Monday, February 05, 2007

MG TD Auxiliary fuse panel installation

If you own a MG TD/TF, you probably know the car is under-fused by modern car standards. There are only two stock fuses. The first connects posts A1 & A2 of the panel and protects the horn circuit. The second fuse connects posts A3 & A4 of the panel. It protects the brake light, turn signals (if equipped), windshield wiper, and low fuel warning circuits. The fuses are rated at 50 amps for A1-A2 and 35 amps for A3-A4. British fuses are rated for the failing current instead of the carrying current of American fuses. The equivalent American fuses are 30 amp A1-A2 and 25 amp A3-A4. Unfortunately, these fuses provide minimal protection for the electrical system of the car. The reasons for enhancing the fuse protection on the car is not only to protect equipment, but to improve the protection of the car's wiring harness and to prevent fires.

Fuses are chosen based on the current load of the device they are to protect. In addition, the size and length of the wire must be considered. The TD & TF have a variety of wire sizes in their harness. The majority of the wire is 14/0.30 or 14 strands of 0.30mm wire. This is roughly equivalent to 18 gauge US wire. The carrying capacity of this wire is only 8.00 Amps.
This is used for general wiring like side and tail lamps. For heavy feeds like horns, 28/0.30 wire is used with a 17.50 Amp capacity. This is roughly equivalent to 14 gauge US wire. For ammeter and voltage regulator connections 44/0.30 wire is used with a 25.50 amp capacity. This wire is roughly equivalent to 12 gauge US.

If you look at the stock arrangement on the TD/TF you will notice several problems. The most obvious is the wiper motor. The wire used to power the wipers is much smaller than the car's standard 14/0.30. The carrying capacity is very low, yet it is protected by a 35 amp fuse. In the event of a short, the wire will glow before the fuse blows. This is just one example of under protection. The bigger problem is the lack of fuse protection for the majority of the cars circuits. The main purpose of a fuse installation is to protect wiring and prevent fires.

To further complicate matters, different harness suppliers use different wire size in their construction. With the help of Dave Braun, Jim Budrow and Dave Rome, I compared the construction of harnesses from 4 different sources. The harnesses were from the factory, a restoration supply shop, British Wiring and one of unknown origin. The unknown and restoration shop harnesses used smaller wire than original in several circuits, while the British Wiring harness met or exceeded the factory size across the board.

When I restored my car, I decided to add fuse protection to every circuit I could. Unfortunately, I had already purchased a new wiring harness and I did not want to cut up a very expensive kit. I like the idea of modifying the car only if the modification is completely reversible. The ideal solution would incorporate a fuse block into the harness. This block would be located out of view with all of the stock connectors in the usual places. Since I didn't have this option, I built a parallel fuse harness that connected to the stock harness in places that are not visible.

The design
I started with the wiring diagram in the factory workshop manual. I followed each circuit and found nearly all had a connection behind the dash. I decided to build my harness so it connected to the stock harness under the dash. A modern car's fuse panel is more of a power center with a main power feed coming in, then supplying power to each circuit in the vehicle. In the event of a short, power would be interrupted near the source by a fuse. While this is ideal, it would not be possible to do in a TD/TF without significantly reworking the entire harness. It would also alter the appearance of the car. I decided the next best solution would be to provide a point in each circuit where the failure of the protected device would blow a fuse before burning the wiring. Since there is no way to cut power at the hot end of each circuit, there is still a potential for failure in the circuit on the hot side of fuse. This area of exposure is relatively small and confined to the area behind the dash. A short circuit on the device end of the protected circuits would cause a fuse to blow.

Before I go on, I will add a disclaimer. I am not an electrical engineer and I make
no warranty whatsoever that any of this article is accurate. Before attempting any of the following, seek the advice of a trained professional. Building and installing this harness could result in significant injury or death to you and/or your car.

To better suit this project, I redrew the wiring diagram for my 1953 TD with turn signals. I traced each circuit and marked the point of fusing with a red bubble. You can click on the diagram below for a larger view.

