jwscab

Hey, anyone got more detailed info about the limp home mode chip, (redundant fuel device), like a datasheet or internal operation or something? I'm interested in putzing around with the chip, but the resistor networks that are in the ecms and memcals leave me baffled. I'm an EE, but for the life of me, I dont know why the resistor networks are configured the way they are, and I haven't found much info on them. resistor voltage dividers with multiple pull-ups to vcc? doesn't make any sense, unless they are some kind of feedback networks..........why do they make it like that???? I've seen some blurbs on diy-efi, and some websites here and there that try and measure the values, but I'm looking for more in depth stuff. thanks!

RBob

iTrader: (1)

The resistors control the injector PW based on the various inputs. To reverse engineer the RFD measure the NETRES pins between both VCC and ground and make up a chart of the values found.

Then drop the ECM onto a test bench, remove the EPROM (to force limp mode), and hook a scope to an injector output (with a dummy load).

Then just temporarily add a resistor and note the changes. Then change each of the inputs (MAP, IAT, CTS, TPS, etc.) and see how each is affected. Two things to look for is the change in the injector PW slope vs. an input. And a change of a knee in the slope.

The actual overall slope can be changed, then where the knee in the slope is can also be changed.

RBob.

jwscab

Yeapp, thats what i figure I'll have to do, I was just throwing out a request to see if anyone knew for sure what the architecture inside was. I kinda figure it to be a bunch of multivibrators inside, with the cal resistors used to set pulsewidth, then pretty much or'ed together, all clocked by the distref. I was just thinking maybe someone had more concrete info, rather than stabbing at each resistor, like, if in fact it is built like I mentioned, I would know, oh, ok, these resistors are for this one-shot, etc etc.....I've seen some stuff on DIY-EFI, I think Grumpy did, at one point, which does sum of some of that stuff.....thanks again....

RBob

iTrader: (1)

I didn't know which ECM you were interested in, yes, on the diy-efi site there was info listed for the '7747 ECM. Which would be the same as the '8746 (LO3 TBI). If you can't find it let me know I should have it someplace.

It would be interesting (actually great) to get some info on the RFD chip. Maybe a de-cap and probe?

RBob.

jwscab

yeah, popping the top off would be sweet. Its a shame, my uncle had the right tools at work a few years back, but hes not working at that place anymore.

Another thing I'm gonna do is it probe the inputs to the chip, where the cal resistors are with a scope. Should see some kind of activity, a time constant could show something like a one-shot, and gain between resistors could be an amp, if not, probably a voltage reference to a comparator or something....hmm...i gotta get that test bench finished.

i'd like to look at both a C3 and a P4 chip....the C3 one is simpler, I think, might be the best place to start......if you do come across the info, just shoot me an email at jwscab@comcast.net thanks!