I have been going back and fourth on using a 749 or 730 ECM for running 8 low-Z 2.5 ohm P+H injectors. In the end, I chose to go with the 730 ECM for several reasons.

1) widely available
2) requires one addition injector driver and two resistors.
3) has the TCC output driver

To use the P+H properly (min turn-on, min turn-off) times you need to add a couple of resistors. Since the pins come out to the edge connector (car plug), then they can be installed here.
One addition injector driver needs to be installed. I am going to use a vanilla 60V, 45A, 2-4V Vg_turnon FET. I don't see any reason to go with anything fancier than that. The ECM has an onboard FET driver (i.e., drives more than a digital output)

When installing the jumper wires add a .070 to .100 resistor (2 watt minimum) from the wire to ground. I am going to use a .070 resistor in my application just to boost the injector current a little bit).

I see no reason to update the injector driver as I see some people have done. The 730 running $8D code flows about 8*(14/12) =~ 4.5 amps. The 730 with P+H will be flowing about the same amount of current on average during the injector on-time. Of course this 4.5 amps is per injector driver. Yes, LC power exists but I don't think it will be a large contributor.

All of this needs to be verfied on a testbench before applying it in a real application. Mainly, to look a the FET temperatures. I plan on using a 2.5 ohm resistor and 3mH inductor as an equivilent injector on the bench. Total cost to upgrade is approx. $5 and parts are widely available.

Sound reasonable??

 

The second injector FET is driven from a chip that does not exist on the '730. IIRC it is U15? Possibly a 44 pin PLCC (haven't looked at it for a while).

The reason for upgrading the FET is for heat dissipation. Hold mode generates a LOT of heat.

RBob.

 

Thanks Rbob. I am going to look tonight in a 730 to make sure all of the surrounding passive components are not missing also. I thought just the FET driver was missing. The 749 would be better if I have to install passive stuff.

I totally forgot about the hold mode needing to drop approx 14 - 2.5 = 11.5 Watts * duty_cycle across it. Yeah, that is a lot of heat. I should probably look around for a FET with a reasonably small Rds value. Thanks for pointing that out.

J

 

I did a few quick calculations to figure out what kind of FET I need to drive the injectors.
The ECM seems to shut down all digital outputs if the vehicle voltage hits 16.9 volts. Using that figure as worst case shows that the Fairchild FET that has been mentioned on this board with worst case injectors (2 ohms) shows that the FET is guaranteed to operate up temps of 185 Faren. degrees.
Using another Fairchild non-ultraFET (non digital gate) will work up to 160 Faren. degrees. It is a sub $2 part avail. from Digikey. I forget the part number but if I decide to use it I will post later.
Assuming the standard ECM FETs are generic early 1990's equates to them working up to about 140 Faren. degrees.

All this was done assuming that the FET case to heatsink had zero thermal resistance and that the heatsink to air had zero thermal resistance. So the the "real" operating temp will be lower than this. A guess is about 30% lower if thermal compound is used when they are installed.
If thermal compound is not used then installing one of the "best" Fairchild FETs is worthless because it would dissipate heat only as well as the factory injector drivers. I wonder how many used thermal compound on thier install.

I haven't check to see if the 730 has all of the passive components installed for the second injector driver. I got too caught up playing with the steel tonight. Maybe tomorrow night I can open up an ECM and take a look.

J

*** EDIT: The above was calculated using 80% duty cycle ***