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MG TD TF 1500 - TF wiring

Hi Folks,
I've about gotten the rewiring done on the 54 TF, using a British Wiring harness(es). Everything came out where it should, but the known diagram problem at the wiper motor apparently isn't known to the harness fabricator. I'll call Josh at BW tomorrow to ask him this same question. There is a hot (green) wire coming to the wiper motor as is proper, but the other wire coming out there is a ground lead with a ring terminal on it. That would make, as shown on the diagram, the motor to run all the time that the ignition is on.
My question, what have others done to make it work properly? I tried taking that hot green wire, which did terminate in a 4 way joint at the dash, and routing it to the wiper gearbox (wheel box?) switch and then running a hot wire to that switch. I'd tested the switch when out of the car, and found that it is a SPST switch, but now find that it is SPST to ground thru it's mount, so I got a direct short to ground when switched on.
I suppose one fix would be to abandon the ground wire to the motor (hook it actually to ground, not to the motor), and run a new wire from the body of the motor to the switch. Maybe that's the best, tho' it would require a non-original wire running outside of the harness. EGADS, what blasphemy (for some folk I suppose).
All thoughts gratefully received. This is all that's keeping me from buttoning up the dash.
A W Parker

The green wire is always hot when key is on. The other wire goes to the switch on the wiper knob which completes the ground when needed.

hi Sandy,
I know that's how it should be, but the harness isn't that way. The green wire is there, hot, but there are 2 black wires (ground) terminating in a ring terminal, no other wires at that location.
A W Parker

Trace the two black wires. the other ends should be fastened BY YOU to the switch on the drivers wiper knob. then the ends at the wiper motor go to the other motor terminal and the ring to the case of the wiper drive.

Thus the wiper motor is connected thru the switch to ground.

You should find two black wires with plain ends coming out of the harness under the dash or they may be completely separate as they are on my TF.

Don Harmer

Same thing as you have Al. Notice the blue wire and the turquoise wire. The motor is always hot with the key on. I ran a turquoise wire to the switch and the blue wire back from the switch and make a connection along with the black wire from the harness to the caase for a ground. The switch on the wiper is a real pain in the a%^ and one of these days I intend to to change it to an unused toggle switch I have under the dash.


When we wired the TF we got a piece of a coat hanger and soldered a piece of black wire to it. Then carefully inserted it into the loom covering and worked it far enough so that it exited after the harness was inside the firewall. Thus not visable from the engine compartment. I believe the black wires that were supplied with the harness were fastened to ground under one of the horn mounting bolts. It's been a while since I did it.
Bob Jeffers

The one thing I could never figure out is why these cars were wired that way, meaning, making an item hot all the time and using the ground to turn it on or off. No wonder these cars were prone to fires. Ford had the same system in the late 30s and 40s, might have even been in the early 50s before they came to their senses and changed it to standardize with the rest of the industry. PJ
P Jennings

I had thought of changing the TF over to negative ground, but I think I'd have to have the clock converted along with the fairly new electronic fuel pump. I think I'll just leave it as is. All new wiring should help eliminate the fire hazard due to shorts anyway. PJ
P Jennings

What LaVerne said about using a toggle switch under the dash is what I did. The "on" switch on the wiper knob is even more of a pain for me as I have locking glove box doors. (and for some strange reason my "switched knob" is in the passangers box).
I converted my TF to neg ground.
David Sheward

David, did you have to have your clock converted or are they not polarity sensitive? That was my biggest concern as I got my clock working and it kept good time. The fuel pump I can change.PJ
P Jennings

Hi Folks,
. Thanks all for your notes, particularly Don Harmer, who explains what I have, and what needs to be done. I can't understand why they would have 2 black wires (at the wiper motor and back at the dash panel) with terminals tying them together at each end. And no explanation with the harness as to what needs to be cut and connected. I'll "ring them out" this AM to be sure. Seems to me that one should be black, and one black with a colored tracer.
. I'd seen the past discussions in the archives and knew about the bad diagram and lousy switch. I think the switch was on the rt side knob when the car arrived here in the Spring, I moved it to the driver's side, after checking it out for operation. I didn't realize that 1 side is probably shorted to ground, but that's no real problem as that side can be connected to the wire that goes to ground back at the motor.
. A toggle switch would fix the switch problem for those who have it, and I think what I had done would fix the "hot all the time" motor, if the switch didn't have the short to ground.
. It appears to me that the whole dash must be removed to get to that switch if needed. In my case I've bolted the dash in, but not the sub-panel, and I have soldered wires to the switch, which I can reach.
. Paul, I haven't seen anywhere that the clock needs to be revised when changing to neg. gnd. This car was neg. gnd when it came, and I'm leaving it that way. I don't know if the clock was working or not, and hadn't worry abt it, yet. Guess I can ck it out today.

