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MG TD TF 1500 - Tail lights mystery

My tail lights are turning on when I turn the key on. Do you have any suggestions as to what would cause this?
C. Eastman

The only thing that I can think of that relates lights with the key is the brake light circuit. Is this a new happening? Has the car been functioning normally? What happens if you step on the brake pedal? Do you have turn signals? Bud
Bud Krueger

Did you just install a new wiring harness? Bud
Bud Krueger

The battery died. I replaced it. I took the generator out and had it tested. It was ok. I just put it back in. The turning signals work fine. The red ignition light will not turn off. Pressing on the brakes does not affect the rear lights which continue to be on. The head lights work. Before the battery died I put in an electronic regulator just to make sure I would have no electrical problems. I resealed the fuel level sending unit as it started to leak. I got that wired right. So where did I go wrong (besides buying a TF)? I guess the last comment shows I'm a bit frustrated.
C. Eastman

Okay, so at this point you have r/r'ed the battery, generator, regulator and fuel sending unit. When you say that "Pressing on the brakes does not affect the rear lights which continue to be on..." are you saying that the brighter brake lights come on in addition to the lights that are already on? Did you disconnect any of the wires in the harness in the back of the car? Bud
Bud Krueger

The brake lights do not get brighter when I press on the brake. I did not disconnect any of the wires in the harness in the back of the car.
C. Eastman

Relay box? What happens when you try the directionals? Brake light switch? Try pulling a wire off the brake light switch.
D. Sander

Okay, if the brake lights do not get brighter it would indicate that it's the brake lights that are on, not the tail lights. When you say "..the turn signals work fine.." does that apply to the rear also? Do the rear lamps turn off and on as turn signals? Bud
Bud Krueger

if indicators are working then either the stop switch is faulty or the master cylinder is not fully returning.Undo a bleed screw on a brake cylinder to release pressure,if the light goes out then clear the recuperation hole in master cyl.see some earlier posts see archive this year "brake light question"
Ray TF2884
Ray Lee

Is the control box/regulator wired correctly, and meeting its specs? With the ignition light on all the time and the generator checking out OK that's my bet.
David Werblow

The brakes lights will only come on when the ignition switch is on... They are on the green circuit which is the white switched circuit after it goes through the fuse box.

The switch at the brake T must be fried, or failed open, or the terminals are touching each other.

Dave Braun

I have had the same problem with an MGB and it was a faulty ground problem involving the ground of the fuel pump and rear lights so that the fuel pump was being connected thru its ground wire to the rear light ground wire thru the rear filiments to the front fender lights to ground. This resulted in the lower resistance brake lights glowing when the ignition was on.

Check the major cluster of ground wires that should be connected via an eye connector to the bottom bolt on the spare tire rack.

On the MGB the was a bolt on the tag frame in the trunk
Don Harmer

Yes, the rear turn signals work. The affected rear light is on, then blinks off, and on again etc. and at the end of the turning signal cycle, stays on.
C. Eastman

C. Eastman

Same with my MGB, sounds like the same problem, the rear cluster ground.

Don Harmer
Don Harmer

The only ignition switched HOT wire going to the rear harness in a TF is the fuel pump wire. Therefore there is some crosstalk with this wire and the rear lights.

To test: Ground the fuel pump firmly to the chassis, the tail light should go out.

If not, be sure the battery is firmly connected to the chassis, and try again

Check all the rear harness connectuons.
Don Harmer

This thread was discussed between 02/09/2012 and 05/09/2012

MG TD TF 1500 index

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