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MG MGB Technical - Deac Bat = Jump = Smoke = ?

Car is a 72 BGT. Managed to leave the lights on when I got to work Wednesday morning. Went to start the car after work an battery was dead (light switch in on position). Called AAA and got a jump start directly on the battery.

Here is the problem and puzzle. First time the jumpers were hooked up started to get smoke from under the hood after a minute or two (might have been trying to start car, but don't remember for sure). Immediately disconnected and opened the hood but could not see source of smoke. Checked polarity of jumper and it was correct. Reconnected and after a couple of minutes was able to start car with no more smoke. After another couple of minutes disconnected jumper and car immediately died. More trying and determined that the car would only run with the jumper connected, and that the tach was not reading but the other gauges were functioning correctly.

I have now pulled the alternator and there is definite signs of overheating at the end of the heavy brown wire at the spade connection to the alternator. I cannot be sure that this was not already in this condition before. Battery is on a charger and is slowly coming back.

Diagnostics so far:

Have pulled air pump (for access) and alternator.
Checked wiring diagram. Battery hot terminal connects to starter solenoid and then branches to the alternator, ignition, fuze box, headlights, and hazard switch. With a partially charged battery the ignition switch appears to work, headlights and hazard lights work. No appearance of problems in the fuze box.

My question is what might have smoked? My initial diagnosis is that the alternator shorted out at first. When the jumper was disconnected the short also went away but left the alternator DOA.

Any other ideas? Comments?

Oh yes, I will be installing the buzzer between the light circuit and the door light switch to warn me not to leave the lights on again. 8^)

Thanks

Larry

Larry Hallanger

Larry. Your diagnosis sounds accurate. Internal short within the alternator caused the smoke. Battery discharged so that it would not run the engine with the alternator not functional--hence the engine died when the jumper cables were removed.

I would fully charge the battery. Remove the brown wires from the main terminal of the starter solenoid and leave the large cable from the positive terminal of the battery still hooked up. Hook up the plug to the alternator. Connect the clip of a test light to the large terminal of the starter solenoid, then, touch the probe end to each of the brown wires. If there is current, as in a shorted out alternator, the light will illuminate. If there is no current flow, the light will not.

If you find current flow on the wires to the alternator, assume the alternator is bad. If you find current flow on the brown wire going to the fuse box, assume there may be some other short in the system and continue checking until you can isolate it.

Les
Les Bengtson

Chain autoparts stores offer free testing of alternators.
Kimberly

Reverse connection of jump leads (or battery) would blow the diodes, and cook the brown wires, with smoke, until they *did* blow. After that there will be no more excess current to cause smoke, but the alternator will be dead as well which is why the engine cut-out as soon as the jump-leads were disconnected.

It is quite a co-incidence for the alternator to short out just as you attached the jump leads, but not impossible. I would need some convincing if it were my car that the leads weren't connected the wrong way round to begin with. You say the tach wasn't registering either, if that is still the case then that confirms reverse polarity of the jump leads as that will have blown the transistors as well.

You should never make the final connection to a battery terminal in case one or other of the batteries explodes, it should be made to somewhere remote like the engine. Personally I very briefly tap the last connection on and off again just to see if I get a spark. You *will* get a small spark if one of the batteries is flat (same with a charger) but if you get a big flash you know you have a problem.

You will need a diode in circuit as just conencting a buzzer between the parking light circuit and the courtesy light circuit will put a drain on the battery while parked, you might like to have a look at http://www.mgb-stuff.org.uk/wn_electricsframe.htm and click on 'Lighting' and 'Interior lights and lights on warning'.
Paul Hunt 2

I just finished reparing a'72 GT with the same symptoms. The car had been fitted with a single 12 volt battery by a PO who had used a red cable as the ground. You gussed it. The gentleman assumed it was the positive lead and when jumpded his wife commented that smoke was coming from under the bonnet. It fried the alternator and melted the large brown wire running from the alternator, through the loom, to the starter solonoid. The difficult part was in peeling back the loom, removing the burnt wire, replacing it and then rewrapping the loom. After replacing the alternator, and black taping over the red ground cable, the car runs like new. Ray
RAY

Have found and fixed the problem. It appears that the main power connector on the brown wire at the alternator was not tight, which I discovered while installing the new alternator this morning. This, combined with what is basically 30 amp wiring in our old cars just couldn't handle what I suspect was a 100 amp jumper current into the dead battery. Heat at the bad connector probably caused (the already marginal) old alternator to fry. Anyway, new alternator and clean tight connectors and all appears to be well.

A suggestion for those of us who do have to jump start our cars on occasion. I have a battery cut-off switch installed on the ground side and I suspect that IF I had turned this off before jumping the car this problem probably would have been avoided. Of course the problem of the poor connection at the alternator would have popped up at some much more inconvenient time.

HTH

Larry
72 BGT daily driver
Larry Hallanger

Whilst there may well have been 100 amps going into the *starter* while cranking from the jump-leads, there would be no-where near that much going into the batteries (which in any case wouldn't have been going through the cars wiring anyway if the jump-leads were attached to the battery as stated), and none at all through the brown lead on the alternator at least until it started. Even then it would hav been no more than the rating of the alternator, which the wiring is designed to cope with.

