Another new project
 

Another new project, I recently purchased a 1994 3/4 ton extra cab 2wd long bed 6.5TD Chevy pickup. I had worked on this truck back in the early 2000s before I went to work for the power company 20 years ago. I had a retail repair garage in our small city of 10,000 people. When the original engine broke its crank at 140,000 miles I advised my client to have the dealer install the replacement engine due to the added warrantee of complete coverage including labor. He brought me the broken engine and I am still running parts of his original engine in my crew cab to this day. This truck had been sitting for the last four years in his son’s field. They live in a remote location and I ran into his daughter in law at a church function and we finally connected. I purchased the truck and the replacement engine needs some work as it is using coolant. This is a common failure point with early GM Diesels, it is usually head gasket failure due to torque to yield head bolt stretch.
I wanted the turbo Diesel for my crew cab. The truck is in nice shape other than the engine problems. i had originally planned to part the truck. It is too nice to part so I will pull the Diesel components and install a carbureted gas engine. This would be a better truck for my grandson than the 68 4x4 to start with. I would much rather he waits on the 68 until he gets into his 30s when he might appreciate the family history with the 68. No smog checks in this part of Oregon so it might get headers and “kids” exhaust. My wife is concerned due to limited parking and limited time for an old man like me.

One issue with putting a non computer engine in a truck with a computer controlled transmission is how to make it shift. I have been poking around some GM computer car forums and it seems the GM used this transmission with non computer controlled engines for a few years. I drove to Portland today about 90 miles one way and bought a computer and harness out of a 92 Chevy Diesel pickup. It had a non computer 6.5 TD and the same 4L80 computer controlled transmission I have. I have to provide throttle position and engine rpm to make it work. Throttle position is the easy part. Rpm will come from a modified HEI GM distributor.

This is like the Chemistry class I took in high school. I was told that the course was just a brain exercise since very few of my classmates would go on to Chemistry. A wise professor.

Sounds like a neat challenge.

why is the transmission computer controlled ?

(warning: my full knowledge of diesel vehiclry amounts to compress--> boom


I have a PhD in chemistry and I write for a living
Haven't touched a chemical since 96.

Not counting car fluids.

GM’s 4L80 is a computer controlled 4 speed version of the famous TH400 three speed automatic. They actually share many components including clutch discs and some planetary units. This transmission is shifted by solenoids and has a force motor providing line pressure. The previous TH400 was completely mechanically controlled. It used engine vacuum to control line pressure and a mechanical governor to control shift speeds.
Progress towards lower emissions and better fuel economy.
I have a 700R4 in my truck, it is a completely mechanical 4 speed version of a TH350. They became 4L60e when a computer controlled all the shift functions mid 1990s.

In my Saturn the 4t45 is a computer controlled 4 speed version of a TH125 three speed transaxle from the 80s.

So to run a computer controlled transmission with a non computer engine I have to provide “digital” data to the computer so it knows what to do. In this case I need to provide throttle position= load and rpm= speed. The computer will use engine rpm and compare it input shaft speed to calculate torque converter slip. The computer will use throttle position to determine a portion of load to regulate line pressure to make sure the clutches apply with enough force to not slip and burn up the trans. They also use manifold absolute pressure on fuel injected engines as part of the load calculations. MAP sensor.

Sounds like a nice project :-) Very smart of you to replace the '68 with this truck as your grandson's daily driver.

My 4-wheeler buddy that lived a couple blocks from me had his grandfather's old '81 C10 for his 1st vehicle. It was absolutely cherry; we loved that truck! 1st thing he did was lift it 12", slap 42" Super Swampers on it, and poked & stroked the sbc. After a year of driving it; he parked it in favor of a not-so-pristine '83 K10 to go mudding with. I understood that decision even when his friends couldn't understand it.

As far as "modern" transmissions go; it's amazing how much you can do with them without opening them up. My tuner for my '05 Grand Marquis asked me how firm of a shift I wanted when I bought the tune from him. He changed the shift points and TV pressure through the ECM. To me that's amazing since we had to pull the trans so we can swap the torque converter for one with more stall speed, and drop the trans pan to change the valve body in order to accomplish those things.

Might just get yourself a hold of a TH400 instead. Those things are absolute tanks, although they drink fuel on highway cruising due to only 3 speeds and no lock-up torque converter. Your grandson will have to really try to break it though :)

The 4L80 is a th400 with overdrive. Why would I want a three speed automatic unless I was drag racing?

Well, GM put that three speed into everything from sedans to motor homes, so it's definitely not just for drag racing. I own a Chevy G Van that came factory with one, still runs great after 34 years. They also run without any computer controls, so it is generally easier to swap into project cars.

I think it's the best auto transmission that GM ever made. One of my neighbours growing up had a pool business with a tree stump removal service 'on the side', and he would remove tree stumps out of the ground with his Chevy 3-speed truck. That poor transmission took an insane amount of abuse and never failed him.

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I replaced the PMD pump mounted driver with a genuine Stanadyne module with a #9 resistor and the truck runs great. Took it for a long test drive and I am very impressed with the performance with the intercooler and 10 psi of boost. The pyro ran up to 900 degrees on the big hillclimb up the big hill going to my street. I am now gathering parts to bolt my 241 transfer case to the 4L80e transmission. I bought an input gear for the transfer case and have a friend looking for a 4x4 output shaft and the adaptor to the transfer case.
This is a long term project, I will disassemble the donor truck this year and go through the engine and transmission. I am going to harden the Diesel with both head studs and main studs. The transmission has about 265 on it and works perfectly so it will get a refresh plus the added 4wd parts. This conversion requires a different evaporator box to clear the turbo downpipe and installing the complete computer and harness into a non computer truck. Both driveline will need length adjustments also. I need to decide on gear ratios also. The truck has 4.56 now and the donor has 4.10. I think it will have enough power to pull 3:73 gears but I only have rear gears in that ratio.

Hope all are doing well, survived another Winter in the NW. 1 inch of rain today, 2 more expected tomorrow. Sure glad I live way up above town. Back in the L200/5, I really love this car.
Andy