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MG MGB Technical - Electrical Gremlin
Have run into one of those Lucas Gremlins in my 67 BGT. Charging system is working perfectly. Car starts and runs fine. Can run with lights on with no problem.
The car can sit a week and start right up. The next time within a day, the battery is low and won't start the car.
I have taken Dave Dubois advice and hooked up a meter inbetween the neg post and the cable. It shows a minor drain which goes away when I unhook the radio and the alternator. Radio is a later model with a chip.
I have turned on and off all the electrical equipment and the meter goes back to zero flow each time.
I would believe there must be a wire or switch that is sporatically causing a drain.
|Bruce - How much is a "minor drain" caused by the alternator? It shouldn't be much more than about 0.025A (or 25mA). If it is much mor than that, then the alternator is probably the culprit. Good luck - Dave|
I agree with Dave, though it doesn't explain why the fault comes and goes.
It is possible that the battery isn't always receiving charge while the car is being driven, so that it is sometimes being left in a low charge state at the end of a run. A voltmeter on the dash (even if only temporary) might be informative, and is relatively easy to connect.
It just might be 'sludge' in one battery cell which sometimes shorts that cell out.
Is the top of the battery clean, and are the acid levels correct ?.
I assume that you have taken a read directly off the battery when it was "low". Without that info, I'd say you might start at the cable connections to see if you have a voltage drop across one or more connections.
|Check your trunk light. Sometimes the switch can allow the light to come on intermittently with the lid shut.|
|Jim. The MGBGT does not have a trunk, nor a trunk light. Les|
|Yes, of course on the GT trunk light, but to chime with Jim, I had my 80 roadster with a 2 day old new battery go dead. By accident at night in a motel parking lot, I saw the trunk glowing. Sure enough, the light trip switch was loose, and not d-cing the circuit when trunk lid closed. "Just amazing" I thought at the Prince's inventiveness, and one more naggle for the books. I taped up the switch to "way off all the time" on the spot and have deactivated trunk lights on the others. This business is tough enough without these whacky things going on. Jeez! Vic|
|An alternator should only exhibit a drain of micro-amps. This is best viewed with a meter connected in place of the battery ground strap (always the ground strap, regardless of battery polarity), switched to its *12v scale*. This is for safety, a meter on a voltage scale will only pass micro-amps in the event of a fault whereas an ammeter represents almost a dead short and will cause high currents to flow and sparks to occur in the event of a fault. An alternator should indicate just a few volts on the meter, if it shows 12v there is a significant drain, which may or may not be enough to flatten a battery over a few days.|
If the car can be left for a week, start OK, then the battery is flat next day I'd be suspecting the battery as much as anything. It could equally well be bad connection anywhere in the heavy current circuit, and these *are* notorious for being intermittent.
|Paul Hunt 2|
|Ok, I am not much closer to this one after a day of invesigating.|
Get 12.75 volts across the battery terminal and also at the selenod battery connection. Checked the battery with a tester and it checks out ok.
I took off the ground terminal and hooked the meter up. Reading is 11.75 volts and dropping.
If I pull the light switch the voltage jumps up to 12.5 (this is without the ground terminal connected). Voltage continues to drop.
Pulled the radio plug and the voltage goes to 0.03.
Can;t believe the radio could drain the battery. So for now, I will leave the radio unplugged and see if the battery drains again
I went through this with my MGC when I added a modern (10 years ago) Sony FM/Cassette unit. The car immediately had battery draining problems. After mis-diagnosing everything else possible I removed the one "always-hot" wire that maintains the radio station programming memory and never had a problem again. It just drives my wife crazy that she has to tune the station every time I shut off the car. I still can't believe it either, but it WAS the radio!
|I think you have a fault in your battery. Voltage figures have little meaning on their own. Get the battery load tested and you will know its condition for sure.|
|Actually, I have a modern am/fm/cd player (Sony) in my VW bug. It sort of has two off posotions. If you hit the off button the radio or whatever is playing turns off but the clock stays on. You have to HOLD the off button down until the clock turns off or the battery gets eaten up.|
No, its not the battery. I've had the radio (and battery) on there for over three years. Car runs well, starts on the first crank, and charges fine. Found out that little bit of trivia after a couple of times hearing click-click-click and checking for drains in the system.
|I am truly humbled to those that really understand the electrical issues. A very frustrating thing to diagnose. |
Luigi: You may have something. This is a panasonic and it has that same feature. I will try this. If not, I will get rid of the always hot wire.
As Kelvin pointed out. It is hard to believe a little digital radio can drain the battery as quickly as it does.
Iain. I plan to have the battery load tested this weekend also.
The rasio is hooked to the accessory side of the key and also the battery side of the key. I noted that there is are two unused large brown wires on the wiring harness in about the midle of the dash *67 BGT). Wonder if this would be a better "Batter" connection?
|Took the ground terminal off where? Measured the voltage where? There is no way a battery would maintain 12.75v connected, but suddenly start losing voltage when disconnected (unless it chose that very moment to fail). I suspect you disconnected the battery then measured the voltage on the wiring. Depending on what electronic circuits are connected like alarms, radios etc. it is quite possible that the falling voltage you are seeing is an electrolytic capacitor in one of those units gradually discharging into the meter and other loads, although if an interior light was still connected it must be a big capacitor. The fact that the voltage dropped virtually to zero when you disconnected the radio tells me that the capacitor was in the radio. As such I think you are on the wrong track, and if the battery *is* being drained then either it is from some other cause, or if from the radio then the radio is faulty. Just because the starter won't turn over the engine you can't assume the battery is flat - although it is often a cause. Are the lights still bright when the car won't start? Or do they dim right down? If the former then the battery certainly isn't the problem. If all power seems to die then it could still be a bad connection at the solenoid, battery posts or battery ground connection. You need to measure the voltage on the battery posts (not connectors) while this happens to determine if it is battery or connections.|
|Paul Hunt 2|
This thread was discussed between 29/10/2005 and 03/11/2005
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