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MG MGB GT V8 Factory Originals Technical - sorting wiring

Well I am in the middle of a motor swap project and have opened up the stock wiring harness. My goal is to sort the harness and then house it in split loom tubing and try to tuck it up into corners so I can hide it.

I have noticed that some wires from the coil and othewr items (I am guessing ignition on/off for the fans?) head forward to the nose of the car. I fear I already have a huge can of worms opened and would like to know what some of you have done. Dit you just cut into it and splice in longer wires, or unravel and re-route like I am trying? I see no way to re-route without adding wiring to get it along firewall then forward on engine..

I am doing a 302 V8 and want to route wiring along the top of the intake manifold inside wire loom to make it all clean and protected. (coil, distrib, temp sensor, alt)
Larry Embrey


I did what you are doing. By shifting the wires just slightly, I was able to reach most locations such as the starter, the fans and fan switch sensor. I had to add an extension to reach the distributor and I left the coil in the original four-cylinder location with a longer coil wire. I disassembled the plug for the alternator and inserted the connecters into a GM holder. I dont know how that will work for a Ford.

I used the split loom tubing because it is fast to make changes with, but it melted and dripped all over the fender wells. Keep that in mind before routing it over your motor!
George Champion

George, a couple questions.

I am guessing you have a rover/buick V8. Did you route the exhaust out the fender or use huggers? I can understand the loom melting on your fenders if it was anywhere near the headers. I am hopping by routing it on top of intake to get it away from that huge heat source. Maybe I will need to use the cloth type housing to be safe. I would think the loom would be as tough as the stock electrical tape, but I guess I will have to wait and see.

I am using a GM Alt right now (kept it after converting the 4cyl), will have to see how physicall fitting goes once Iget the block firmly mounted.
Larry Embrey


Yes, I have a Buick with block hugger style headers with no louvers in the hood (bonnet). Without checking, I will guess there may be only 6 inches between the header and alternator, but even as far away as a foot the split loom cover melted. I tried to measure the air temperature under the hood with a device sold at auto parts stores with a remote sensor to give in car and outside temperatures. Unfortunately, the under hood temperature exceeded the capacity of the device. I dont remember the maximum that it would register, but it was something like 135F.
George Champion

WOW that is hot. Course you do not live in the coolest state either.. I will be calling the Mfg of the tubing and asking thier recommendations. I know it is used in the harness for my Mazda truck, might just be getting the right "model" of tubing..

Larry Embrey

Thats true; it was 104F (official high, actual temp is always 5 10 hotter) yesterday when I got home my wife asked me if I was crazy to have taken the MG for the forty-minute commute. Its nice outside now, only 97 with 7% humidity.

At the time I checked the under hood temperature of the MGB V8, I also checked the under hood temperatures of two other cars and neither one of them exceeded the temperature capacity of the device as did the MG.

I would certainly be interested in what the manufacture has to say on the melting temperature or anything else. Please share.
George Champion

Well, I called Taylor Inc. The guy I spoke to said it would not work. I mentioned that my truck had some in it, but he said he would not risk it with his product. DRAT! I will be emailing a couple mustang customizing people to see what they do as well. I know someone has to make a product more heat resistant to do this.. Maybe I just need to go buy some blue elect tape..
Larry Embrey

Larry, there has to be a way. My wiring loom is just sitting out there on the right fender, in the stock location, and it looks pretty dumpy even though I neatened it up quite a bit. (NB it's just wrapped in that black "cold shrink" tape -- and that hasn't melted despite being only a few inches from the headers.) I think it would be a lot cooler to get it out of sight -- surely you could use some of that heat shield tubing wrap that they sell at Summit, etc. and get the heat down to a tolerable level(?) The other thing that masters of the domain do, is, I think, run a tube of wires up inside the fender. Then there aren't any heat issues. I'm sure it is easier said than done, but it sounds like a good call to me.

I talked to my mustang buddies (I get much of my info from them since they run 302's..) and they said they run the standard split loom over the intake with no problems. The hitch might be that our V8 B's do not release the heat as well...

I will be working on neatening things up, but want to get her on road ASAP so I can get running sorted out before upcoming trips. I like the fender idea, just need to get it so it stays dry inside.
Larry Embrey

Your engine bay temperature may stay much cooler in Washington than mine does in Arizona. You could use some nylon ties for now and snip a one inch piece from your Mazda as a test when you get it running for a melt test.

