backup Fuel mode

[chevss442]

I have a 1996 GMC. I am trying to find out exactly what the PCM does in backup fuel mode. Specifically, does the PCM keep the torque converter always unlocked and prevent shifts into 4th gear in this mode?

[JohnC]

Define "backup fuel mode".

[Shikaroka]

Perhaps he means "Limp Home" mode?

[chevss442]

Yes, the GM shop manuals call it "backup fuel", AKA limp home.

[DmaxMaverick]

It depends on the reason (DTC) for the "limp", if it is a limp. "Backup fuel" can mean a couple things. The "default fueling" is a stored static fuel curve, like when sensor inputs are absent, like the resistor or ECT. This won't effect the tranny or TCC.

In true "limp mode", fuel is limited to ~60%, and the TCC and OD is disabled. If it is a tranny code, it will restrict the tranny to one gear, usually 2nd, for starting out and it won't upshift, staying in that gear. If it's the 2nd gear circuit throwing the code, you get 1st and it won't upshift.

There are several combinations of power, timing, and tranny operation, depending on the DTC and model year. For '96, there weren't many combinations, and more came with later years. If you have the DTC's, you can look them up, and see what the table says for operation.