Tuning '94 chev Suburban Big Block

[delcowizzid]

we dont veiw voltages but the resistance withen the netres chip that changes the voltage on the pins.TBI to pfi v8 actually looks like it needs a couple of mods with resisters to be factory correct. anyone care to measure the resistance of the pins above on a TBI memcal to see if it has the 7.5K between the 2 pins of the v8 memcal setup

[rsicard]

we dont veiw voltages but the resistance withen the netres chip that changes the voltage on the pins.TBI to pfi v8 actually looks like it needs a couple of mods with resisters to be factory correct. anyone care to measure the resistance of the pins above on a TBI memcal to see if it has the 7.5K between the 2 pins of the v8 memcal setup

Thanks much for the diagram. Don't understand the purpose for the resistors in the Memcal. Will try to measure the resistance between the two pins on the Memcal. Heard something about the Memcal providing the limp home function of the ECU.

[rsicard]

we dont veiw voltages but the resistance withen the netres chip that changes the voltage on the pins.TBI to pfi v8 actually looks like it needs a couple of mods with resisters to be factory correct. anyone care to measure the resistance of the pins above on a TBI memcal to see if it has the 7.5K between the 2 pins of the v8 memcal setup

Don't understand HOW netres chip voltage dividers are converted to discriminate 2nd or 4th Distributor Reference Pulses. Which two pins does the netres resistance need to be measured. Need a memcal pin layout reference to start with to be certain where to measure.

[dave w]

The only Memcal information I have is for the '7727, '7730, & '7749 ECM I downloaded from Thridgen.org years ago. See attached pic. I DO NOT KNOW if the '7427 Memcal is exactly same? I'm not sure it's a good idea to post the pic of a different Memcal? I strongly suspect that GM would not re-invent the wheel when it came time to design the '7427 Memcal. It's likely GM would leverage / re-purpose a Memcal it already has / had in production for the '7427 Memcal with maybe a different component (limp home mode) on the Memcal for TBI?

dave w

[1project2many]

we dont veiw voltages but the resistance withen the netres chip that changes the voltage on the pins.TBI to pfi v8 actually looks like it needs a couple of mods with resisters to be factory correct. anyone care to measure the resistance of the pins above on a TBI memcal to see if it has the 7.5K between the 2 pins of the v8 memcal setup

I have made a series of measurements including TBI memcals and posted the results. TBI memcals show 7.5k between the two pins.
http://www.gearhead-efi.com/Fuel-Inj...ull=1#post3374

The images above are for specific chips in the memcal. Those chips are connected to numbered pins on the memcal. Understanding how the ecm uses voltage on pin 56 to determine fueling mode involves studying the ecm schematics and / or having access to GM documentation. It may be a distraction if the goal is simply to make your engine run well.

Here is some work done by Cliff Harris which shows the memcal pinouts which correspond to the specific chip in the pictures above. Cliff's work has been very beneficial to this hobby. Notice how pins 53, 56, and 56 work together to provide the desired voltage for fueling mode select.

http://www.misterpeachy.com/VettePics/MEMCAL-ECM.pdf

[95GMC_sierra]

Gents,

I am doing a similar conversion on a 454 and the edelbrock chip worked OK but high end performance was bad and ran lean at high RPM's Since the original TBI to MPFI conversion I have rebuilt the motor with significant changes and could not get it to run decent at all. I then came to this forum and was directed to the memcal jumper and PCM resistor shorting solutions and I did my homework on them. As I understand them...


