T-Series Electrical Problem: headlight switch turns starter

[jiml]

i have a 1976 mgb with an electrical problem. the headlight switch will engage the starter. all wires are correctly attached. i installed a new starter relay that i purchased at Advance for less than $5, however, i see Moss sells them for $25 or more. did i make a mistake here?
any suggestions will be appreciated.

 

[Mickey Richaud]

Hello, jiml -

Welcome to the Forum!

Wish I could help you but wanted to alert you that you'll probably get more response if you post your question in the MG forum as well.

Be sure to introduce yourelf in the "New Member Introduction" Forum as well.

Mickey

 

[Dave Russell]

Re: Electrical Problem: headlight switch turns sta

 

[borjis]

Re: Electrical Problem: headlight switch turns sta

You turn on the headlights and the starter turns?

Wow! thats a new one.

Do the headlights come on?

 

[Mickey Richaud]

Re: Electrical Problem: headlight switch turns sta

Hello, jiml -

Welcome to the Forum!

Wish I could help you but wanted to alert you that you'll probably get more response if you post your question in the MG forum as well.
Mickey

I think he IS in the MG forum?
D

Yep, he is. Thread was moved from "BCF Wiki" Forum after I posted this...

 

[Kimberly]

Re: Electrical Problem: headlight switch turns sta

Wiring diagrams are available at https://www.advanceautowire.com click on stock schematics.
Make yourself a couple of enlarged copies of the wiring diagram for your car.

Something is not connected properly or you have a couple of damaged wires. Damaged insulation on the white/red wire and the blue or red/green wire maybe allowing the headlight circuit to energise the starting circuit.

What color wires did you connect to what terminals on the relay?

Do you have a test light or a multimeter?

 

[Rob_P]

Re: Electrical Problem: headlight switch turns sta

I had a similar problem on my 76 B where the Light switch would turn on the ignition circuit (although it would not engage the starter).

Mine was caused by bad wiring from the DPO.

At the fuse panel on the passenger side fender under the hood, the lights were plugged into the ignition circuit.

If it were me, i would disconnect the starter, turn on the lights and follow the power with a test light/volt meter from the starter to the light circuit. Eventually you should find the culprit.

Rib

 

[AlanT]

Re: Electrical Problem: headlight switch turns sta

I just noticed this post. You may already have the problem solved. But if not.....First of all,did the replacement of the starter relay change anything or is it still doing the same? If you still have the problem then post back and I will tell you what to do next.

Alan T

 

[jiml]

Re: Electrical Problem: headlight switch turns sta

this problem existed until i replaced the starter relay with one purchased at Advance Auto. once the new relay was installed all lighting systems worked and the light switch did not activate the starter solenoid. feeling confident that the problem was resolved i started the engine with the ignition and flicked on the headlight switch. the headlight switch activated the solenoid making an awful sound. i flipped the switch off to deactivate the solenoid, the engine continued run fine. i can start the engine with the ignition. if i turn off the ignition and put on the headlight switch, there are interior lights and tail lights but no headlights.
thanks/jim

 

[AlanT]

Re: Electrical Problem: headlight switch turns sta

OK..Here goes.. Assuming this is a SIC car and pre-electric cooling fans. The problem is most likely in one of the blocking diodes. There should be two diodes in close proximity behind the dash that are located about where the passenger's left knee would be. The culprit is most likely the one with green with pink trace on one end and white with red trace on the other. Check the diode to see that it is still a diode and not a connector. Or just disconnect to diagnose. I think you will only lose a warning light in the process. Let me know what happens.

Alan T

 

[jiml]

Re: Electrical Problem: headlight switch turns sta

the diode is there without any wires connected to it. a white/red wire is connected to the green/pink wire by a 1/2"x1" blk lucas connector. i didn't find a second diode.

 

[Kimberly]

Re: Electrical Problem: headlight switch turns sta

Have you checked the wiring going to the headlight switch? Disconnect the the white/red wire(s) going to the starter relay. Turn on the headlight switch and check the voltage on the white/red wires you just disconnected. If you have voltage, then the white/red wire from the ignition switch is crossed with the headlight wires. You are going to have to physically and methodically check the wiring. Look for incorrect connections, burned or damaged insulation.

Make an enlarged copy of the wiring diagram to chart your progress.

Failed diodes supply the starter with power from the green circuit. Your problem seems to involve the red and blue circuits (lighting), not the green circuit. Check to make sure someone didn't mix up the white/red wire (starting circuit) and the red/white wire (part of the lighting circuit) and connect them together under the dash.
Methodical testing is the only way you are going to know for sure.
Wiring diagrams are available at https://www.advanceautowire.com click on stock schematics

 

[DrEntropy]

Re: Electrical Problem: headlight switch turns sta

the diode is there without any wires connected to it. a white/red wire is connected to the green/pink wire by a 1/2"x1" blk lucas connector. i didn't find a second diode.

but you may have found the problem! That "connection ~NEEDS~ the diode in place. A direct connection will start the car with the lighting switched to "ON" every time!

 

[AlanT]

Re: Electrical Problem: headlight switch turns sta

Does that connector happen to have a part # on it? Something like 83225 (AorB)? Maybe a flat plastic looking piece with 1/4" male and female spades?
And is your service interval counter still hooked up to the speedo cable?
Alan T

 

[Kimberly]

Re: Electrical Problem: headlight switch turns sta

the diode is there without any wires connected to it. a white/red wire is connected to the green/pink wire by a 1/2"x1" blk lucas connector. i didn't find a second diode.

but you may have found the problem! That "connection ~NEEDS~ the diode in place. A direct connection will start the car with the lighting switched to "ON" every time!

