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MG MGB Technical - Weird Electrical Problem

I have a 78B that is basically all original with the exception of an electronic ignition. The PO installed an aftermarked radio and installed speakers in the doors. However, it looks like he did a decent job on the install - used the factory wires and grounded the radio through a ground strap to the transmission tunnel (so, I don't think the ground is a problem). Here's the problem - occasionally, the radio will lose all power. Completely shut off. This will last anywhere from 10 seconds to a couple of minutes, then come back on and work normally the rest of the drive.

Here's the weird part. It happens when I'm downshifting while braking. Not all the time, but, sometimes, as I'm braking to make a turn, as soon as I push the clutch, the radio quits. Less frequently, starting the car does it. I generally turn the ignition switch to acc and wait a few seconds for the fuel pump to build pressure and the radio works. As soon as I turn the ignition full on to start, the radio dies and stays off for a minute or so.

I've traced the wires, checked the connections, stuck a voltmeter and ammeter on every wire I think of and can't find the problem. I'm on the second radio, so it isn't that. At first I thought it might be the brake pedal switch, but, since I'm not depressing the brake pedal when I start it, I have generally ruled that out (although the emergency brake is on). Otherwise, I have no other electrical problems. Any thoughts on where to look next? It isn't a big deal since nothing else is affected, but, it is annoying.
J. Lawrence

That is a strange one, and - as an old radio man once told me - we can immediately narrow it down to one of two possibilities: "It's either the fuse, or something else."

It sounds like the problem is possibly mechanical rather than electrical, since it appears that stressing the motor's position in the car creates the problem. Obviously starting the engine and downshifting do stress the mechanical objects in the car, and it adds vibration and torque to the radio, too. The first thing I'd do is to wire up a 12V bulb, like an aftermarket dome light, to the same places you're getting voltage to the radio, by tapping into the wiring harness close to the radio. You might have to up the size of the radio fuse slightly to run both loads.

This will tell you if voltage is still reaching the radio when it quits. You did not say if the dial illumination on the radio goes out when the radio stops playing, so you may already have the indicator you need. I've seen similar issues that were eventually traced to a bad solder joint inside the radio, so don't assume it MUST be in the car, though the second radio swap DOES point that way.

If you lose power to the dome light, you can move the tapped points back further along the lines to isolate the problem, like getting the dome light ground from the chassis, then tapping the power before the radio fuse, etc.

All troubleshooting is a process of "divide-and-conquer", and the verification of good A+ power and ground is an essential first step.

Best of Luck - - Alec

PS: Just a wierd thought - how long is that grounding screw through the tranny tunnel? Could the transmission case be hitting the bottom of that screw?
Alec Darnall

J. I had a similar problem with my 69 roadster twice. When I slowed for a light and down shifted the radio would go off for about 5 or 10 seconds. My problem was the alternator was on the way out. It seems these newer CD/radios need a minimum of power and when they don't get it they shut down. When I slowed and the revs droped my weak alternator was not pushing out enough juice to keep every thing encluding the radio happy. After getting a new alternator things were fine. For a while. Then the problem came back but this time it was not the alternator. One of the connections that I had made at the alternator (small brown with yellow stripe) broke away. Once I got that taken care of things were back to normal.
Dana Wilson

Thanks guys - you may have hit it.

Alec - The illumination on the radio goes out - it is clear that there is no power getting to the radio. it never ocurred to me to consider the stress on the engine as perhaps straining the wire loom.

Dana - My alternator recently died so that has been replaced. I never connected the two b/c the radio has done it for at least a year, but, the alternator went all at once. The car has been in the shop getting some engine work done, so, I haven't driven it enough to know if the problem still exists since the alternator swap. I should have the car back this week, so, I'll pay close attention and see if the problem still exists. I'll follow up and let you know.

thanks. Jeremy
J. Lawrence

You don't say how new the radio is, but most newer radios need two power sources to work. One will be the main power and the other will be the "memory" power. Also, if it has a removable face, you might check the contacts between the face and the radio. Sometimes they need to be cleaned as well.


Jayme
77 MGB
Jayme

If the alternator isn't the problem, try another "ignition on" source to power the radio. It could just be an intermittent fault at the ignition switch or elsewhere. You can also try wiring the "ignition on" wire to the "battery wire temporarily to rule that out, but don't forget to shut off the tunes when you park the car.
Jeff Schlemmer

I had this problem with my truck. It turned out to be a loose, slipping alternator belt.
Kimberly

This thread was discussed between 14/05/2005 and 15/05/2005

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