Rookie Questions & a short story.

[MichaelW]

First, the short story to set up the questions. 7 years ago I stroked a 91 Silverado 5.7, retaining the top end and no computer mods. Runs well enough for a commuter. Empty nest and the urge to try for more HP got me to this point. I have another 383 on the stand, almost ready to go. I'd like to learn the programming end while it's on the stand. I bought an 89 gmc truck for the harness & computer. I also have the GM books for the 91. 89/91 are both 700R4, 7747 comp. While digging into the 89 harness, I discovered several anomalies. I found that most of the inputs and outputs to the 89 ecm differ from the 91 GM wiring schematic. Is that the norm? I expected to see more similarities than differences. I also discovered that the 91 has something called a "hot fuel handling module" Can this be omitted/ ignored if I decide to re-land the 89 pins to match the 91 schematic? I'm leaning hard this way, anyhow. The excuse to keep everything 91 is to keep apples with apples and not use the oranges, since I already have books for the 91.
Please, keep your responses long and drawn out. I have the rest of my life to figure this out :) HP = fun

[1project2many]

The hot fuel handling module is a timer that operates the fuel pump after the engine has been shut off. It's designed to prevent fuel from boiling in the fuel lines after the engine is off which could cause a hard start or even a no-start. It can be omitted but in some areas of the country with trucks that work way too hard it's a good idea to leave it in.

Not sure about all the inputs and outputs being different. Are you working with the same ecm in both cases? Which circuits are in different places?

[MichaelW]

That's good news. I think I can omit/ bypass that module. I was concerned that it could throw a code at some point. The ecms are the same (7747). All input sensors land pins differently. Most of the outputs land differently. I have access to the ALLDATArepair site and they verified that the 89 is different from the 91 and their 91 data also concurs with my GM schematic. Also found a resistor buried in the 89 bundle just inside the firewall. That will be tonights project. Thank you for the help. MW

[1project2many]

I don't remember any resistors in the harness. The 4wd trucks have a diode in the 4WD circuit which is usually in a small, black or gray box on the RH side of the cab. Very surprised as to different pins as I don't remember it that way, but I am old enough to have forgotten what age they say you start to forget things.

[Fast355]

The hot fuel handling module is a timer that operates the fuel pump after the engine has been shut off. It's designed to prevent fuel from boiling in the fuel lines after the engine is off which could cause a hard start or even a no-start. It can be omitted but in some areas of the country with trucks that work way too hard it's a good idea to leave it in.

Not sure about all the inputs and outputs being different. Are you working with the same ecm in both cases? Which circuits are in different places?

After having one practically catch on fire one day for no particular reason, I avoid the Hot Fuel modules now.

[MichaelW]

Got it! Fire/smoke bad, tire smoke good. I'm going to omit/bypass that module. Could the pin difference be 89 GMC versus 91 Chevrolet? If so, I'm going to re-pin to the 91 schematic and move forward. Next task will be to upgrade the 7747. I've been to this site often as a guest, and I appreciate the info that helps guys like me get past common problems. Really cuts down on the frustration factor. Thanks to all MW