With the post about Internal or external voltage reg.I share this info. The external unit uses a mechanical relay that turns the voltage from the alternator on/off to the battery.The internal uses a zener diode or avalanche diode as it is sometimes called.This is a small resistor looking electronic unit that acts as a relay.When you use jumper cables to start a car with low battery,the "good battery"car senses a charge is needed whne the cables are attached and goes into charge mode.Once the low battery car is started and the cables are removed that voltage must go somewhere.On the mechanical unit the relay opens to stop the charge mode.The internal unit passes it thru the zener diode.Its a pretty hard hit that the mechanical can handle.The zener,not as forgiving,takes a beating.The way to save the zener from damage is simple.Turn off the "good battery" car before you remove the jumper cables.This will save premature failure of the internal VR.

:classic: Thanks . Some vary good information. I do have a question on the process of jump-starting . I always attach the jumpers from the good batt. car + to + of the batt and - to ground on the engine of the discharged car . I have been told that this will help protect the alt. . I have done this several times in the past with out difficulity . Have I just "dodged the bullet" or is this good info. and a method to follow?? Good info on the shutting down of the charged car before disconnect, this I dident know but makes vary good sence.

Ever have an old fuse just pop for no real reason?Being it is made to be the weak link,they just wear out after time.Same with the zener.Thats why your home has mechanical circuit breakers,They are more forgiving.My point was each time you jump a car incorrectly it takes a little less "mileage"away from the zener.As for your neg to engine question,That is because the battery gasses are combustable.When the connection is made you do not want a spark to be made at the battery that could cause it to explode.You make the connection away from the battery to eliminate the danger potential.

I am aware that hydrogen gas is emitted from the batt. and is vary explosive , that's why its wise to disconnect the neg. lead first. I was of the impression that connecting the neg. to the ground of the secound cars engine that in some way helped to safeguard the alt of the power supply . Gary

No as they say" ground is ground" (guys I am aware of the difference frame vs chassis ground--. it's just a saying!!)Your post reminded me over a story I think you will enjoy.In the days of metal bumpers I had a loaded pickup die in a bad location for battery access.The fix was to connect the positive and neg jumper cables together to form one long single jumper.We pulled the vehicles together until the bumpers touched.Then we connected the long jumper to the battery positives.I know that goes against the spark lesson of earlier,but we didn't want the arc to damage the bumpers like a little welder.Cool huh? Sometimes I out cleaver myself!!!

/? ROFL