I want to replace the under dash wiring harness, several wires have areas where the wire casing is bunched up and wavy ( I'm thinking this is from the wires getting too hot ?) . My 75 Dart has a factory 65 amp alt with the alt wire going directly to the amp meter (after market) .
My first question is can I use a 18 gage wire in place of a 20 gage ?
Second question is where might I find the proper terminal ends for the bulkhead and other connectors ? I found some Dorman/Motormite numbers but they only go to 14 g wire I need some that go to 12 & 10 g wire.

Thanks, jeff

You are not going to find common wire terminal connectors for 18-20 gauge wire. 18-20 gauge wire is about the size of telephone wire and I would avoid using it in your car for a number of reasons. Stick with the 10 to 14 gauge. If you need to replace your bulkhead connectors you can get them from any junk car of a similar year and splice them into your existing harness. Or just replace the whole harness altogether.

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You can always increase wire size (larger diameter conductor, lower gauge number), and in many cases it's a very good idea. You must never decrease wire size (smaller conductor, higher gauge number).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Not quite true, there are lots and lots and lots of different kinds of terminals available for these smaller wire sizes. Overall, you'll find a limited number of types and configurations for most all wire sizes at normal consumer-level sources of supply (hardware stores, auto parts stores, etc). A lot of the more esoteric stuff can only be found from e.g. Terminal Supply—and usually at that point, you're looking at having to buy large quantities.
Certainly high-current lines (such as the charging system power circuit) need to be nice and thick, 10ga is preferable there. But there are a lot of places where 16ga is appropriate, so it should definitely be on your list. Remember, sometimes 14ga is going to be too bulky/large for the application in mind. There is actually a fair amount of 18ga wire present in many of our cars (depending on year) and for low-demand circuits such as the dashboard lights, glovebox light, etc., it's fine.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for the replies,

At this point I'm not sure if I'm going to replace just the bad wires or make a copy of the old harness by replacing the existing wire with new wire and terminal ends (keeping the factory connectors). I will probably make a custom fuse block, keeping all the circuits the same as the factory ones.

Thoughts ??

Thanks jeff

Here's what I'm doing. I don't know that it's the best way, or even a good way for that matter but I'm committed to it at least until I know whether or not it's going to work. My headlight harness is in excellent shape so I will probably use it, with appropriate modifications to support relays. The under dash and body harness are in good shape so I will use them although I'm inclined to put new terminals and connector housings where I can. The bulkhead connector is OK and should work fine with some clean up and new terminals. That leaves the engine harness. I contacted Evans and they will make a new harness with the upgrades to elctronic ignition and electronic voltage regulator for less than I would spend buying wiring and terminals. The cost is $130.00 - $135.00; I'm inclined to go that route. The high current circuits in the car are already 10 or 12 gauge and I see no benefit in going larger than that. If the terminals are clean and protected from moisture and corrosion that should be adequate. I did order a Bussman 24 circuit fuse block with 2 feeds ( switched and unswitched ) which I will use in place of the old corroded stock fuse block. My only point of indecision is what to do about the ammeter. I will definitely add a voltmeter and oil pressure gauge and I will leave the ammeter in the circuit but I think I will bypass it with either a straight run to the starter solenoid and back to the battery or I'll try the active shunt. I've seen too many meltdowns under the Mopar dash and with antipated demands due to additional accessories; I'd rather be safe than sorry.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Yeah, this is appropirate. The original headlamp wiring was marginally OK when it was new for feeding the original low-wattage lamps, but add in four decades' age and higher-wattage bulbs, and yup, you'll want to use the original headlamp harness to trigger the relays, and run new heavy-gauge wire for the headlamps' power and ground. See here. Don't do it the half-baked way that MADelectric chucklehead suggests, which involves cutting into the original harness, splicing in relays, and using the factory thin-gauge power and ground wires to the bulbs, for that way solves only about 1/3 of the problem.
I would treat this on a case-by-case basis. Many of the original terminals and housings are of quality that cannot easily be duplicated, so if there's nothing the matter with the existing terminal or housing connector, I'd leave it be. As far as the actual wires go, as long as you are examining and working with the wiring to the rear of the car, I would definitely run a ground wire of 10 or 12ga from any good engine bay ground (battery negative terminal, engine block end of negative cable, alternator housing...) clear back to the rear of the vehicle. Sheetmetal grounds are schlock enough even when they're brand new and located near the front of the car, but 16 feet away from the actual ground? Pfft. Running this ground wire will make your brake lights come on faster and brighter when you step on the brake (always a good thing for safety performance) and reduce the chance for bizarre electrical faults later on down the line.