Memcal jumpers

he memcal changes the threshold voltage on a comparator chip within the PCM which changes the DRP to injector bank firing number. Once this memcal mod change is done, the base pulsewidth at idle and battery voltage at idle vs pulsewidth adjustments are needed to compensate for the halving of the injector firing per cycle. I was told this is the best way to use the bank fire application since it insure a better mixture at idle, and prevents the changing of the firing sequence at idle that occurs in the TBI mode.

current sense resistor shorting

The current sense resistors are tied to ground on the low voltage side of the driver switch FET in series with the injectors and the injector drivers. The jumpering of the current sense resistor adds about 600 Milliamps of drive current at 12 volts and about 750 milliamps of drive current to the injector driver circuit at 14 volts by reducing the overall circuit series resistance by about a 1/2 ohm. The low impedance peak/hold TBI injectors are about 1.5 ohms each and the bank of 4 saturated high impedance (16 ohms each with 4 in parallel) injectors presents about 4 ohms to the driver circuit. The ability of the saturated injectors to turn on consistently and rapidly is a function of the drive current and the shorting of the sense resistor adds additional current to insure a rapid and repeatable turn on time of the saturated injectors coil. The current sense resistor shorted enables the injector coils to transition to fully open with less voltage drop/heat developed across the injector coil. The rapid saturation and shorter/crisper/faster switching time of the coil alters the actual pulsewidth (vs. commanded Pulsewidth) of the injector fuel pulse which requires initial tuning to compensate. I used an oscilloscope and a clip on current probe to analyze the current waveforms through the injectors with and without the sense resistors shorted and the shorted resistors were firing repeatably with a consistent rise time versus voltage. With the current sense resistors installed n(stock condition), the leading edge of the current pulse to the 4 injectors was slow and inconsistent, and varying pulse to pulse. The varying leading edge and "tearing" of the current injector pulse waveform means the coils were not opening reliably and repeat-ably.

I endorse the shorted current resistors and the associated retune required after viewing the current pulses on the O-scope.

I am returning to tuning this engine now and will keep you all up to date on the results. 1995 GMC 454 (30/100ths over (460 ci)), 7427 PCM with $31 mask, edelbrock MPFI with 36 lb pico injectors, edelbrock performer RPM heads, Thorley tri-wye headers and 3 inch single exhaust.

john

[JeepsAndGuns]

Nice info. Glad I did the memcal and resistor mods when I did my conversion.

[dave w]

Nice info. Glad I did the memcal and resistor mods when I did my conversion.

What he said X2!

dave w

[rsicard]

Really enjoy reading the post by 95GMC_sierra. It is very interesting. Have put my 454 suburban TBI to PFI Edelbrock conversion on the back burner. Right now concentrating on the 1995 Chevrolet C1500 pickup with 350 V8 to be swapped out for 350 Vortec with 454 throttle body. May just end up putting higher pressure capable in-tank fuel pump and use 94-95 TBI two original refurbished injectors. Then tune 16197427 ECU for this combination. Just need the appropriate TBI fuel regulator spring for the higher pressure.

[rsicard]

Gentlemen:

Would like to resolve the TBI to MPFI (batch) injection situation wherein jumpering from 2nd DRP to 4th DRP. Going back to March of 2013 the following was posted:

"Mod'd the 16197427 ECU. Put jumps across the two current sense resistors. Also put a jumper on the Memcal per the photo from HP tuners. Installed the ECU in the Suburban and started it up. Wow does it run shitty. Shut down the truck, removed the ECU, opened it up and removed the Memcal jumper, re-installed it in the Suburban. Ah, back to running smooth. Don't know the pin layout of the Memcal so I will stay away from jumpering it for now. Set up TunerPro RT to record BLM values at idle and off idle. Open loop at idle, it fills the BLM cells with 128. Set up TP to record off idle BLMs. Ran the Suburban for several miles to warm it up, varied the speed, RPMs, gears to fill in as much of the off idle BLM cells as possible. Then came back and parked at the house. Went back to idle BLM map, varied throttle to fill in as much BLM cells as possible. Captured all with MS Excel spreadsheet."

Getting better at tuning GM TBI 7427 ECMs. Trying to reason what happened when going from 2nd DRP to 4th DRP. Wow dId it run shitty. Must have been too lean? Still trying to figure out why it would run lean.

Any ideas you fellows? Please advise. Thanks.

Rolly