ecm mod

[BlackBandit]

I was just wondering if someone could tell me what resistors need replaced in the ecm and what resistance resistors to use in order to make my ecu compatable with low z injectors. TIA

 

[ijames]

It's not that simple. The injector drivers need to be replaced with peak and hold drivers and some components (mostly resistors) added that they need. Bob Bailey (TurboBob on this board, you can go through Mike Licht at Full Throttle Speed&Style, a board advertiser, if you want) and Red Armstrong (Quad Air in Ohio, he's not online) both do this mod for about $125.

 

[norbs]

Just use it as is. It will take at least 6 months of use until it frys. if your in a jam just use it, bring a spare with you toavoid the tow

 

[BlackBandit]

I'm not really in a jam. I just figured that since electronics is my job I'd just do it myself so that I could save the downtime of sending out my ecm and spend the money I saved on something else......like groceries.

 

[bruce]

Originally posted by BlackBandit
I'm not really in a jam. I just figured that since electronics is my job I'd just do it myself so that I could save the downtime of sending out my ecm and spend the money I saved on something else......like groceries.

Just as a matter of working on an old car, having a spare ecm around isn't too bad of idea.

 

[BlackBandit]

Originally posted by bruce
Just as a matter of working on an old car, having a spare ecm around isn't too bad of idea.

I'll definately start doing that. I keep an extra crank sensor, and I bet ya'll can guess why! Especially out here where everything is freeway and traffic everywhere. Never really cared if I broke down back in good ol' Michigan. By the way, what is a "peak and hold" driver. Is that a differant circuit than stock or just different caps, inductors and such?

 

[ijames]

The simple way to drive a coil is with an npn transistor that is either off or fully on so it is saturated and the current is limited by the resistance of the coil. That's how the stock "high impedance" injectors are controlled, and hi in this case means about 17 ohm for stock and anything over 12 or so for bigger replacements. With 13.8 V at the alternator, assuming no voltage drops in the harness, and Vce(sat) of 0.2 V that means the stock power transistors carry (13.8-0.2)/17 = 0.8 A each while the injectors are open, after all the inductive transients have died away. As injectors get bigger it takes more power to quickly open them, and the next step the manufacturers went to are "low impedance" injectors, about 2 ohms. If you use one of these with the stock driver, the current will now be (13.8-0.2)/2 = 6.8 A (again, after being open for a few milliseconds so the transients are done). I've heard that the stock transistors are rated at about 4 A, and some people have them blow immediately and some get them to last many months. It probably has to do with how much time you spend at wot, since that's when the pulse widths are long and the current is at maximum the longest. One approach is to go to bigger transistors, but this has a couple of problems. Lots of power is being dissipated in the injectors which can only be cooled by flowing fuel, so if you run them dry there is the risk of burning them out. Also, it takes a lot of time for this current to die away when the injector is turned off and so the closing time suffers. As a practical matter with 72 lb/hr low impedance injectors on a 3.8 L motor idling at 650 rpm and controlled with a saturated driver the minimum pulse width is about 2 milliseconds which is about what is needed, so the control isn't the best (but it does work). The better way to control the injector is with a peak and hold driver. This is a circuit that ramps the current up to some level (4 A typical) over a millisecond or two (basically controlled by the inductance) to get the injector open, then drops the current to a holding level (1 A typical) to keep the injector open. Then when the injector is closed the current only has to drop from 1 A to zero, instead of 6.8 A, so the closing time is much better and now that 72 lb/hr injector can operate down to about 1.5 ms or a little quicker. Also, much less power is dissipated in the injector, but now the rest goes into the driver in the ecm so it gets much hotter. Oh, changing to a fet driver won't really change anything in saturated mode, just a small difference in voltage across the injector and power dissipation in the ecm. Cherry Semiconductor made the CS452 and CS453 p&h drivers in a 5 pin to220 package; their data sheet gives some good info. The 453 is for the big TBI injectors and provides twice the current of the 452. Last I heard these were discontinued but maybe some other company is second-sourcing them now.

Anyway, that's the short version.

 

[BlackBandit]

ijames,

Thanks for the info. I did some digging and found a couple company's that still have the cs452's and 453's around. Also took a look at the data sheets and everything looks pretty straight forward. If I decide to try this should be able to figure the rest out myself. If not at least I have a better understanding of how the ECM drives injectors. Again, thanks for taking the time to point me in the right direction.