Ill fess up to a very stupid wiring mistake I made that melted a wire under my dash and kept blowing fuses. I scratched my head over this for a long time. Sometimes the fuse would blow as soon as I used the circuit and other times I could be driving down the road and hit a small bump and lose the gauges, brake and tail lights etc. I finally found the problem while trouble shooting with the parking lights on and closing the hood blew the fuse. The hood prop rod was hitting the fuse block! The reason the fuse block was too close to the prop rod was because I spaced it away from its original location to relocate the wiring harness behind it and farther away from the exhaust.
George Champion

If you have a RB loom then the coil wires will snake back and fore a bit because of the ballast for the 6v coil. The ballast is a length of resistance wire, hence the snaking back and fore.

Paul Hunt

yeah mine has the snake big-time. To bad everything ties in up by the fuse block, menas everything thing so far is a few inches short when re-routed to firewall the over center of engine.. Sounds like it is time to rip open the parts car harness for donor wiring. I will even be able to get matching guage and colors!!

Thanks for the help folks!!
Larry Embrey


I started to ask whether you had seen Bill Jacobsen's car, and then I remembered you had a whole page of his car on your website..duh... Anyhow, Bill's engine bay has got to be the nicest on the planet, as well as the rest of his car.

Your dilema is easy, just do what Bill did! Seriously though, good luck on the completion of your 302.

Steve Watson

p.s.; thanks for the excellent web site, it provides great inspiration for us wannabe's. I did notice, however, that Bill's page loads VERY slowly; the top 2 photos are .jpg's, and the next 2 are .bmp's, maybe that's why.

Good catch, I will check those files out.

Bill had an advantage also, the 215/buick intakes have a tunnel under them, my performer intake does not. BUT I have already torn the stock harnes opne, disected it and cleaned up the routing bigtime. I should have pictures in a week or two..

Thanks for the kind words on the site. I can't wait to actually see Bill's car in person, those picts came from a friend..
Larry Embrey

Think I'll chime in here guys, maybe some of my suggestions will make sense to you. At least Kurt seemed to appreciate the tips.

Since things are tight where the harness goes, it makes sense to keep it as compact as possible. Also, although accessibility of the wires is handy, that corrugated stuff is a less than ideal solution. Odd as it may sound with all the modifications I've done, the old way is pretty darned good and I'll try to explain what I mean. First the wires. Using wires from a spare loom is a pretty good idea. The wire itself is of a fairly fine strand which promotes flexibility, durability, and current carrying capacity for a given size. The insulation is vinyl, and the wire used in new cars is very much like it because of it's durability. Over time if exposed though it will harden and crack but under the hood there's not much that will hold up better at anything close to a reasonable cost. The size of the wire matters too. Circuits which will be carrying a heavier load may need to have the wire size increased, and some switches may need to be upgraded, but wires in signal circuits and other low load circuits should be kept at the stock size to keep the wire bundle size small on one hand and to promote durability on the other.

The primary reason for enclosing the wires is to prevent the vinyl of the insulation from drying out, and so that the wires are not exposed to oxygen which will corrode the copper. Otherwise a simpler solution would make sense. On these cars a non-adhesive flat wrap was used, which is also composed of vinyl like the wire insulation. Plastic outgases throughout it's useful life, losing volatile elements such as plasticizers and losing flexibility in the process. If the wire bundle is tightly encased in the same material the insulation is made from though, the plastic in that layer will outgas both to the outside and the inside, causing the wire insulation to outgas at a much slower rate. This is the reason why there is such a marked difference between the wire ends and the wire in the middle of the harness. Effectively sealing the harness jacket improves this phenomenon, but the wire ends are the weak point. Anything that can be done to protect the wire end will help, and this definitely extends to soldered joints and sealing the connections to the insulation, as this is where the wire strands are attacked by oxygen. Incidentally, a needle type probe should never be used to poke through the insulation of a wire to detect a signal for similar reasons.

