear you got the ignition wiring figured out

Did you make any changes to the alternator harness and cab harness to hook up the ammeter in the gauge cluster? If you have not yet made any changes to that part of the wiring, but did add the black wire for the ammeter to the engine harness, I believe that would create a situation that would result in a battery drain. With that added black wire, you will now have constant power going to one leg of the ammeter (as it should). But if you haven't changed the cab/alternator wiring yet, the other leg of the ammeter would end up connected to the #4 terminal on the voltage regulator. And that terminal should only be powered when the ignition is on. But with the other side of the ammeter hooked up, it would feed power through the meter and keep the regulator/alternator energized all the time, draining the battery. If this is the case, simply leaving that added black wire disconnected from the starter solenoid (and taped up so it won't short against anything) should eliminate the battery drain until you can get around to re-configuring the rest of the wiring to work with the ammeter.
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Old 01-18-2018, 09:39 PM   #19
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Perfect! Drain solved.

Could I have burned out my voltage regulator? I don't have headlights now. Dash and sone lights are fine.
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Old 01-18-2018, 10:32 PM   #20
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Re: 66 C10 Engine Wire Harness w/ Gauges and HEI

Nice! That's one less problem to deal with

Having power continuously applied to the regulator's #4 terminal would keep the regulator coil energized all the time and potentially burn it out. You can measure the voltage while the engine is running to see if the regulator is still functioning okay. It should be somewhere in the 13.5 to 14.5 V range (give or take a little) if the alternator & regulator are working okay.

As for the headlights not working, that wouldn't be directly related to a potential problem with the voltage regulator. To check the headlight circuit, you can use a voltmeter or test light to check for power at various points along the circuit. For example, check to see if the red feed wire has power going into the headlight switch. Then check the light blue wire that runs from the headlight switch down to the floor-mounted high/low beam switch to make sure that has power with the headlight switch on. And move on to checking various connection points along the tan and light green (low & high beam) wires as they run from the high/low beam switch out to the headlights. Also, check to make sure both headlights have good ground connections to the grille / front sheetmetal and that those parts have a good ground connection to the rest of the truck.
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