250 cid firewall wiring harness layout

[WB72]

I am trying to determine the heavy gauge red wire that goes to the engine. It isn’t the one that goes to the battery junction block. I have not seen a diagram for the 6 cylinder, there are many for a V8 though.

This is for the engine portion on the two piece harness, not the one that goes to the headlights.

Will post a picture later for clarity. I can’t seem to attach one via iPhone.

With the way the harness is routed, it could go to a solinoid on the mono bore carburetor or to a temperature sender located either on the head, or the thermostat housing. It seems like a large gauge for any of these options.

['68OrangeSunshine]

I don't think there's a heavy Red wire in the stock wiring bundle that comes over the hump from the bulkhead connectors. One Red line comes out of the other half going to the radiator wall, and feeds the EVR and Alternator, splits and another part goes all the way along the front gutter to the pass side fender terminal and joins the fusable link off the batt pos lead.

Could it be a 12 vdc feed for HEI that a PO laced into the harness? Especially large gauge. Hotrodders used to say '12 ga wasn't big enough, ya gotta use 10 ga.'

If you have the 1968 Chevy Truck Service Manual, the L6 schematics are on the next page from the V8 plans.
A factory line to the Temp Sender should be Green. Don't know about electric choke on a Monojet. Mine was mechanical/manual.
Year of truck, HEI or points? Pix help us see it too.

[WB72]

Is there a diagram showing the harness connector and what each pin connects to.

I would assume this would be standard and the only thing that would change is what’s pins are used depending on which options were in the truck.

I will add photos.

[WB72]

These are the pics.

10-12 gauge wire, faded red, darker than pink, looks like factory install into the bulkhead connector. in the same position as the Pink that goes to the Starter (changed to yellow by PO as seen in picture 3).

Three possible connection points determined by length.
Red = Sender in the thermostat housing
Yellow = Temperature sender in head, Green wire exists so I don't think it is that one.
Green = solenoid connected to the Carburetor.

Wire routing, behind engine, around the head past the coil, around the front of the engine and back to the drivers side.

also a better picture of the sender in the T-Housing.

['68OrangeSunshine]

OK. Yes it looks like OEM. It's not like my '68 at all.
I do not have these wires.
Guessing they are for Carb Choke, Idle Stop Solenoid and Water Temp?
I have a Manual Choke. No Idle Stop Solenoid for Model Year 1968. I now use a mechanical Water Temp gauge.
I do not know why there are electrical terminals coming out of your Thermostat Housing. Never seen that.
Also why is your Water Neck pointing to the Passenger Side? Mine points left.

Idle Stop Solenoids came in by MY 1969 [IIRC]. They kept the engine from ''dieselling'' [running-on after shut-off] caused by bad gas.

What year is your truck? Automatic?

[Jason Banks]

Wow that temperature sensor in the Tstat housing is wired.

I would almost bet someone was running an HEI with that. I don't see the braided resistance wire that runs to the starter and then onto the coil.

I'm not by my truck right now, but I'm pretty sure that position right next to the brown wire that goes to the brakes is the one where the resistance wire usually is.

[Jason Banks]

Here's the best diagram I know of:

http://67-72chevytrucks.com/vboard/s...&postcount=302

Of course it says V8

[Jason Banks]

Brown to brakes

Dark Green(below the brown) goes to temperature

Your Fat red one appears to be the one that goes to the starter and then on to the coil but I bet it was HEI and not run at all the the starter anymore. That's what you do when you convert. Are there 2 wires in there?

The next 2 slots are empty because the 2 green for the backup switch go to your column instead of a switch on the transmission.

I'm not sure what the brown with the white trace is.

Purple goes to the starter.

[Jason Banks]

You are not alone. Look at this thread.

http://67-72chevytrucks.com/vboard/s...d.php?t=329561

You don't mention what year your truck is, but judging by your picture under your name it looks like 71-72

['68OrangeSunshine]

PO's conversion to HEI was my first thought for the fat Red wire. But it should drop off at the distributor, not wrap around the valve cover.
And WB72 clearly is set up for points now.

[Jason Banks]

Look here:
http://www.mediafire.com/file/zyngg5...uck_Wiring.pdf

Page 39 of 50 in the PDF

12ga Pink Goes to Idle Stop Solenoid

20ga Pink Goes to the Starter and Coil. Must be the resistor wire.

20ga Brown With White goes to Transmission Controlled Spark Solenoid.

20ga Tan Goes to Brakes

20ga Dark Green goes to Temperature Sender

12ga Purple goes to Starter for cranking.