Let's look at the connection points I used for my fuse installation. You can compare the numbered bubbles, in the the diagram above, to the numbers that follow.
I will describe the wires and the connection locations in the car. Think of each bubble as a point where we break into the circuit. We connect a wire to each end of the bubble and run the wires back to a fuse box. Connecting these wires to a fuse has the same effect as placing the fuse at the location of the bubble for protecting the circuit. For each bubble there is a place to tap into the circuit without cutting the harness. Here's a description of each:

  1. Wiper Motor - The wiper motor is powered by a dedicated two wire cord that runs from the motor through the wind shield frame to a hole in the right side of the scuttle and behind the dash. From there it runs through the firewall on the left side to the fuse bock and voltage regulator. The run behind the dash provides a good hidden place to connect a fuse. I fused only the hot wire in the aux fuse block. More on the wiper wire later.
  2. Ignition coil - The ignition coil is powered by a white wire that emerges from the harness a few inches below the harness grommet in the firewall . The source end is in the group of wires that connects to the voltage regulator and stock fuse block. This wire connects to the "A3" terminal of the block. The regulator/fuse wires exit the harness very close to the firewall harness grommet. Close enough to allow the wire to be removed from "A3" and fed though the grommet, under the dash, and soldered to a fuse block wire. Heat shrink tubing can be applied. A return wire of the same color can be connected to "A3." Once completed, this wire is hidden under the harness and visually undetectable. This is one of two circuits where connections are not made behind the dash.
  3. Flasher circuit - By protecting the flasher, we will protect the flasher, part of the turn signal relay box and turn signal lights. The flasher normally connects to the main harness via a solid green wire with a bullet connector. I installed a bullet connector on the wire to the fuse block and plugged it into main harness. I installed a bullet and a 1 way connector on the return wire to plug the flasher in.
  4. Dash lights - The dash (instrument) lights are connected to a rheostat via a red wire with a white stripe. To protect the lights I removed the wire from the rheostat. I ran a wire from the rheostat grub screw to the aux fuse block. I soldered the return wire to the dash light wire and covered the connection with heat shrink tubing. Note: this does not protect the rheostat. I considered making the connection between the ignition/light switch and the rheostat, but the problem is the fog lights are powered from the hot side of the dash light rheostat.
  5. Running (Side) Lights - The running lights are powered by a red wire connected to the "T" terminal on the light/ignition switch. Like the dash lights, I removed the running light wires from the "T" grub screw. I ran a wire from the "T" grub screw to the aux fuse block. I soldered the return wire to the running light wires and covered the connection with heat shrink tubing.
  6. Low (Dip) Beam Headlights - On cars with a foot operated dip switch, there is a small sub-harness that mounts to the switch. It connects to the main harness with bullet and 2-way connectors. I connected wires to and from the aux fuse block between the main and sub harness using like connectors. The low beam wires are blue with a red stripe. Note: on cars with a dash mounted dip switch, the connection could be made at the 2-way connectors near the dip switch.
  7. High (Main) Beam Headlights - The connection for the high beam is the same as the low beam above. The high beam wires are blue with a white stripe.
  8. Inspection (accessory) socket - The powered end of the inspection socket connects to the ammeter with a brown wire. I replaced this wire with brown wires to and from the aux fuse block. one wire connects to the ammeter and the other to the accessory hot post.
  9. Fuel Pump - The fuel pump is powered by a white wire connected to the stock fuse block at terminal "A3" like the coil above. I used the same method as the "Ignition coil" above.
  10. Brake lights - On cars with turn signals, power for brake lights comes from terminal "A4" (green wire) of the stock fuse block to the brake light switch. From the switch it runs to the turn signal relay box - terminal "5" behind the dash (green wire with a purple stripe). To allow for behind the dash connections, I connected the wires to and from the aux fuse block between the green/purple wire and terminal "5." I removed the spade terminal from the green/purple wire and replaced it with a bullet connector. Using a 2-way connector, this wire is connected to the aux fuse block wire (also with a bullet connector.) I then soldered the spayed terminal to the other wire from the aux fuse block. The spade terminal is reattached to terminal"5." Note: this does not protect the brake light switch. On cars without turn signals, connections could be made at the stock fuse block like the ignition coil and fuel pump above.
  11. Low Fuel Warning - Power for the fuel warning circuit comes from terminal "A4" of the stock fuse block via a green wire. This wire connects to fuel warning lamp. I removed the green lamp wire and soldered it to the aux fuse wire, then covered it in heat shrink tubing. I connected the other fuse wire to the lamp.
  12. Fog Lamp - Power for the fog lamp comes from the hot side of the dash light rheostat to the fog light switch (red wire). I removed this wire from the fog light switch and soldered it to a (red-yellow) fuse block wire. The other (red-yellow) fuse wire is connected to the fog light switch.
Construction of the auxiliary harness
To build the auxiliary fuse harness you will need to find and purchase a suitable fuse block. I searched various auto parts stores and found a modular fuse block that could be expanded in multiples of 4. This block also uses modern ATO (blade style) fuses. It includes terminals that are crimped and soldered to wires then snapped into the back of the block. When searching for a suitable block, find a non "power bus" type. Make sure the hot side of each fuse slot is not bridged together. This type of block is used to create a power center and will not work for this application.

