engine dies when unhooking battery, alternator dead right??

Foxfan88 · Apr 23, 2006

Hey last night my volt guage went to like 8 and the battery light came on.

so i parked it, unhooked the battery and it died. This means a bad alt right?
i washed the engine earlier that day so maybe i killed it doing that,

it always was weak and never put out enough, then after elec fan and pullies it got bad. SO maybe it was going bad from the start.

Roland69 · Apr 23, 2006

sounds like you need an alt.

Roland69 · Apr 23, 2006

oh yea and while your at it go for the 3G I'm going to when my alt takes a poop

_jb_ · Apr 23, 2006

Never disconnect your battery when the car is running. The jump in volts could short electrical components.

You could take it into Autozone and have it checked.

Daggar · Apr 23, 2006

_jb_ said:
Never disconnect your battery when the car is running. The jump in volts could short electrical components.

You could take it into Autozone and have it checked.

I'm not sure why folks keep saying that. Our alternators are internally regulated and the EEC is capable of surviving a spike more than twice what our electrical systems are able to produce. Has anyone actually seen an EEC or other components destroyed this way?

Edit: It seems to me, that jump starting a car would be allot more of a threat than disconnecting the battery, particularly if the cause of said dead pony was a shorted battery or alternator internal short.

Roland69 · Apr 23, 2006

I don't know why but my dad always told me if I was boosting someone and my car was Fuel injected to leave my headlights on so I always have but never understood why.

Roland69 · Apr 23, 2006

Oh and for testing alternators I have always tested the voltage of the battery with the car off and again with the car running .

HISSIN50 · Apr 23, 2006

The battery acts like a damper to the system. I've read that one can see like a 40 volt spike running without a battery in the system. I cant back this up with firsthand experience - because testing is free, I do that instead.

I know quite a few folks have done this old-school method of testing and were apparenly alright. But I wouldnt say that it will always turn out that way. The risk is not just to the puter, but to all electrical devices in the system (radios, etc etc).

Just my humble two cents.

Roland69 · Apr 23, 2006

HISSIN's 2 cents is like our Dollar (worth alot more) I mean by that that his 2 cents is worth more then he probly knows

Mustang5L5 · Apr 23, 2006

I've blown an EEC in a Mustang during a jump start.

Don't ask me how but it happened.

jrichker · Apr 23, 2006

Daggar said:
I'm not sure why folks keep saying that. Our alternators are internally regulated and the EEC is capable of surviving a spike more than twice what our electrical systems are able to produce. Has anyone actually seen an EEC or other components destroyed this way?

Edit: It seems to me, that jump starting a car would be allot more of a threat than disconnecting the battery, particularly if the cause of said dead pony was a shorted battery or alternator internal short.

The sudden removal of the load from the output causes a spike in output voltage just before the voltage drops off.

As far as having a filter in the power supply input of the common A9L computer, there isn't much of a filter network except a couple of caps to dampen some of the high frequency electrical noise.

For any kind of power filter, I would expect a 10 MFD soild cap, a toroid coil and a .1 MFD cap on the back side of the toroid. That isn't the case with any of the computers I have opened up for inspection.

See the attachment and you'll see what I meant. It is part of the actual A9L schematic. I will be glad to share the link or the diagram with anyone who is interested.

Roland69 · Apr 23, 2006

^^^ another one who's 2 cents is worth more to us than he probly knows

Daggar · Apr 23, 2006

Although I'm aware that there is a spike, what I'm not aware of are any spikes sufficient enough to cause elsectrical damage. Anyone ever see damage as a result of disconneting the battery on a Fox?

baskin · Apr 23, 2006

An alternator is regulated, but how it’s regulated is what makes the difference. The alternator’s comprised of basically four components, the armature, the field winding, the diode bridge, and the regulator. The armature generates an AC voltage that’s dependant on rpm. The current, however, is dependent on the magnetic field that it sits in. Since the final output voltage is a product of output current and load, it’s then dependent on that magnetic field. The regulator then adjusts the final output voltage by adjusting the current through the field windings, which determines the magnetic field that the armature sits in, which determines the output current and, in the end, the output voltage. The problem with disconnecting the battery lies in the fact that it takes 2-3 thousandths of a second for the magnetic field generated by the field windings to collapse, so even when the regulator shuts off the current, the armature output current remains what it was for a short time. The battery is a very low resistance source, so as the load changes, the battery keeps the voltage stable.

Keep in mind that although 2-3 thousandths of a second seems to be a short period of time, it’s excruciatingly slow when compared to most electronic circuits, and although it may not damage the EEC immediately, the alternator can easily spike to 120V and exceeding the breakdown voltage can very well lead to a failure at a later time du to damage to one or more of the P/N junctions of the components.

blackstangt · Apr 23, 2006

also check your wiring, i changed my battery and alternator once and it turned out to be the wire going from the starter solenoid to the alternator, which corroded because it's right next to the battery.

Foxfan88 · Apr 23, 2006

battery was fine, alternator was fine, but i felt it was bad still, or starting to go bad. replaced the alt and still doesnt show much

the guage is down by the red line almost, but the car acts fine, acts like its getting good voltage.

Roland69 · Apr 23, 2006

did you use a volt meter to see what your battery volts are not running and agian when the car is running.

baskin · Apr 23, 2006

Check the ground wire from the engine to the chassis. The gauge reads chassis voltage, which won't effect the battery voltage or how the car runs

_jb_ · Apr 23, 2006

Just a thought, but since you have UD pullies your power will be lower than a normal setup.

Foxfan88 · Apr 23, 2006

ok guys i got it! it was wire going from the starter solenoid to the alternator.

corroded so i spliced some new wire in. it shows up higher on the guage now. But still acts like its not quite up to par. Battery is fine and the Alt is new, How much do UD pullies really affect it. its on the 2nd white line from the bottom.

I have UD pullies, I had the UD alt pulley but now i have a factory sized pulley (changed it today). But its still underdriven since the UD crank pulley is still on the car.

i might try and go back to stock pulleys and see what happens.
but is there anything else to check that would affect it?

what sucks is now i have a new alternator that i dont need. My old one was fine. But i have already used the new one and the core is at the shop so its almost impossible to get it back.

well i got a new alt

engine dies when unhooking battery, alternator dead right??

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