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Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes

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Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#1
VC venezia Avatar
VC venezia Vito Venezia
Calistoga, CA, USA   USA
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1968 MG Midget MkIII "Imp"
1976 MG Midget 1500 "Zip Ties"
2003 Ford Focus
I have seen other topics with the same problem, but I did not find a solution. Basically, my tail lights light up when I use the brake, but they don't turn on when I use the light switch. The light switch turns on the headlights just fine, and it's a brand new one; I just replaced the old switch, so that can't be the problem. Is it one of the wires that go from the switch to the glove box?

Thanks, and I'm sorry if this is just a cloned topic. I just don't want to resurrect old topics as such an act is frowned upon in most forums.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#2
Brad76Midget Brad H
Salem, MA, USA   USA
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I had the same issue when I redid the wiring in my 76 last year.

Also check whether the running lights go on. Mine went on when I only put them on, but when I put on the headlights, they went out.

Start at the switch. It may be that you have the wires on the wrong poles -- there are three poles, and it's easy to have your wires on the wrong ones.

I hope that helps.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#3
VC venezia Avatar
VC venezia Vito Venezia
Calistoga, CA, USA   USA
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1968 MG Midget MkIII "Imp"
1976 MG Midget 1500 "Zip Ties"
2003 Ford Focus
In reply to # 3072114 by Brad76Midget I had the same issue when I redid the wiring in my 76 last year.

Also check whether the running lights go on. Mine went on when I only put them on, but when I put on the headlights, they went out.

Start at the switch. It may be that you have the wires on the wrong poles -- there are three poles, and it's easy to have your wires on the wrong ones.

I hope that helps.
Could be as simple as that, I wasn't sure where each wire went. I'll take a look tomorrow.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#4
Brad76Midget Brad H
Salem, MA, USA   USA
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In reply to # 3072132 by VC venezia Could be as simple as that, I wasn't sure where each wire went. I'll take a look tomorrow.

I would bet a finger that is the problem, since the brake lights are operated by a different switch, and you replaced the switch on the dash.

That was the same problem I had too. When I fixed that and checked that it all worked, I marked one wire with a dot from a paint pen, and made a corresponding dot on the back of the switch, so I won't run into it again.

In reply to # 3072109 by VC venezia Thanks, and I'm sorry if this is just a cloned topic. I just don't want to resurrect old topics as such an act is frowned upon in most forums.

Oh, right...you'd almost think repairing our MGs is such an ever-changing and fluid science that what someone offered as an answer to someone else with the same problem four years ago may not still be a valid answer or something today...THAT is an issue about which I've never read a good explanation.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#5
Ormolu Avatar
Ormolu Barrie Jones
Montgomery, Powys, UK   GBR
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1955 MG TF 1500 "Emily"
1973 MG MGB GT V8
It sounds like a bad earth (ground) to the lamp. Open the boot (trunk) and trace the BLACK wire that comes from the back of the rear lamp. Either lamp will do. Run a temporary wire from the BLACK terminal to a good earth.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#6
TBLACK49 TOM B
ALAMOGORDO, NM, USA   USA
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1977 MG MGB
You might like to check the fuse sometimes the clips that hold the fuse in place stretch and the fuse doesn't make contact!

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#7
Ormolu Avatar
Ormolu Barrie Jones
Montgomery, Powys, UK   GBR
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1955 MG TF 1500 "Emily"
1973 MG MGB GT V8
It is not the fuses. There are two lighting fuses on the Midget, one for the driver's side and another for the passenger side. You have a problem with both rear lights but the front ones are both OK. If a fuse was blown you would lose one rear light and one front light.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#8
VC venezia Avatar
VC venezia Vito Venezia
Calistoga, CA, USA   USA
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1968 MG Midget MkIII "Imp"
1976 MG Midget 1500 "Zip Ties"
2003 Ford Focus
In reply to # 3072163 by Brad76Midget
In reply to # 3072132 by VC venezia Could be as simple as that, I wasn't sure where each wire went. I'll take a look tomorrow.

I would bet a finger that is the problem, since the brake lights are operated by a different switch, and you replaced the switch on the dash.

That was the same problem I had too. When I fixed that and checked that it all worked, I marked one wire with a dot from a paint pen, and made a corresponding dot on the back of the switch, so I won't run into it again.

In reply to # 3072109 by VC venezia Thanks, and I'm sorry if this is just a cloned topic. I just don't want to resurrect old topics as such an act is frowned upon in most forums.

