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MG MGB Technical - More Drained Batteries!

Here's another story similar ot Mack's regarding a drained battery with a few different twists. I have a 77B that I recently finshed painting and have just put everything back on the car. Initially, it would not start, and for some reason the starter had gone bad over the last 3 years while it was being restored. I got a new starter, and it fired up so I drove it around a bit to make sure everything was working. The next day the battery was dead. I have been chasing what I have thought was (is?) a short but I cannot find anything. I have found some dubious parts, and here is what has been replaced:
Battery
Starter
Battery cables (+ and -)
Rectifier in Alternator

The latest tests I have done:

TEST 1) Disconnect connector from alternator, disconnect ground cable from battery, connect voltmeter between ground cable and -ive terminal:
- 12V reading initial reading.
- disconnect hot line from stereo amp to + connection: 12V reading
- disconnect clock and stereo: 50mV reading
- disconnect fuse # 4: reading jumps around between 150mV and 50 mV

- I traced all of the brown wires, changed a few connectors that looked dirty. - no shorts were found, no signs of insulation worn through.
- Traced the purple wires from the same fuse, and they checked out OK.

TEST 2) Disconnected the connector to the alternator, turned the ignition on, ran a wire between the brown and yellow wire from the alternator to ground, and got the ignition light to come on. (Ignition light does not come on when ignition is turned on. It's been so long since I drove the car I don't remember if this is supposed to come on when starting the car or not?)

TEST 3) Hooked up the alternator again and started the car. Checked the voltage between the brown wires at fuse #4 and ground. I am getting about 10.5V at idle, and no increase in voltage when I rev the engine up to about 4000 rpm. (I started the car with the help of the battery charger, so the battery was not fully charged to 12V. I would still expect the alternator to kick it up to 13V-14V though.)

I recently had the alternator checked out and the guy said I had a bad rectifier which he fixed ($66). I am thinking about taking the alternator back in and watching him check it out to make sure it's putting out some juice.

Any suggestions on where to go from here?

Thanks,

-Dan
Dan Scharpf

Dan. A lot of information. First, yes, the ignition warning light should come on when the key is turned to the run position. If not, there is a problem. One quick and dirty check is to fully charge the battery, leaving the battery post connectors off, then let the battery sit for a day. Reconnect and try to start. If the battery is dead, bad battery. If it starts, it is a problem elsewhere. If your battery has also been sitting for three years, it may well be bad. But, your alternator should also be putting out 13.5 to 14.5 volts when it is charging, so you probably also have an alternator problem. On the later model cars, there is always a slight drain on the battery from the clock. Some stereos also are constantly hot. A battery, in good condition, should be able to go more than a week with these slight drains on it and still start the car strongly. I believe Paul Hunt's website has some good troubleshooting tips on how to find a short. But, I suspect battery and alternator are your primary problems. Les
Les Bengtson

Have you tried disconnecting the +ve lead from the (charged) battery, and connecting an ammeter in series to determine if there is any current flowing out when everything is apparently turned off? This may throw further light on to the problem. If there is current flowing, you should be able to trace it's path to the point where it is being consumed. if there isn't any current flowing, I would suspect the batterys.
Phil

Thanks for the comments. I do have a new battery in the car, and I did the check the battery itself. It can hold a charge overnight when nothing is connected. I went and had the alternator bench tested again this morning and it's putting out about 13.5V.

Things are now pointing to the wires running to the alternator since I am not getting the ignition light to come on when the car is turned on, and I am also not getting the 13.5V when the car is running at high rpm.

Here are the questions I have now:

1) I will try the current test between the +ive battery terminal. What kind of numbers should I expect from a "good" system vs. one that has a drain somewhere?

2) It seems that if I can get the ignition light to come on by touching the brown/yellow wire at the alternator to ground when the connector is off the alternator, then somewhere within the alternator the loop isn't being closed. Does this make sense? Does the alternator just use its casing as the ground? The only wires going into it are 2 browns, and the yellow/brown.

3) Other than checking the voltage at the battery, how can I check to see that I am getting the 13.5V out of the alternator since I can't get to the alternator terminals when the connector is attached?

Thanks,
-Dan

Dan Scharpf

1. Rather than trying to measure current put a *voltmeter* in place of the battery ground strap - much safer for instrument and self if there is a big drain. If this shows 12v you have a drain. 6 volts or so is normal and is the unavoidable leakage from the alternator diodes. If you do see 12v unplug the alt. If it goes then you have one or more alt diodes short. If not then it is elsewhere, remove the purple fuse (interior lights, horns), then various browns to track it down.

2. Yes and yes. Almost certainly your alternator is bad.

3. Measuring the voltage on any brown should show you the battery voltage, there are plenty of these around the fuse-block. Purples will do so too - as long as the fuse is good. If the ignition is on so will whites (unfused) and greens (fused).

PaulH.
Paul Hunt

Here's the latest update on the drained battery:

While reading some old notes I noticed a sketch I made of the alternator connector. Apparently after the rebuild I had the connector on backwards which switched the brown/yellow and the thin brown wire on the alternator.

Good News: The ignition light came on as it should before the car was started, and the battery maintained 12V overnight.

Bad News: There is still only 12.4V when the car is running, even though the alternator puts out over 13.5 when bench tested. The ignition light has not come back on since the initial cranking when the connector was switched to the correct way.

Hopefully I'll have some time this weekend to continue more of the diagnostics, but there is still something screwy going on.
Dan Scharpf

This thread was discussed between 17/09/2001 and 20/09/2001

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