Warning, typical long boring Carl post alert! Skip now if you value your eyes!
Our stock injectors were about 16-17 ohms; in the injector world I think down to about 12 ohms is considered high impedance. These can be driven with a simple npn transistor used as a saturated switch, so if the alternator voltage is 13.8 volts and the drop across the transistor is about 0.2 V, there is 13.8 - 0.2 volts = 13.6 volts across the injector and at 16 ohms that means the current is 13.6 V / 16 ohms = 0.85 amps and the power dissipated in the injector is 13.6 V * 0.85 A = 11.6 watts. That's a small enough amount of power that even with no fuel flowing through the injector to cool it, it can survive indefinitely without burning up, so the simple driving scheme is adequate. Low impedance injectors are down around 1.2-2 ohms so let's use 1.5 ohms for example. If you use the same drive scheme you now have 13.6 V / 1.5 ohms = 9.1 A flowing through the injector (all of these currents and powers assume the transient response is over and everything has reached steady state) and now the power dissipation is 13.6 V * 9.1 A = 124 watts. That's enough to burn up the injector if no fuel is flowing, and enough to get pretty hot even with fuel flowing, so the simple saturated switch doesn't work so well anymore. It can be used (I have done it in a few ecm's with Siemens 72's) but it ain't the best way. Instead, a peak-and-hold driver is used. This is a chip (or circuit) that ramps the current up from zero to 4 amps over the first millisecond to get the injector open, but then it cuts the current back to 1 amp which is enough to hold the injector open but which cuts the power dissipation back to 1 amp * 1 amp * 1.5 ohms = 1.5 watts, a nice safe low value for the rest of the time the injector is open. This works because it takes more magnetic force to yank the injector plunger up than it does to hold it up, so you can turn down the current and thus the magnetic force to save power. As Larry Niven put it, however, TANSTAFL (there ain't no such thing as a free lunch
). In this case that equates to lots more power dissipation in the ecm. With the stock set up the dissipation in the driver transistor is 0.2 volts * 0.85 amps = 0.2 watts which is less than the indicator light bulbs in the dash. With the peak-and-hold drivers, however, they now have 13.6 volts - (1 amp * 1.5 ohms) = 12.1 volts, times 1 amp = 12.1 watts dissipated in each driver. That's 12.1 / 0.2 = 60.5 times the power disspated in the stock ecm (and a total of 6 * 12.1 = 72.6 watts, equal to an old fashioned 75 watt light bulb in the lamp on your desk), which is why you frequently see people posting here asking if their modified ecms are supposed to run so hot after a long wot blast. Yes, they are, it's basic physics. This all assumes "linear" circuit operation. You can get fancy and use high frequency switching schemes just like in a switching power supply, and cut the power dissipation in the ecm, but now you have to worry about electronic interference and more expense and so far as I know no one is using such a scheme in the popular aftermarket ecms or in modifed stock Buick ecms; I mention the possibility just for completeness.
The peak-and-hold drivers can drive high impedance injectors just fine, so a modified ecm will work with either kind of injector safely. If you open up a stock ecm there are six injector driver transistors lined up along the right side bolted to the case as a heat sink, along with a few other things. Each of these has three leads. Now open up a typical ecm modified with peak-and-hold drivers for low impedance injectors in place of those transistors. Each of those drivers has five leads, and there are six large resistors wired one to each driver, plus a snubber diode, so there are parts glued to other parts and a fair amount of wire going from part to part so once you have seen one it is easy to tell at a glance if an ecm is stock or modified. I hope this answered your questions sufficiently.
On the count of three you will awaken feeling refreshed and much more knowledgeable. One ... two ... three!
