Rain and Power

I just purchased my first MG and am a mechanical newbie, but I am hoping someone can help with my electrical problem. When I get caught in the rain I am having a problem with the electrical system charging. I can hear an electrical humming noise when the car idles and can drive 15 - 20 minutes before I drain the battery. This is happening only when it rains. After I charge the battery and on dry days the humming noise and the charging paroblem is gone. Any ideas on where to start to look?

The first suspect would be the wiper motor and/or wiring. I'm assuming that since it's raining, you've got the wipers on - so that would be an obvious variable. Fifteen or twenty minutes - even if your lights and wipers are on - is a pretty serious battery drain (especially if your charging system is working properly).

Actually the 15-20 minutes is with only the lights on. I have been driving with the wipers off the few times I have been stuck in the rain since I noticed this problem.

Paula: I am afraid that your car's problem may be more than we can diagnose from a distance. However, my guess that the "humming noise" you hear could be electrical arcing underneath the car. I haven't a clue where this could be coming from, but check the wires at the starter for openers as these wires are mounted low enough on the car to suffer from rainwater on them. Other sources could be the fuse box (can't imagine how rainwater would get on that, though) or the battery main cables. It has to be something that gets wet while you are driving in the rain. I'd love to see this one and try to figure it out, though! Sorry I can't do more. AL Bradley

Paula, first what year is the car? Second, is the red alternator light on the dash staying on all the time?

What happens when it's NOT raining and you have your lights on?

It is a '77 MGB.

When it is dry and the lights are on I can't hear the humming noise and it charges without any problems.

Sorry, forgot to mention that the alternator light does not come on in either case, wet or dry but the dash lights slowly dim. After I stop the car, I only get a clicking noise when I try to start it.

Check the altenator belt. If its lose, when wet it may not even turn the alt. under load...

It sounds like your alternator is charging ok but you can't know without checking it with a voltmeter, which you should have in you minor toolkit any case.
As other will point out, it very well could be a case of poor electrical grounds. The typical ones are the battery's negative cable where it meets the body in the battery compartment, or the multi-connection one the right side of the engine compartment. Several black ground wires come together there, near the fuse box, and are screwed to the inside of the fender.
Check the batteries negative cable connection first.

I agree with Al on this one. If there's goo or dirt on the posts of the starter, getting it wet will cause an intermittent short and discharge the battery. Crawl under there and clean all the goo off, and check that the connections are good and tight. Disconnect the battery before you do this, don't want to let the smoke out!

Paula,

You need to isolate the humming sound somehow such as, if the humming continues while the car is just sitting and idling, open the hood does the sound increase? The sound is coming from somewhere. I think when you find the source of the humming; you most likely will have found the problem. You may not need to wait for rain, use a water house and wet areas down till the humming starts. Could the humming be coming from your radio when this electrical problem occurs? Let us know what you find?

Again, with your keen intellect and rapier-like analysis of the situation, I think you may have nailed it.
Glad you switched to "Iggy".

I will try the hose trick as soon as I get the car back from the shop. I had to bring it in because it starts to misfire and die when the engine warms up. I wasn't able to figure that one out, but I am hoping to solve the this on my own. Thanks for the ideas on where to start!