A W Parker

My clock wasn't working when I got the car. Sent it off to the "Clock Guru"... Hal Kramer (sp?) with a note telling him car was neg ground. My clock was also missing a "hand" and he fixed that.
Because I have a Battery switch on my hot lead, I ran a fused line from batt to clock so I wouldn't have to reset it each tyme. It is the only thing that stays on when I, (to put in words one might have found in our T car manual)
"engage the dissengage" for the battery.
(or should that be "dissengage the engage"...LOL)
David Sheward

As I said before, the clock is my only real concern, everything else is secondary and replaceable. Because it works so well, I'm very concerned of it's polarity. I do have another, non working, clock that I could send out if needed. PJ
P Jennings

Whilst your under there...
The turnsignal indacators are very hard to see on a sunny (remember those) day.
I got a little piezo beeper from radio shack and put that in the circiut (with a toggle switch to engage/dissengage) it beeps at me to let me know my signal is on.
My right signal is "haunted" ....have had it out of the car about a dozen times and re-built (works fine on the bench)...but sometimes it just "decides" we are going to turn right now.
Wife got sick of following me with the turn signal on wondering where/when I was going to turn.
David Sheward

Hi again, an update,
. I was thinking there were 2 black wires at the motor, there's only one, and I can't find the other end of it. There are 2 black wires coming to a ring terminal behind the dash panel, I separated them, one goes to ground, but the other does not go to the wiper motor black wire. (and it appears that the ground wire, or the pair, is intended to connect to the sub-panel to provide a ground there, which I had done.)
. I also had been thinking that the case of the motor was part of the circuit and was rubber mounted to keep it from ground. It's not connected to either motor terminal.
. I have left a msg at Brit. Wiring this AM, hopefully Josh will call back & can enlighten us. So far it appears that I need to run another wire back to the switch, which will ground it to complete the circuit, as several others have done.
. I'll keep you posted. Thanks for your help.
A W Parker

Another update - I talked to Josh at Brit. Wiring just now. He was somewhat familiar with the harness arrangement, I think he had an actual wire layout to look at, as he knew where the other end of the black ground wire at the wiper motor was terminated. He did not know where the one of two blacks wires terminating in a ring terminal behind the dash came from. It is not grounded anywhere in my car yet, at least. He did not know of a problem like several of us have had in needing to run another wire from the motor to the switch. He said he will get in touch with the folks in England who make the harnesses to see if they have heard of the problem. The only explanation he could come up with was that maybe a few cars were made "that" way because they ran out of something, etc. So, no resolution to it all. He did thank me for calling, and said that if nobody reports such problems nothing gets changed/fixed.
. I told him I had just picked out a nice purple #18 wire to run from the motor to the switch. I don't think I'll be able to hide it inside the braid cover.
Thanks again to all, this is a great forum.
A W Parker


On my TF the wiper motor has two wires, not in the harness. One from the motor terminal to the switch. This may have been black with a tracer.

Then a black wire back from the switch to the case of the motor via a ring under the cover screw. NOTE: Neither of these wires is in the harness!!!!!

The case is grounded thru the wiper mechanical cable.

It may be that your harness is wired exactly as shown in the faulty wiring diagram in the workshop manual, with a wire directly to ground from the motor and no provision for a switch.

The wire at the motor in your harness should test out to go to ground, if so connect it to a cover screw and add a pair of wires to and from the switch as it is in my TF.

Note that the horn, the wipers, and the gas gauge all are switches in the ground side of the circuit, so a short in these wire only turns on the device and is not a hot lead to ground. It is actually safer.
Don Harmer

Hi Don, Yes, I agree, my harness is as the (wrong) diagram shows, and I just realized this PM that the motor is grounded by the mech. cable.
What I plan to do is run a new wire from the motor to the switch, which will ground it to operate the motor. My switch seems to have one side shorted to gnd, but I'll run a wire from that terminal to ground at the panel just to be sure. Then ground the motor case with the existing wire which runs to the ground terminal that's under the side panel uppermost bolt. I guess putting the switches in the ground side results in less "hot" wires running around, particularly in the case of the gas low level switch.
I thought I'd get it done this PM, but ended up fixing the headlite switch which had a poor connection between the grubscrew post and the internal copper trace to the contact. I silver soldered it, after re-staking it didn't fix it. I think all wiring and switching tests out OK now.
thanks again,
A W Parker

This thread was discussed between 09/12/2010 and 10/12/2010

MG TD TF 1500 index

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