I retract my previous HEI guess.

['68OrangeSunshine]

Originally Posted by Jason Banks

Look here:
http://www.mediafire.com/file/zyngg5...uck_Wiring.pdf

Page 39 of 50 in the PDF

12ga Pink Goes to Idle Stop Solenoid

20ga Pink Goes to the Starter and Coil. Must be the resistor wire.

20ga Brown With White goes to Transmission Controlled Spark Solenoid.

20ga Tan Goes to Brakes

20ga Dark Green goes to Temperature Sender

12ga Purple goes to Starter for cranking.

I retract my previous HEI guess.

I was thinkin TCSS but the OP never came back with the year. Can't be HEI since we see a points set up.

[WB72]

I haven’t had time to open the manual to page 39 but will during a break.

Truck is a 72. 250 cid 3 speed on the tree.
There has been no signs of HEi, but could have been in and out again.

Thanks for the input so far guys.

['68OrangeSunshine]

Originally Posted by WB72

I haven’t had time to open the manual to page 39 but will during a break.

Truck is a 72. 250 cid 3 speed on the tree.
There has been no signs of HEi, but could have been in and out again.

Thanks for the input so far guys.

Poobably never had HEI. You might think about putting it on in the future. Great upgrade.

The TCSS is obsolete. Never did much good anyway.

Keeping the Idle Stop Solenoid is probably unnecessary, but don't throw out the stuff if you're going to run a Monojet. If it runs on after shutting the key off, you will want to put it back on. Also called an Anti Dieseling Solenoid.

If you're going to an aftermarket intake, 2- or 4-barrel carb and headers, then all this old wiring is obsolete.

[WB72]

For sure it is for the TCS thanks for the diagram, Jason.

By the looks of the diagram, the temp sender on the t-Housing is to work with the TCS. I will leave it in place and remove that wiring.

This truck is being built with this drive train due to sentimental reasons. I want to have the same style of truck that I shifted my first gears on. with that being said, I also have an LS in my shop to transplant if things don't work out as planned, so I think I will delete the TCS system. i am not too attached to that. I don't think I will set up HEI since I have alternate plans in the works anyway. I did that conversion in my other 72 and am pleased with it tremendously.

As for my water neck, I may have installed it incorrectly during the rebulid of the old 250. will be an easy swap if it doesnt line up with the radiator. I will keep an eye out for theat soon as to not delay the build when I get to that portion of it.



I have another question that doesn't really have anything to do with this post, can i buy resistor wire if i wanted to rework my stock harness?

here are a couple of pics to show you what I'm working on now.

['68OrangeSunshine]

Originally Posted by WB72

For sure it is for the TCS thanks for the diagram Jason.


I have another question that doesn't really have anything to do with this post, can i buy resistor wire if i wanted to rework my stock harness?

here are a couple of pics to show you what I'm working on now.

Possibly M&H fabricators could make up one for you. You may already have it in that bundle. It's a Pink 20, that splits into two Yellow 20s, one to Starter and one to Coil. For a lot less you could use a regular wire with a big ceramic Mallory resistor, if you want to keep points. Seen top of pic.
[I sometimes wish it was as easy to tune an HEI, as Points were with a Dwellmeter.] I forgot the p/n: "700, 701, 7000"?... possibly. I used to run it when I had a big square Mallory coil and points. Also tried Pertronix. Worked great for 7 years, then would black out at 3500 RPM completely shutting off the engine while at speed in gear. I went back to points for a year, until I got an HEI.
[M & H:]
www.wiringharness.com

[Jason Banks]

I find it interesting that M&H and American Autowire only offer 2 different harnesses for the 70-72 6cyl engines. They have 20850 for Manual Trans and 23435 for Auto Trans.

It would be interesting to hear if they include the wire for the idle stop solenoid.

This site says to just remove the idle stop solenoid and replace it with a bolt:
http://www.carburetor-parts.com/Mono...nt_ep_353.html

These new harness are only $119 but if they include the idle stop solenoid wire you might end up with an extra wire if your solenoid doesn't work and you can't come up with one.

I wasn't able to find anyone selling just the resistor wire, but I bet if M&H is making new harnesses they have the wire they could sell you.

[davepl]

M&H will customize the harness to your spec (at least within reason). So you can get with or without HEI or with other wires added/removed/modified. I ordered mine with a power lead to the distributor ala HEI and no resistor wire because I'm running Pertronix. But it looks nice and stock, no splices.