[Sirrotica]

The diesels from 91-93 have a factory standalone transmission ECM because the engines had no electronics for engine management, they used a fully mechanical injection pump. I believe that's where the "fully mechanical" term, the OP is talking about, is being misunderstood. It's a fully mechanical injection pump, not a fully mechanical transmission. Although there is aftermarket support that can make the 4L80E fully manual without a onboard ECM. That means the way to upshift and downshift is to move the gear selector manually, totally different modification.

Mark, "Half Inch Stud", member name on PY, has a 68 GTO with the full manual 4L80E conversion in his car. He also has a write up showing the progression of the swap documented on PY.

In 1994 GM switched to an diesel engine management system, and a different injection pump. Post 1993, the ECM both controlled the injection pump, and the transmission. Pre 1994, the ECM only controls the transmission. I believe this is where the misunderstandings lie about "fully mechanical" the OP talks about, it's the injection pump, not the transmission. the transmission wasn't changed, the injection pump was the change.

The case pass thru wiring harness plug in the 91-93 also had a tendency to leak fluid as they aged. The updated plug (94 and later) can easily be updated with a 94 and later internal transmission wiring harness. If being used in it's original application (91-93 chevy/GMC truck) you also need to splice a late model (94 and later) plug onto the wiring harness in the truck to complete the update. Transmission parts suppliers sell just the updated plug, and wires externally, as well as the 94 and later internal wiring harnesses to update the 91-93 OEM harnesses in the trucks.

The aftermarket transmission transmission ECM's use the 94 and later plug on their wiring harnesses, so if you had a 91-93 transmission that hadn't been updated to the later plug, you would need to change the internal wiring harness inside of the transmission to correspond.

The only difference in the 4L80E between diesel and gas, is the diesel torque converter is tighter, less slippage than the gas engine converter. The rest of the transmission is the same as far as internal parts.

In 1997 the Hydramatic division made a case change to better lubricate the center support by moving the location of one of the cooling line connections. In a tightly confined transmission tunnel, the 91-96 transmissions are usually somewhat easier to hook up cooling lines in a conversion in a car, due to the revision in 97 that moved the rearmost connection further up, and to the rear. To interchange a 97 and later transmission into a 96 and earlier truck, the aftermarket makes a short tubing adapter to extend the later cooling line location the the early cooling line position, making a swap of a late transmission, to an early truck possible. It may also make the 97 and later transmissions easier to hook up cooling lines in a car.

There is one other variation of cases, the later cases that bolted to a LS engine had a seventh bell housing bolt dead center at the top, the other transmissions only had the 6 standard chevy bell patterns. I'm not certain why that 7th bolt was added, but it's out there and should be noted to make this list comprehensive. It has no bearing on a better case and the cases still bolt up to the 6 bolt pattern.

Jake (Jakes Performance) says that the earlier transmissions (91-96) are fine to use in a HP application. He states that the Hydramatic case update (97 and later) had little if any better results in supplying better lubrication to the support. Since I drive a 1993 K3500 6.5 turbo diesel dually with a 270,000 mile drivetrain, I would tend to agree that the cooling line update had little, if any effect, on transmission longevity.

No matter which transmission you have, pre 97, or post 97 they are an excellent basis for a HP, OD build.

Hopefully that will clarify the misunderstandings concerning the terminology of "fully mechanical", and "electronically controlled", the OP has encountered.