While you're doing this, examine the existing wires and consider upsizing the most ridiculously small ones. If I had everything all stripped down, I would be very tempted to increase the gauge size of the brake lamp wire and the reversing lamp wire (especially the latter, which is something dumb like 20ga). Also, if you're planning on adding a centre brake light or adding amber rear turn signals, now's a great time to do those wiring mods!
That is a tremendously good price, and Evans' work has a good reputation. I would talk over upsizing all smaller-than-10ga circuits by one gauge size (18s to 16, 16s to 14, 14s to 12, 12s to 10). It shouldn't add much of any cost, and will make the end product more robust and less problem-prone. Also remember these cars didn't have much in the way of circuit protection. There's no main fuse or fusible link! This is a serious omission that ought to be remedied. A few minutes' work calculating the maximum possible legitimate current draw and figuring out what sort of a fuse to use can easily save you many tears and dollars sometime in the future.
Good! The original fuse block is too small and not of very good quality.
I'd go for the active shunt. It's more work, but a working ammeter is more useful than a voltmeter.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dakight ,
where did you buy the Bussman fuse block ?

SlantSixDan,
I've seen the total amp draw caculation before but I don't remember where. Also where would the best place be to install the main fuse?

Thanks, Jeff

This ought to get you headed in the right direction.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Jeff, my receipt for the fuse block is on my desk at work. I'll ook for it tomorrow and post the name of the place. It was something like $38.00. I ordered it last Monday but don't have it yet so if I were you I'd wait until I actually get it so we'll have some gauge of the level of customer service. This particular block, however, support up to 24 circuit with a maximum of 30 amps per circuit. I think the total maximum current capacity is 200 amps which is way more than I would ever expect. Dan's advice for a main fuse or fusible link is a good one as well; I think I will look for a maxi fuse holder and put one of those in the main feed. There's a power distribution block in my Dakota that I thought might be a nice way to go but it only supports about 8 circuits in addition to the various relays; the remainder of the circuits are in the dash. It does have a number of maxi fuses including a 100 amp main fuse. It would make a neat installation if it just supported a few more circuits.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Dan,
In order to try and keep cost down would replacing 20g wire with 16g wire make any difference? The cost of the wire isn't so bad , but the cost of the terminals is where it gets expensive. Terminal Supply has a 100pc min. on the terminals, @ approx $17.00 per 100.

20g wire --------------------------------------------
18g wire $9.00 + m&f terminals $34.00 = $43.00
16g wire " " " " "
14g wire " " " " "
12g wire " " " " "

By replacing the 20g wire and18g wire with 16g wire I could save some money.

Thanks jeff

In general, replacing smaller-gauge wire (higher number) with larger-gauge wire (smaller number) presents no problem at all. There are only two things you need to watch out for:

1) Fusible Links. These are lengths of special wire, designed to burn out and protect the rest of the circuit in the event of overload. They are very carefully selected for length and gauge (fuselinks are generally 4 numbers smaller than the wire into which they're spliced, e.g. a 12ga wire is protected by a 16ga fuselink, a 16ga wire is protected by a 20ga fuselink, a 10ga wire is protected by a 14ga fuselink, and so on. You must use fuselink wire, not ordinary wire, and you must use the correct gauge and length. Your '74 should have only one or two fuselinks under the hood, and none under the dash, but this precaution bears keeping in mind.

2) Sometimes you'll run into a situation where there's no room (physically) for a larger-than-original wire or its terminal within the factory connector. You should be OK in most cases going from 18 or 20 to 16, but keep an eye on it. You may have to do some adaptation if you run out of room in one of the connectors.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks Dan,

Right now I'm going to start working on the under dash wiring harness. I did not find any fusible links. - I tried going to the "here" link above a few times and could not connect.


Thanks, Jeff