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MG TD TF 1500 - Electric woes

After the usual amount of aggravation, frustration, and select four letter words I managed to install LED lights. They are in the taillights, brake lights, Front stoplights and dashboard lights. They are amazingly bright, much brighter than the incandescents. All was going well or so I thought. I was having a bit of the ignition problem so one of the club members followed me home from an event. He reported my turn signals were faulty. I will try to describe what is going on.

No running lights on.
If I signal a turn the front and rear stoplights on that side perform perfectly. However, the front and rear stoplights on the opposite side also flash but at a much lower intensity. The dashboard lights also flash. If I apply the brakes, both brake lights function correctly. One indicating a turn, the other a bright brake light. The opposite side and the dashboard lights continue to flash.

With stoplights and/or headlights on everything is perfectly normal.

The flasher relay is specifically for the LED circuit.

When a turn signal is used there appears to be a low voltage leak to the opposite side. My first thought was a bad turn signal relay. I replaced it with an extra unit I have and have the same issue.

The next thing that came to mind was a bad ground but each individual light fixture is grounded and the turn signal relay is screwed directly to the chassis. This is not a single fixture problem since it affects all four fixtures and the dash at the same time.

The turn signal indicator light on the dashboard works normally in all scenarios.

Any thoughts would be most appreciated.


Mort Resnicoff

Did you not check everything before driving it? Its not clear exactly what you have done, but with LEDs its best to install them in pairs for a particular function, then check that it works and doesn't interfere with anything else before moving on.
If you have separate LED indicators I would remove them first and return the flasher relay to standard and check everything else functions correctly. In reality you don't need LEDs for flashers anyway, because at 21W they are more than adequate as standard bulbs.
Dave H
Dave Hill

You are getting electricity feeding out in to the other circuit. Does not take much to excite a LED. I would try installing resisters to the turn signal circuits and see if that solves the problem. When you figure this out (and you will) you can include it in your seminar "Möbius, and the six million dollar idea man" at GOF 102. :-)

D. Sander

David, I thought about resistors, since the LEDs do not take much voltage to get excited. I know many people used them in the early days of LEDs but I did not think I would need them since I installed all LEDs and the proper relay. I did not think it would be necessary. The LBC gods are laughing at me again.

Terminal"T" on the back of the ignition switch feeds all the stoplights and the dashboard lights. I would think this is the proper location for one resistor.

Would anyone have calculated the value of this resistor?

Would anyone to know of a variable resistor I could introduce into the circuit to determine the value needed?
Mort Resnicoff

Where did the LEDs come from? Not all LEDs are the same?

I bought mine from Classic Car LEDs in the UK, they develop their own products. Mine work fine, the flasher is having an issue of no output to the P terminal so I may well replace it soon.

Moss LEDs have had a history of issues for twice the price.

Try going back to regular lamps on the front and test it that way.

My 02c worth.
P G Gilvarry

It is my understanding that the LED "bulbs" actually have "driver" circuitry built in that regulates the current/voltage going to the LED part. Adding a resistor will not change the power going to the LED unless the voltage drop in the resistor is large enough to lower the voltage to a point outside the regulator range. I have seen bulb specifications that allow for voltages between 6 and 24 Volts.

It does sound like you have some voltage leakage between Left and right sides.

You should be able to sort this out with a Multi-meter.
I would suggest that you use an analog meter however, not a digital one.

Jim B.
JA Benjamin

The most common problem found when converting to LEDs is that one side has the power going to the shell of the lamp and the other side (or the front on the same side)has power going to the center contact. This will cause a ground loop and result in the symptoms described.
This is not a problem with incandescent bulbs as they are polarity neutral (they don't care). However, since LEDs ARE polarity sensitive, all manner of bizarre behavior can result.

Double check each of the sockets and verify that all are wired the same.

On the other hand, if you are using light boards and not led bulbs, they should be clearly marked as to which side is positive and which is negative and should avoid the problem.

As a matter of interest, I design and manufacture direct replacement LED light boards for many British cars including virtually all MGs from 1934 to 1962. All are manufactured in the US and are specifically designed for every particular vehicle and are less expensive that some others. They are not a "one size fits all".

See my web site at

Lew Palmer

Okay..... Here is the bottom line.

All the lights are working perfectly.

A note of caution to interested parties.

A prior owner added turn signals to the vehicle. All four running light sockets were incandescent and everything functioned properly. (Sort off).

When I converted to LEDs I was unaware that the internal circuits have a different illumination brightness. It is quite easy to tell. Just put 12 V to each terminal in the base of the 1157 and you will see the difference.

The dimmer side is for the running lights and the brighter side is for the turn signals and brake lights.

As it turns out, the circuit for the running lights went to the brighter side of the LEDs. This was not an issue with the incandescents but caused all sorts of havoc with the LEDs.

I reversed the wiring going into each socket and with the proper electronic flasher everything is fine.

Mort Resnicoff

This thread was discussed between 19/04/2018 and 27/04/2018

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