Whilst a bad connection at the alternator *would* cause heat and volt-drop, and hence a reduced output to run the cars electrics and recharge the battery during normal running, there is no way that would cause enough heat to damage the alternator or, I suspect, do that amount of damage to the brown.

I'm not sure what you mean by the reference to a battery cut-off switch as far as jump-starting goes. If you had one and it was off, then connecting the jump-leads directly to the battery (as was stated) would not allow you to start the car. It would only have put a relatively low charge current into the battery, and would take some time to put enough in to start the car off your own battery, which would need the switch to be turned back on anyway. If you had the switch off and somehow connected the jump-leads to the starter, it would not have made any difference to reverse connection. And whilst it would allow the car to start and crank you would *have* to turn the switch back on before disconnecting the jump-leads, or risk doing more damage due to over-voltage. As such there are more risks than benefits, and any battery cut-off switch *must always be on* before connecting jump-leads and attempting to start the engine. Recharging the batteries whilst still in-car and with the charger connected directly to them is the only time you *might* have a cut-off switch off.
Paul Hunt 2

If anything a high impedance at the alternator would protect it from current surges on jumping. I agree with Paul H that this is most likely to have been reverse polarity. The counsel of perfection is to check the brown wire all the way to the alternator in case of hidden damge within the loom.
Stan Best

Paul,

When I initially read your post about the buzzer, I couldn't imagine the necessity of the diode as I've been using my chime for two years without a diode and it works fine.

Having thought about it, I realized the courtesy lights are hot all the time since the switches only interrupt the ground, thus the ground pigtail on my courtesy lights simply becomes a hot lead in a series circuit going through the courtesy filament, going through the Radio Shack "chime" unit I bought, back through the filaments in my panel light circuit, ultimately finding the ground on the panel light ground.

Clearly, you are right! No question about that. So, why does mine work? Here in the US North, my GT has been in storage (battery connected) for over a month. After reading your post, I started it - just to reconfirm. It showed no signs of battery drainage.

Again, my own sense tells me you have to be right about this. I'm not challenging you. But I am curious. Do you suppose my "chime" unit is polarity sensitive? I'm running all-halogen bulbs in my panel lights, but I'm not aware that they are polarity sensitive.

Any ideas what could be going on here?

Many thanks, and Happy Holidays,

Allen
Allen Bachelder

Allen

I assume that you have the same Radio Shack chime that I just installed in my B, "Model: 273-071 Typically used in car alarms, the 80dB Chime operates on 6-18VDC, carrying 125mA current at 12V." I included the diode in my installation but noticed when checking the chime operation prior to installation that it only worked when connected to the correct polarity. I did not check for reverse current flow in the "backwards" configuration.

FWIW

Larry
72BGT
Larry Hallanger

Paul

I agree totally with your logic. Only one question: IF the jumper polarity was reversed would the car start and run with the jumper connected? And what effect would it have on the battery, which has taken a charge and is operating the car with apparently no problem (I drove it to work this week).

Thanks

Larry
Larry Hallanger

On your chime, if internally it is polarity sensitive (and anything but a simple buzzer almost certainly is) it almost certainly has a protection diode to stops reverse current flowing through it, which is what happens when connected between the courtesy light circuit and the parking light circuit when the lights are off and the door closed. If it didn't then the reverse current would probably destroy the electronics in the chime!

Larry - Ignoring the battery for a moment, there is nothing to stop an engine starting and running with reverse polarity, just as you don't *have* to change anything but the tach electronics when convering from +ve ground to -ve ground.

Connecting two charged batteries together +ve to -ve and -ve to +ve would result in a huge current and possibly exploding batteries, if the jump leads could carry enough current. When the cars battery is flat, it will still take some current out of the donor, although nowhere near as much, but I still wouldn't like to try it! I don't know enough about the chemical reactions in a flat battery when voltage is connected correctly and incorrectly to say any more than that.

Is the tach working again or still faulty?
Paul Hunt 2

Paul,

Many thanks for your explanation. I don't recall much detail about my installation - whether the chime leads were marked + or -, or if I just did it randomly, with a 50% chance of getting it right. By my nature, if one lead was marked + I would have connected it to the panel lamp (white/red) circuit, with the other lead going to the ground switch at the door. But the current back-flow problem was not on my mind, so I did it right for the wrong reason.

I also believe it is possible to charge a totally flat battery with the wrong polarity and it will function that way - but not well or for long. This happened about 20 years ago, so I don't recall the details. I was either jumping or charging a low battery in a '67 B (+ ground). When I tried putting the jumper (or charger) leads(+) to (+) and (-) to (-) as marked on the battery top, I got huge sparks, so I tried reversing it and it worked. I wondered if the battery was mismarked at the factory, but local garage mechanics told me it was possible to charge any completely flat battery the wrong way, although this was certainly not advisable.

FWIW,
Allen
Allen Bachelder

Paul

Tach did not work when running in the "jumped" mode. Working correctly now. I have come to believe that the AAA jump was with reverse polarity. So much for depending on AAA to be right.

On the chime, a pack of 2 diodes is only $0.99 from Radio Shack. Cheap insurance at twice the price 8^)

Thanks

and HAPPY HOLIDAYS to everyone!

Larry



Larry Hallanger

This thread was discussed between 14/12/2007 and 20/12/2007

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