4L80E TCC lockup speed.

[56pan]

I've just had a new, (mil. surplus), 6.5 engine put in my truck. Got a freshly rebuilt Jasper 4L80E trans. also. My TCC engages at about 55 mph, and the TCC disengages at what seems to me way too little increase of throttle. The new engine has much more power than the old one, and I'd like for the TCC to engage at about 45 and not disengage with the very small increase in throttle that's required. I put 265/75R-16 tires on the truck. Did the circumference measurement, and reset the jumpers on the VSS buffer. My speedometer is dead accurate. If anyone has any advice, or information on a shop that could reprogram the PCM, I'd sure like to hear of it. Thanks in advance.




'93 C2500 Chevy, 2wd. Nothing special. New engine, Fluidampr, 4" exhaust, fresh Jasper trans.

(Posted the above in the drivetrain forum with no results.)

[JohnC]

If this is in a '93 you don't have a true PCM, just a TCM as it were. IIRC, Jet makes an aftermarket unit that is somewhat programmable, but I don't know if it will do all you want.

Changing axle ratio or tire size will affect the lockup speed. Taller gears or tires make it lock up later.

Search the old posts. Someone else went through this a few years back, although it was with a later PCM.

[56pan]

Thanks, John. Just had the time to do a search and found what I'm looking for from a place in SC. $400, and much cheaper than a couple of other programmable units. ~$1100.

[midniteplowboyy]

Check your TPS. What voltage is it set at, at what rpm does it idle?

If you have the 6.2 IP, crank the pump all the way, that way you get more fuel with less throttle. It may be taken more throttle to do the job, than what the TCM is programmed for.

I have no problem with my lockup on my 93, and I have 3.21 gears in it. IIRC, my voltage is around 0.5-0.55v @ around 600-650rpm's. It'll stay locked down to 1300rpm around 45mph in town, as long as your easy on the throttle.

I have my 6.5 pump cranked all the way also, I put a 45ohm resistor across the transmission pressure solenoid wires to boost pressure about 5psi. I actually have a six position rotary switch to change resistance that I built, open-45ohm-35ohm-25ohm-20ohm-10ohm. I just use the 45ohm position with my Transgo-HD shift kit, shifts about right.

Jet used to use your very TCM for there controllers back in the early 90's, then used the van TCM for awhile, they would just program the EPROM to your needs.

You could put you a manual lockup switch on there. I used an old school floor mounted dimmer switch for mine, works slick, just give it a tap with your left foot, though I never use it.

[56pan]

Good info midniteplowboy. Is your '93 a 3/4 ton? I thought the lowest numerical ratio for the diff. was 3.42 on a GM 3/4 ton, which is what my ratio is. Yeah, the Inj. pump is the right one, a DB2-4911, but it's set up for an N.A. 6.5, not a turbo. Just haven't had time to turn it up, and I'm thinking that's why my TCC is disengaging too soon to my liking. And I do have 265 size tires on the truck, and I suspect you're right and that's keeping the TCC disengaged until 55 mph. That's going to have to be fixed, if it's not better after I turn up the pump. Been checking around for programmable TCM's and the cheapest I could find was $400.

[midniteplowboyy]

I put the 3.21's in, it originally had 4.10's, since the 93 is my play truck/daily driver I didn't like the low gears. Its a 2wd 4-door dually with some machined down alcoa's to 22" with 305/40-22 tires(pretty heavy), real close to the same height as a 235/85-16 or 265/75-16 tire. I'm running a Holset with IC on the 93, with marine injectors and the injection pump is turned up all the way.

3.21's were an option in the 70-80's, before OD trans were available, for the 14 bolt 10.5" ring gear trucks(very rare). I ordered my 3.21 gears new though off the internet, sold under the Yukon name, made by U.S.Gear.

I have a 92 6.5 4wd 5sp reg cab with factory 3.42's, the 3.21's are a little steep but great for interstate driving, around 1900rpm @70mph, IIRC.

If you have pyrometer, crank the injection pump all the way. Check TPS voltage also. I'd put a manual lockup switch on there first before buying a controller.