Next you will need wire to find wire of the proper length, size and color for each circuit. If you have access to a new TD or TF wire harness you can use measure the distances for each connection. I did this during my restoration harness. You will need to select a mounting location for your fuse block, then measure to the connection point listed above on your car. I chose to mount my block under the dash behind the tool box. There are already holes on each end of the tool box from the factory. I ordered wire from British Wiring Inc., in the same color code and size as the main harness. (Your length will vary depending one where you mount the block.)

If you decide to mount the fuse block to the back of the tool box, you can use this parts list. (Click on the list for a larger view.) The wire lengths listed will provide extra wire to easily reach the connection location. The 4 meter length need to be cut in fourths and the 2 meter lengths cut in half to provide the "to and from" wires for the fuse block. (Note: British wiring sells their wire in meters.) Heat shrink tubing and cloth harness tape should be available locally in hardware or automotive parts stores. Bullet connectors are available from British Wiring or the other restoration suppliers. In addition to the parts list, you will need a soldering iron, solder, wire cutters, wire stripper, needle nose pliers and a source of heat for the heat shrink tubes.

Once you have the wire and fuse block(s), you can assemble the pieces. If your block has a optional power bus configuration, cut the common fuse terminals into separate pieces before assembly. Separate the wires into pairs based on the color codes and circuits above. It might be helpful to tape and mark the wires in these pairs. Solder one wire to each fuse terminal and install them into the block. I also covered the soldered end of the terminal with heat shrink tubing. Once you have all twelve pairs of wires soldered and the terminals in place you can bundled the wires and form branches to reach the connection points. Once you choose a mounting point for the harness, you can measure the distances route for your connection branches and wrap them in harness tape.

I used five branches in my harness. (Click on the picture below for a full sized view.) The branch on the left is the wiper wire. You will notice bullet connectors on the end of this wire. I installed this disconnect behind the dash to allow for possible windshield removal in the future. Since the TD/TF have the wiper wire routed though the windshield frame, a point of disconnect would be needed if the windshield would ever need to be separated from the car. Moving to the right, the next branch contains the wires that connect to the back of the instrument panel. This includes; the side/running lights, dash lights, fog lights, inspection socket and fuel sender. The third branch contains the wires that connect to the flasher/turn signal harness and relay box. These are the brake light and flasher circuits. Notice the connectors as described in "design" above. The fourth branch contains the headlight connections for a car with a foot operated dip switch. Notice the connectors as above. The final branch contains the stock fuse block connections. This includes the wiper wire, coil and fuel pump.

The installation of the fuse harness is fairly straightforward. For my harness, I mounted the block to the back of the toolbox using factory holes. Once you choose a location and mount the block, run each of the branches to their connection points.
Connect the wires to their circuits. For the connections that use bullets, simply unplug the existing connections and connect the fuse harness connections to these ends. For the connections behind the instrument panel, make the connections as described in the "design" section above. When connecting the fuse wire to the circuit wires, solder the ends together and cover with heat shrink tubing.
Pull the wiper wire through the harness grommet in the firewall. Connect the wire you fused to terminal "A4" on the fuse block and the ground side to terminal "E" on the voltage regulator.
For the coil and fuel pump, remove their connections from "A3" of the stock fuse block. Feed these wires thought the firewall harness grommet to route them under the dash. Solder these wires to your fuse harness wires and cover with heat shrink tubing. Route the return wires back though the grommet and connect them to terminal "A3."

The last step in the installation is to tie the wires and support them. Make sure they are away from moving parts and clear of the drivers feet.
It is also a good idea to make a fuse key and place is next to the fuse panel. I used the key on the right. I covered it with a piece of clear packing tape and stuck it to the back of the toolbox, next to my panel.

Fuse Selection
It is important to select fuses that will provide good protection. Dave DuBois helped me determine good starting values for each circuit. You can see the values on the wiring diagram above. Unfortunately, we could not find good data for current draw, so educated guesses were used.
I recently acquired a heavy duty digital ammeter. I will be testing the cars circuits for current draw in the near future. Watch for a follow up article with better values.

I hope this article helps you better protect your T series car. It is not perfect, but it provides a good measure of improvement. I accidentally tested mine soon after installation. I had a contact problem in my turn signal relay box. When I removed the cover to make an adjustment, I heard a "pop" nearby. The cover had shorted one of the relays to ground. The fuse blew and prevented any damages to the relays or wiring.

Thanks to Dave Braun, Jim Budrow, Dave DuBois and Dave Rome for their help with this project.

Please post any ideas, suggestions or comments.