Oh, right...you'd almost think repairing our MGs is such an ever-changing and fluid science that what someone offered as an answer to someone else with the same problem four years ago may not still be a valid answer or something today...THAT is an issue about which I've never read a good explanation.
I tried all possible wire combinations, and the tail lights didn't turn on regardless. It's most likely a ground issue, I think. What's funny is, I think the tail lights only light up via brakes when I'm moving. Still need to test whether or not this is true. Also, only one flasher light in the back works when I flip the hazard switch. The one on the left. There are no flashers in the front because I haven't installed them.

UPDATE: The brakes DON'T ALWAYS turn the tail lights on. Switch doesn't do it at all. It's a 50/50 chance for the brakes. Strange.



Edited 4 time(s). Last edit at 2015-08-29 11:23 PM by VC venezia.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#9
KenAdkison Avatar
KenAdkison Gold Member Ken Adkison
Shelton, WA, USA   USA
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1975 MG MGB
This sounds like a bad 4-way hazard light switch. Both the turn signal and brake lights power go through it and it is a common problem.
The very best schematic is at: http://www.advanceautowire.com/mgb.pdf
If you have LED tail lights, insure you have a aftermarket load compensating flasher.

Check for power at the flasher socket. I think the 4-way flasher is mounted in the console by it's switch and the turn signal flasher is next to the wiper motor, to the right of the glove box.
Ken

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#10
refisk Rick Fisk
Frankenmuth, MI, USA   USA
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Except Vito's car is a Midget, not an MGB.

In reply to # 3073067 by KenAdkison This sounds like a .....<snip>.....
The very best schematic is at: http://www.advanceautowire.com/mgb.pdf
.....<snip>.....
Ken

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#11
Brad76Midget Brad H
Salem, MA, USA   USA
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This helped me a lot while sorting all of my wiring. I bookmarked it on my phone and had a good reference as I was crawling around in the car.

http://www.mgexp.com/article/lucas-colours.html

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#12
VC venezia Avatar
VC venezia Vito Venezia
Calistoga, CA, USA   USA
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1968 MG Midget MkIII "Imp"
1976 MG Midget 1500 "Zip Ties"
2003 Ford Focus
Well, I discovered that the tail lights work if I either use the brakes or the rocker switch ONLY when the car is going downhill. On flat ground it doesn't work, only rarely and it sometimes flickers, especially with the switch. What would that indicate? Ground?



Edited 1 time(s). Last edit at 2015-08-30 03:39 PM by VC venezia.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#13
Brad76Midget Brad H
Salem, MA, USA   USA
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That's weird, and I think it means something is grounding that shouldn't be.

How had the switch you replaced go bad? Did it crumble, like it had gotten too hot?

I'm wondering if there's a wire behind the dash that is bared of its insulation, which may have run through the switch, and it's just touching something.

Can you get a look at that part of the harness to see?

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#14
VC venezia Avatar
VC venezia Vito Venezia
Calistoga, CA, USA   USA
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1968 MG Midget MkIII "Imp"
1976 MG Midget 1500 "Zip Ties"
2003 Ford Focus
In reply to # 3073330 by Brad76Midget That's weird, and I think it means something is grounding that shouldn't be.

How had the switch you replaced go bad? Did it crumble, like it had gotten too hot?

I'm wondering if there's a wire behind the dash that is bared of its insulation, which may have run through the switch, and it's just touching something.

Can you get a look at that part of the harness to see?
I'm not sure what happened to the stock switch. It just stopped working entirely. Which wire would I be looking for under the dash? Anything from the switch that seems abnormal? Also, is there a way to remove the dash or something under? It is very cramped.



Edited 1 time(s). Last edit at 2015-08-30 04:25 PM by VC venezia.

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Re: Tail Lights Don't Turn On When Light Switch Used; Only When I Use Brakes
#15
Brad76Midget Brad H
Salem, MA, USA   USA
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In reply to # 3073333 by VC venezia
I'm not sure what happened to the stock switch. It just stopped working entirely. Which wire would I be looking for under the dash? Anything from the switch that seems abnormal? Also, is there a way to remove the dash or something under? It is very cramped.

I am not thinking about any wire in particular, but if the switch melted or crumbled, the heat would build up from a short nearby. And, if it moves with gravity, the harness back there may wiggle a little around the gauges.

But, it may also be in the trunk, where the wires are also able to move around and may short against something. I would check anywhere they can move around with gravity or when the car moves.

Finding a fault without a continuity tester is going to be nearly or completely impossible.

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