Posted by Peter Florance on July 23, 1998 at 02:46:01:
In Reply to: Re: Coil posted by wietse wullink on July 23, 1998 at 00:31:11:
: if I used a DVM in the current measure mode, what should I expect to see???? Understand the fundamentals of the coil and ignition system, but not sure what inductive means???
Well that's the question. The coil is an inductor (wire wrapped around a core). When we were in science class, we wound wire around a nail and put a battery on it. That formed an electro-magnet and you could pick up paper clips with it (I know, what's that got to do with you car?). Well, the magnet field that's formed by current through the wire is stored energy. The same thing happen when the motronic brain grounds one end of the coil, current starts flow, starting at 0, but ramping up for the period of time called the dwell (very important in point ignition, where you had to set that). The reason the current isn't instantly large but ramps up, is because while the coil is building the field, instead of all passing through, it's building the field. When the brain stops connecting the coil to ground, the field collapses and is converted to current, well sort of, only there's nowhere for it to go, so It becomes a large voltage which discharges through the secondary winding (which is in the same field, and it has a path through the spark plugs, and ZAP you got spark!
The point if all this is your DVM could go crazy trying to chase that saw tooth shaped current
/|
/ |
/ |
/ |
___________/ |______________
Current ^ZAP!
______
| |
| |
__________| |_______________
Voltage
> < Dwell time
But the voltage should be a square wave a above. What you may have is a bad coil, but on the secondary side. Low resistances like .5 ohms are tough to measure. Just getting could contact (let alone the resistance of the meter leads) really makes tough. Maybe some with motronic car can measure their secondary to ground and give you and idea. I can check my non-motronic tommorow at work but I don't know how well they compare. If you hook up a dwell meter and you have dwell that's near spec and a stiff +12 volts on the positive coil lead, the coil is bad. Just make sure the +12 volts is really there on the coil while the car is cranking; you could have an open resistor.
Whew!
Peter