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Electrical problems
#1
  This topic is about my 1957 MG MGA 1500 Coupe
MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
I repainted mga 57 coupe and had to remove headlights, tail lights, and front turn signal lights. I replaced the turn signal lights and the tail lights with new hardware. The headlights I had to cut the wires and remove the complete fixture. I cut the wires such that they only would go back together one way. I spliced them back together and soldered and taped them and installed the head lamps.

In the time it took to paint car the batteries went dead so I did all the re-wiring with no power. I replaced the batteries with one 12 volt battery and kept the positive ground. Everything worked before I started and in theory it should work now. However.... I have no headlights, brake lights, turn signals, or running lights. The 15 amp fuse blows when I press the brake pedal. I removed the bulbs from the rear tail lights and the fuse still blows when pedal is pushed. I thought it was possible I may have one of the 3 wires in the wrong place and by removing the bulb the wires are 'not connected' to anything.
I have no 12v with good fuse to rear lights checking with meter.

I also replaced the high beam floor switch because I had a brake fluid leak which may have damaged the switch. Didn't help any, but did clean up the contacts to those wires. I also disconnected the 'ground' that is connected at the front fender bolt on the right side of the car and cleaned with wire brush to make sure good connect after paint.

Not sure where to look next for problem. Any suggestions would help. Been driving without brake lights for a couple of months now while trouble shooting...

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bobs77vet Avatar
bobs77vet bob K.
northern Va, VA, USA   USA
lets start in the back, how do you know those wires you put in the crimping tubes of the rear lights are not grounding out?

you have a power wire grounding out some where or you have it running directly to ground, do you have a test light?

i would disconnect the rear lights and see if the fuse blows

edit....i reread your post....your brake light power wire at the rear lights is grounded....its grounded .before the light bulb thats why taking the bulb out makes no difference



Edited 1 time(s). Last edit at 2018-02-12 09:40 PM by bobs77vet.

MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
That is the same thought I had.. . the bulbs being removed effectively disconnects the rear lights, however if I turn on the headlight switch the fuse does not blow. Only when I push break pedal. It also takes out my fuel gauge when the fuse blows which is how I can see its related the the brake pedal. I guess my next step would be to find the brake light switch and trace back from there.

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barneymg Avatar
barneymg Barney Gaylord
(Somewhere in USA), Pick one (or more), USA   USA
1958 MG MGA "MGA With An Attitude"
These circuits are related to the green wires on the switched fuse. The short to ground will be found downstream from the brake light switch. For the MGA 1500 see circuit here: http://mgaguru.com/mgtech/electric/circ_f2.htm

Follow the Green/Purple wire from brake switch to relay terminal #5, Notice this connects through the relay to terminals 3 and 7, which go to the rear lamps for brake lights

Disconnect wires 3 and 7 from the relay. Connect a test light to power on the fuse bock, and touch it to relay terminals 5, 3, 7. Those should all be open circuit and make no connection and no light. If you get a light, then the wire from brake switch to relay is grounded.

Connect a test light to power on the fuse bock, and touch it the wires you have disconnected from the relay. Bright light indicates short to ground, Slightly dimmer light indicates correct connection through the brake light bulb filament. This will narrow it down to which rear lamp has the short to ground.

If you connect a hot jumper wire to White/Purple and White/Brown (one at a time briefly), the rear lights should light up. The one that has short to ground will cause a fat spark and will not light up. Following that wire downstream should lead you to the short. This may be a mis-connection of the power wire into a ground connector (black wires), or may be a short of the power wire to ground inside of the rear lamp fixture.



Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com

bobs77vet Avatar
bobs77vet bob K.
northern Va, VA, USA   USA
Barney nails it as usual....you could also disconnect the rear lights and have some one step on the brakes and use a test light to determine the brake light power wires.

did the lights have a clear plastic shielding sheath near the connections to stop the power supply wires from shorting out?

3066james Avatar
3066james Gold Member Jim Cheatham
Amelia, VA, USA   USA
The 15 amp fuse might be part of your problem. I'll have to check but I thought both fuses are 35 amp slow-blow fuses.

Also, after repainting, you need to scrape some paint under the fastener that the ground wires hook to. Just cleaning the end of the wire or the connector on the end of the wire is not enough. Don't ask how I know. smiling smiley

Jim

Zur Avatar
Zur Dave H.
Amarillo, TX, USA   USA
I also put a small smear of dielectric compound (silicone grease) on new, or 'disturbed', or cleaned old connections. This waterpoofs them and avoids corrosion & ensures good contact.
It may be "belt & suspenders" but it makes sense to me!

In reply to # 3680530 by 3066james Also, after repainting, you need to scrape some paint under the fastener that the ground wires hook to. Just cleaning the end of the wire or the connector on the end of the wire is not enough. Don't ask how I know. smiling smiley

Jim



Dave
"WTF?"

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MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
I used a drill with a wire brush on it to clean area down to metal. So that connection should be good. And yes, it was painted over after paint job because I forgot to tape it off and it got painted just like everything else....

MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
Thanks Barney.... I looked at the diagram you posted and will trace and check switch from brake pedal... I had a leak from brake master of fluid and am thinking this is the source of my problems ...

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MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
The bottom fuse is a 35amp slo-blo .... the top one I replaced (a package of 10) was a 15 amp so that is why I came up with the 15 amp fuse. I looked at the manual today to try to see what is suppose to be there and could not find a value for the fuse anywhere. I will have to check other sources to get the proper value. I blew the last spare fuse today testing for shorts ..

MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
I am out of time today for more testing, however I did find that there was no voltage at 3, 5, 7 ... using a volt meter. I connected a hot wire from fuse to 3 and 7 and found that the driver side rear lights up... the other side did not..... I will have to remove the fixture and see what I can see and go from there...

also .... seems my car was wired before they invented color wires... everyone of them is black ...

Stephanie Eddie Lorenzo
Lakeland, FL, USA   USA
I have a 1979 MGB roasters,I installed all the electrical sistem,wire, spark plugs, everything new,coil everything,and doesn't want to start.

bobs77vet Avatar
bobs77vet bob K.
northern Va, VA, USA   USA
In reply to # 3680796 by Stephanie I have a 1979 MGB roasters,I installed all the electrical sistem,wire, spark plugs, everything new,coil everything,and doesn't want to start.

spray starter fluid in carb and try and start it.....if it rumbles you have a spark.....no rumble no spark, make sure your points are not grounded

MikeConlisk Avatar
MikeConlisk Christopher Conlisk
Hephizbah, GA, USA   USA
Thanks everyone.... I now have brake lights and fuse is holding. I found a short on the passenger side under the rear spring hardware where it connects the spring to the frame. The wiring harness was routed thru the tight space between frame and hardware. I moved enough and taped off worn bare wire.

Brake lights now work.... problem with turn signals.. right side works .. left side lights up both right and left bulbs with no blink. ... I was going to try switching the wires that go to the left and right side bulb and see if that is the problem... otherwise I can live with only making right turns...

EDIT: .... I now have turn signals left and right ...front and rear ...couple more things to check and then I can button up the brake light and turn signal problem...



Edited 1 time(s). Last edit at 2018-02-14 06:22 PM by MikeConlisk.

Stephanie Eddie Lorenzo
Lakeland, FL, USA   USA
But the car's was running. One day I started when I was going to move it the engine shut up and didn't start anymore

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