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MG MGB Technical - Ingition switch?

Hi Folks - I'm new to this board and the car. My father has a 1971 MBG (2x6 VDC batteries - negative earth) with 58k original miles - that I'm going to buy from him. It has at least one electrical issue - not able to start. By that I mean that there is no response whatsoever to turning the ignition key. The ignition light is not on, there is no response from the fuel pump, or the starter solenoid. The car has been in storage for 5 years. It was run for the 3.5 years, then the ignition stopped. Batteries are quite low - 2 Volts on one, 4 on the other...

Additional indications: no ignition buzzer, door buzzer, hazards, turn signals, etc. The only electrical device that I noticed working was the courtesy light in the dash below the radio knockout.

I took a couple of measurements and here's what I find:

- Starter relay (appears original): expected contacts are open, coil resistance is about 51 ohms.
- Coil (appears original): resistances measured from + to - was 3.4 ohms, + to center or - to center was 6.7 ohms.

Next, working with data from mgb-stuff.org/spanners/electrics I was looking for voltages and find:
- Voltage across both batteries = 6.32 VDC
- Voltage at starter relay with ignition switch closed was 0.6 VDC on the white/red wire
- Voltage at the fuse box with ignition switch closed was 0.7 VDC on the white wire
- Voltage on the brown wire in the fuse box was 3.7 VDC.

My recollection of trouble is that while the car was in storage, the batteries stopped holding a charge and were later replaced. This did not solve the problem and my dad had to unhook a battery lead between runnings. About 18 months ago, the ignition stopped working and he's been unable to run the car since.

Also, he described that the brake test light wouldn't go out, so he disconnected the circuit at the point inside the left side of the engine compartment - some device.

Any comments to help interpret things are appreciated.

Thanks,

Ted
TMC Clayton

Ted: Your batteries are dead. I would start by replacing the twins with a single Group 26 12V, located in the passenger side battery box. The ignition light should now be coming on. If not, possibly the bulb itself is burned out. Next, clean the the connections at the starter solenoid. The main feed from the battery is connected here. The brown wires here carry unfused power to the fuse box and alternator. Consider replacing the fusebox. It is very cheap to replace and a corroded one is the source of most MG electrical problems.
With the car running is the ignition light still on? If so the alternator is not working. Check voltage at the battery with the engine running. If it is under 14 volts you have a problem with the charging circuit.
If your dad had to keep the batteries disconnected then there is probably a short somewhere in the system.

My best advice: Check the "Electrical Help" link at the far left at the top of this MGB Technical Forum page. It is packed with great information that should get you running.

With regard to the brake light, it is triggered by a switch located in a cross tee with brake fluid lines running into and out of it, mounted on the drivers side inner fender. This switch detects a difference in brake pressure between the separate front and rear brake circuits. If there is a leak it will come on, so check for leaking wheel cylinders, brake lines, etc. If there are no leaks it can come on if the brakes were not bled properly. There is a procedure to center the valve described in the workshop manual. Many cars, with otherwise perfect brakes, have this switch disconnected.
Regards, Andy
Andrew Blackley

Hi Andy,

I agree with you that the batteries are certainly dead. Just interpreting the other voltage variance as an indication of some short or other trouble. I'm interested in the possibility of converting the batteries to a single 12 V model. Will a "group 26" fit in the existing opening - or will some modification be needed? I also appreciate your suggestion regarding the fuse box - it's an easy enough replacement. Not worth keeping if it tends to be a source of trouble.

The ignition switch still troubles me, though. I am not able to start the car - even with good batteries - there is no response. I suspect that needs to be replaced before much progress will be made. We are, in fact, unable to jump start the car. Gotta think it's the ignition switch itself.

Thanks very much for your additional comments - it all helps and just having another set of eyes looking over the symptoms is helpful. I really appreciate it!

Ted
TMC Clayton

With decent batteries turn the ignition on and measure the voltages at the bottom two fuses - both should be 12v. If the bottom one is but the other is significantly lower check again at the ignition switch brown and white wires. Make sure the multi-plug by the steering column is pushed together properly, sometime the pins get partially pushed out the back and only their tips make contact.

If the voltage on both fuses is zero or very low you have one or more bad connections. Check on the solenoid where the main battery cables and browns connect, the clamp on the hot battery post and the battery post itself, all these should show 12v with the ignitoion on and about 10v when cranking. Similarly test the interconnecting cable clamps and battery posts if you are still using twin-6v, these should show about 6v and 5v with respect to ground. The battery ground clamp and post should show 0v with respect to shiny metal on the body. If where you should be reading 12v and 6v is reading low, or where you should be reading 0v is high, you have bad connections.

When you have the courtesy light still glowing brightly when the ignition is on and the key is turned to 'start' but doesn't crank we can start looking at the starter circuit.
Paul Hunt

If you decide you want to keep dual 6v batteries, I was told that tractor batteries are fairly inexpensive (at least compared to what your auto store cost you) and you might be able to get them at agricultural stores.
Norman Fair

This thread was discussed between 09/06/2002 and 10/06/2002

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