How? The red and blue wires are independent of the green circuit. The red and blue wires (lighting circuits) are energized when the headlight switch is closed. If the diode is missing, and the white/red wire is connected directly to the green/pink wire, the starter will engage whenever the ignition switch is turned to the run position. The green wires get power from the white wire circuit which is energized when the ignition switch is turned to the run position.
jiml's complaint is the starter engages when the headlight switch is turned on. Meaning either the red or blue wires are supplying power to the starter. jiml is going to have to physically check connections and the wiring to make sure everything is connected properly and not damaged.

 

[AlanT]

Re: Electrical Problem: headlight switch turns sta

OK,I've held out long enough. But first I must tell you that the last time I heard of this problem was about 1985 and the time before that I was a technician at the BL dealer. The following is what I remember. The Service Interval Counter goes closed circuit every 25000 miles . That turns on the EGR light on the dash or console. If the diode fails , the circuit feeds back through the EGR bulb all the way to the starter relay. It has to be the 'Perfect Storm' though. That is why it happens so rarely. The same thing happens to the later MGB's with the handbrake. Pull up the handbrake lever and the starter engages. The car that gave us so many problems was about six feet away from a tree that was struck by lightning. I wish they had told us that to begin with.

Alan T

 

[Kimberly]

Re: Electrical Problem: headlight switch turns sta

OK,I've held out long enough. But first I must tell you that the last time I heard of this problem was about 1985 and the time before that I was a technician at the BL dealer. The following is what I remember. The Service Interval Counter goes closed circuit every 25000 miles . That turns on the EGR light on the dash or console. If the diode fails , the circuit feeds back through the EGR bulb all the way to the starter relay. It has to be the 'Perfect Storm' though. That is why it happens so rarely. The same thing happens to the later MGB's with the handbrake. Pull up the handbrake lever and the starter engages. The car that gave us so many problems was about six feet away from a tree that was struck by lightning. I wish they had told us that to begin with.

Alan T

The EGR/service counter lamp has a green/pink wire going to it and the dash brake lamp has a green/orange wire going to it. There are diodes connecting white/red wires to the two above lamps. If either diode fails then the green circuit will energize the white/red wires causing the starter to engage. Look at the wiring diagram for a '76 at https://www.advanceautowire.com click on stock schematics.

The green circuit and the lighting circuits have nothing in common. If closing the headlight switch causes the starter to engage, then the blue or red wire circuit is energizing the starter, not the green wire circuit.

As far as the diodes go, I have looked at the wiring diagrams for several years and I have come to the conclusion that the diodes allow the white/red wire to energize the EGR lamp and the brake dash lamp when the ignition switch is in the start position, a bulb test feature. Therefore I have advocated the removal of the diodes and insulating the terminals on the wires to prevent shorts, because of what may happen when they fail.

 

[jiml]

Re: Electrical Problem: headlight switch turns sta

it's just the way you described the connector and the counter is connected.

 

[DrEntropy]

Re: Electrical Problem: headlight switch turns sta

ISTR a circumstance just about like AlanT recounts. At about the same year, too. No diagram in front of me now but there was feedback tru the lighting circuit engaging the starter. A thorough inspection of the connections and a diagram are indeed the only way to sort this, agreed. And likely diode replacement.

 

[Kimberly]

Re: Electrical Problem: headlight switch turns sta

ISTR a circumstance just about like AlanT recounts. At about the same year, too. No diagram in front of me now but there was feedback tru the lighting circuit engaging the starter. A thorough inspection of the connections and a diagram are indeed the only way to sort this, agreed. And likely diode replacement.

I do have a wiring diagram in front of me.
How does diode failure in the service counter light circuit or the brake warning light circuit affect the headlamps and taillights?
The circuits are independent of each other.

This what happens when the diodes fail.

Service counter lamp diode:

Service counter reaches a specified mileage point and closes illuminating the lamp. If the diode fails, feedback from the green/pink wire energizes the white/red wire causing the starter to engage. The green/pink wire gets power from the green circuit which gets power from the white wire circuit.

Brake warning lamp diode:

The brake warning lamp is illuminated when the handbrake handle is pulled up closing the circuit between the green wire and the green/orange wire. If the diode fails, then feedback from the green/orange wire energizes the white/red wire causing the starter to engage.

Removing the diodes and insulating the unconnected terminals will have no effect on the electrical system with the following exception, the bulb test feature when the ignition switch is in the start position will be removed.

The exterior lighting circuits are completely independent of the white/red wire circuit and the green wire circuit.

Exterior lighting is wired as such;

brown wire > headlight switch > branches to two circuits,
1.) the taillights, dash lights, and marker lights - red/green wires, red/white wires, and red wires.
2.) the headlights - blue wire, blue/white wires, and blue/red wires.


If everything is connected the way it is suppose to be, the only way I can see that the exterior lights would have any effect on the starter circuit would be that if the insulation on one of the white/red wires and one of the wires in the lighting circuit were damaged allowing the lighting circuit to intermittantly energize the starting circuit.

If someone had worked on the wiring under the dash, then who knows without looking if a couple of wires are cross connected, especially the white/red wires and the red/white wires.