MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Question about radio potentially draining battery

I have just recently started working on restoring my 1978 MGB and, while I helped my dad rebuild/restore a similar '78 B while I was in high school, we never ran across this problem.

The gentleman I bought the car from had previously let his son drive it and the son replaced the radio. No biggie but the radio NEVER turns off. There is a power on/off button on the radio but it doesn't work. Even when the car is off and I push the on/off button, the radio stays on. Because of this, I've had to replace the battery twice since I bought the car in May.

My question is this: I went to work on the car last week and the engine wouldn't turn over, which is no surprise, so I tried jumping it off my other car, which usually works. Not this time. This time, the engine wouldn't turn over...nothing. Could the radio have COMPLETELY drained the battery? And if so, short of ripping the radio out completely (which I'm prepared to do but really don't want to as there are other things I need to concentrate on where the car's concerned) what can I do? Would taking this radio out and putting a new radio in (thus making sure the wiring is done correctly since the guy I bought it from did tell me his son didn't exactly know how to work on the MG) stop the battery drain?

Sorry if these are stupid newbie questions. Like I said, my Dad and I owned a '78 MGB 10 over 10 years ago and, while I remember how to work on an MG, I remember how to tackle the BIG STUFF, like the carburetor, alternator, clutch...things like that...but we never had any problems with the radio. And what problems we had with having to jump the car off was due to the alternator or starter and not the battery itself.
D. Jackson

Firstly the radio is obviously faulty if it can't be turned off.

Secondly on a 78 it should be wired to the accessories circuit of the ignition switch so that even if the radio is inadvertantly left on when you turn off the car and remove the keys, that will turn off the radio as well. Without either of those then yes of course a radio left on will eventually completely drain the battery, although how quickly depends on various factors, there maybe something else draining it as well.

Thirdly whilst flattening a battery is not good for it and will shorten its life, simply recharging it should have been enough, not replacement twice in 5 months.

Fourthly even if the battery *were* completely flat, the use of jump-leads should still have got the car started again, assuming they were used correctly and the donor car had a fully charged battery. So you would seem to have at least three problems to deal with, but maybe more.

The first thing to do is find where the radio gets its power from and disconnect that to stop any further drain, you can think about reconnecting it later. It may be wired to a purple feeding the courtesy light behind the console.

Then I'd disconnect the battery (always remove the ground connection first and reconenct it last for safety) and recharge it off-car. When fully re-charged you need to check for any further drains, the radio may not be the only one. Reconnect the 12v cable to the battery, and instead of reconnecting the ground cable conenct an analogue voltmeter on its *voltage* scale in its place to reveal any drains. It is normal for an alternator equipped car to show a few volts on the scale, which represents the microscopic leakage of the alternator diodes. If this is all you have there are no other drains. But if you see a full 12v registered you have other drains. Make sure you have everything switched off like courtesy lights, ignition etc. and if still present unplug the alternator and remove the bottom fuse between the brown and purple wires. If the drain drops to zero the drain is on the purple circuit. This feeds things like boot light, courtesy light, map light, horns, clock, and the time-delay buzzer.

If the drain is still showing you will have to start disconnecting brown wires from the main lighting switch, ignition switch, ignition and starter relays and finally the starter solenoid until it goes.

When you have found and fixed any other drains you can reconenct the radio supply to the white/green wire which lurks behind the centre console and this will ensure the radio is off when the key is removed.
Paul Hunt 2

This thread was discussed on 06/11/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.