PCM thinks transfercase in low range

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Dec 22, 2025 | 03:06 PM

#31

geoffunkel

Mountain Pass

Joined: Mar 2012

Posts: 170

Likes: 12

Quote:

Originally Posted by Mark Kovalsky

The software can look at load, but only to delay the shifts, not schedule them. Except for what I explain below, all shifts are based on pedal position and vehicle speed. Pedal position is used to represent load. There is no other load input to the shift schedules.

There is a patented feature in the software called prevent powertrain hunting. Older transmissions had problems where the trans would upshift on an uphill grade, the vehicle would lose speed, and it would downshift. Then it could accelerate again, upshift, and do it all over again. This bit of software looks at what tractive effort would be available in the higher gear, and if it isn't at least what is available currently, it blocks the upshift. I can see it wasn't tuned as well as I would like in my '16 6.2L/6R100. It delays the upshifts on level ground.

Honestly well done on your shift programming! Having owned mostly manual transmissions over the years, I usually get annoyed by the shifting logic of most automatics that I've driven, but I actually like the shifting logic on my E4OD.

Dec 22, 2025 | 03:20 PM

#32

geoffunkel

Mountain Pass

Joined: Mar 2012

Posts: 170

Likes: 12

Quote:

Originally Posted by jas88

Back in the day, the C6 had a vacuum line to tell it manifold vacuum. The modulator was adjustable and that would affect how late or early the shifts were, at least to some extent.

A lot of old school transmissions (Torque Flight 727, GM 700R4, Ford AOD) didn't use a vacuum modulator. They used a TV (throttle valve) cable or linkage instead. Throttle position (via Throttle valve) and vehicle speed (via governor) were the only inputs the transmission needed to determine shift points. Obviously using hydraulic pressure instead of electronic control, but same idea. Two different ways of skinning the cat but you can infer the load the engine is seeing from the throttle position.

Sort of in the same way that speed-density fuel injection systems can infer the amount of air entering the engine based on MAP and RPM. Or you can do the same thing with a MAF sensor.

Jan 3, 2026 | 01:48 PM

#33

TCliff

Thread Starter

4wd Low

Joined: Dec 2025

Posts: 11

Likes: 4

From: Indiana

PCM fixed it

I found a place on e bay that had good reviews that repaired PCM’s. I sent it to Keyworks in Henrico VA, he repaired it and got it back to in a reasonable amount of time. Everything now works as it should. Thanks everyone for all the help

Cliff