Electrical tape has no place in an automotive wiring harness. Although it is made of a vinyl material, the adhesive deteriorates rapidly into a gooey mess which attracts dirt and is difficult to remove from the wires, as well as causing the tape itself to move around on the wire bundle. Better alternatives are available. Non-adhesive vinyl flat wrap is readily available (even in blue) and has many advantages. It has been proven to be as effective as any reasonably available means for protecting the wire, It adds essentially nothing to the diameter of the wire bundle and in most cases will constrict it to a smaller diameter, making it much easier to route the harness (no small consideration). One often overlooked advantage is the ease with which it can be used for repair work. Let's say you only want to rework one section of the harness. It is a very simple matter to remove the flat wrap from that section, make the necessary repairs and upgrades, and then rewrap that section along with part of the original harness so that there is some overlap. Self vulcanizing rubber tape can then be used to secure the ends of the wrap, as well as to protect the wire ends. In this manner, you can in fact re-wrap an entire harness, overwrapping untouched sections of the original harness, to greatly simplify the renewal of the entire harness to nearly new condition and appearance. If done well it can be very nearly impossible to tell the difference.

If a longer wire is needed in the harness than is at hand, the best connection between two pieces is a "Lineman's Splice" which requires about 1" of stripped wire on each piece. This splice should be soldered, and covered with heat shrink before being wrapped inside the harness. Before applying the heat shrink it is also a very good idea to coat the joint with "Liquid electrical tape" or a similar vinyl coating. This liquid is also very good for sealing crimped and soldered terminals to the wire insulation, and can be followed with a self-vulcanizing rubber tape in those cases to make a very durable termination.

The highest quality terminals available should be used on the wire ends. You will only be doing this once if done right, and the expense is small compared to the labor involved. The factory bullet ends are really pretty good if you take the time to clean them when they need it, but here again anything you can do to keep oxygen out of the junction will help tremendously. Many options are available here such as the aforementioned liquid vinyl tape.

The entire harness can be removed if the car is being stripped down for paint, but otherwise it is quite a bit of work and most of the harness won't need it. However it isn't too difficult to just do the visible parts of the harness, and this gives you the chance to clean up the engine compartment. If you don't have the same exact color to splice a wire longer, it is probably better to splice in a close color and use the original wire end, making two splices, than it is to splice in the other wire and use it's end. This way you can use the factory wiring diagram if you need to troubleshoot. The solderable bullet connectors (not crimped) also can be re-used. For soldering you should try to get a roll of "eutectic" solder, (63/37 composition, available at electronics supply houses), make sure the connection and soldering gun or iron are clean, pass the tip across a wet sponge before doing each joint, and position the wire so it doesn't move while the solder transitions from liquid to solid.

That's about it. Work neatly and you'll be proud of the results.
Jim Blackwood

Try calling Techflex. Phone number is 1-800 3235140. They sell all kinds of braided coverings for wiring harnesses and hoses. I picked up the catalog at the International Boatbuilders Exhibition with an eye to using their products on hoses and wires in engine rooms of USCG approved boats. They have stuff in Kevlar ($$$$, I'm sure) and also some marked fire retardant and also some in fiberglass. There is one type that is self extinguishing. They also make one style, probably not flame retardant but in 16 different colors, could maybe color code the harness branches in the cooler parts of the engine bay and the rest of the car.. They had a picture of some pickup truck with their stuff in it. The truck looked like something an auto customizer would come up with while on a bad LSD trip but that's only my opinion.

See if they'll send a catalog. Sone of the styles would also be good for shielding fuel lines.

Good luck and best regards, David Walworth
David Walworth

TEchflex also has a website, I haven't gone to it but you could probably check out the styles there.

Best, Dave
David Walworth

One more and then I'll shut up. For any situation whre you are not using the bullet connectors but using fittings such as spade connectors or ring connectors consider using the crimp connectors used in the marine industry. These type have a piece of adhesive lined heat shrink tubing already attached. After crimping just get out the trusty heat gun and shrink the tubing closed. If you go out and get the heat shrink tubing alone make sure that it is the type with adhesive lining, not all of the types available come with adhesive lining. If you follow Jim Blackwood's excellent advice you will do fine, just wanted to offer up an alternative, especially if you aren't as confident in your soldering skills.

Best regards, David Walworth
David Walworth

David, thanks for the info.

I am using an AMP circular 9 min connector for the firewall, but I will look for the adhesive crimp connectors for the other ends.
Larry Embrey

...and I suspect you can solder the marine connectors (after crimping of course) from the ring end too, which would also cause the insulation to shrink down, and then finish off with the heat shrink gun. An excellent recommendation, btw.
Jim Blackwood

One more note. Check out Dan Masters' article at this link:
It is very good, and includes specifications for the wire sizes, linking to a source. Most colors and sizes appear to be available. Very good news I'd say.
Jim Blackwood

This thread was discussed between 15/05/2001 and 22/05/2001

MG MGB GT V8 Factory Originals Technical index

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