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MG TD TF 1500 - Questions on Wiring Harness

After successfully replacing my water pump (based on the advice of this forum), I then decided to bite the bullet and replace the wiring harness. The clincher in my decision is when I showed a friend the results of my water pump work and he said, "Oh my ..." when he saw the wiring ---my PO was somewhat creative and sloppy with his home grown wiring "harness".

I had previously ordered the Moss Motors harness. I purchased the color diagram which is available on the web, and I had access to a bunch of previous threads for advice.

I took my time and everything went very well. I had a little trouble snaking the wire for the license plate light, but I ended up using a braided steel line from my sailboat which was flexible yet stiff enough to force through.

The Moss harness was great. I tested most of the high amp connections by hooking up to a battery trickle charger. I also inserted an in line fuse in the hot lamp wire.

So the fun began when I hooked up the battery and went for a test drive. All the essentials worked perfectly, ignition, coils, fuel pump,headlights, brights, fog lights, dash lights, etc.

I did notice a couple of quirks -- the side lamps turned on when I applied the brakes. I was beginning to think this could be a feature, but traced it down to a loose ground wire (at least it seemed to make the problem go away).

My biggest issue now is that the turn signals do not work. They used to work with the old wiring. I verified the wiring into all the relevant turn signal. The PO had the turn signal controller box (with the 8 wire input) locaed inside the engine compartment. I moved it to the inside of the passenger compartment, but it is not yet attached to the fire wall. Does the box itself need to be grounded by physically attaching this box to the firewall? Is there anything else I need to check to determine why the turn signals are not working?

Since all my repairs resulted in the car being off the road for three weeks, I have decided that the car works well enough to drive. I have been using hand signals to indicate my turns and have determined that most drivers, all who are younger than 30, do not have a clue what I am doing. They wave back, or swerve to avoid me. So I really need to fix the turn signals.

Any advice as usual will be valued.


Mike Iandolo

Yep, you have to ground the box for the turn signals.

Dave Braun

And I suppose I could have been a bit more clear...

See Dave DuBois for a really good brake light relay to preserve the pressure switch and to make good grounds. visit his website at

You also might want to look at my installation on and look under Electrical and Electrical restoration.

good luck,
Dave Braun

Check your wiring to the flasher unit. X is the hot wire, P goes to the light on the panel, and L goes to post #1 on the turn signal relay.

L Karpman

I'm gonna make you feel better. Here's what my PO did.
Must have been a sale on purple wire. Did the whole car with it.


That is a great one LaVerne. Guess if you do one wire at a time, that is not all that hard to do, but WOW, I can just hear the conversation at the hardware store. What do you have that is cheap and a lot of. Well, I got this purple wire that no one ever buys. Great I will take it.

My mechanic just rewired an MGB that had been done with all 'lamp wire'...the previous owner even pulled the two strands apart..... everything brown!!!!

The wiring diagram you purchased off ebay is wrong when it comes to the flasher unit. I have the same one and my signal light did not work until I wired the flasher as indicated in the original wiring diagram out of the workshop manual. I sent an email to the web site where I bought the colour diagram but I guess they didn't correct the error.

Joe Hine
53 TD
Fredericton, N.B.
J.D. Hine

Colors? We don't need no stinkin' colors!

Honestly, what I did for a wiring diagram, especially since I have a hybrid car... it has the dash mounted dipper, turn signals and a main beam indicator light... I'm sure it was a factory special (just kidding).

I took my workshop manual and copied the wiring number key and put the key at the bottom of the five post non TS car and the TS equipped car diagrams, sized them to fit 11x17 paper (about 135%). I made about 10 copies each. I just scribbled on them as I went, and have used them to wire friends' cars (yes, I have more than one friend...) Eventually, I started thinking of colors in terms of numbers and it got even easier. Like 4 is blue, and 17 is green, 9 is white... etc.

I had British Wiring make my harness to my specs for the hybrid I was building (location of the flasher unit and turn signal box, dip switch and panel light and a long wire for the TS panel light as I was thinking of doing something different, I did end up with a pizeo buzzer). They only messed up one area. I had to modify the head lamp circuit slightly to bring power to the dip switch properly.

I pretested the harness and my turn signal system to assure that I had everything pretty much ready to go.
Everything worked on installation. The only hic-cup was that I never realized that the TS were wired through the fused ignition switch circuit (green) and I was confused when they wouldn't work at first... until I turned the key!

I just wired the car pretty much so everything worked at the dash level, and then incorporated the turn signal circuits. I really, really liked the separate dash harness, and it is good to remember to fish a line through the spare tire rack for the license plate lamps BEFORE you mount the rack.


Dave Braun

Thanks everyone for your suggestions. I will be back home next weekend and will attach the box as Dave suggested. I will also verify the wiring as Joe and Larry suggested. By the way, I also concluded that the color diagram I purchased on the web wasn't exactly accurate and several times I found myself refering back to the diagram in the Workshop Manual to make sure that I was connecting the wiring correctly. I have actually come to appreciate the workshop manual as a good source for many of my repairs.

Again thank you for your help. I am sure that this problem is now behind me, and the car is preparing to give me another challenge.


Mike Iandolo

This is indeed strange. Why were so many cars restored using a single color for the wiring? Mine was done with pink wire! There must have been a time when harnesses were scarce.

It really bugs me but I admit that it works, and seems to work well.

Tom Balutis

I assume one just went to the local auto supply/hardware store and asked for a bulk roll of wire... if you do one wire at a time it wouldn't be too bad, but attempt a repair and it becomes a bit daunting?

Agree Gordon

There really are not all that many things that have multiple wires. Even easier if you have an early TD without turn indicators.

I just made two simple corrections and now have my flashers working. Thanks to Dave, Larry and Joe for your suggestions. I verified Joe's comment about the color diagram I purchased on the web. They indeed had the "P" and "X" connections reversed.

It feels good to have a solid wiring harness in place.

Mike Iandolo

Me too.



I miss the old days... fried bullet connectors, wires with no color, frayed insulation, bypassed wires with crimped splices and add-on devices.


Dave Braun

Joe Hine,

Could you please contact me off the BBS.

Gord Clark
Rockburn, Qué.

gordclark at securenet dot net
Gordon A. Clark

This thread was discussed between 09/10/2008 and 14/10/2008

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