4l80 questions

[Billett]

Got my TCC wired up the other day hoping to improve the 14.5 mpg I calculated on my last trip. Filled up yesterday at an estimated 12.2mpg... now I try not to consider myself an mpg freak, but I could have done 12 with my 350/ granny gears. Anyway...This is a 2 question post:

1: what kind of rpms are you guys turning at highway speeds? I know I am running with 3.73 rear but 2400 at 70mph seems high compared to what I've read here. It calculates out about right with my 28" tires but I've read under 2000 from some on here...

2: how can I test that the TCC is even functioning? Or that I have one? 4l80 is from a 95 van, pcm is 2003 van. I heard that if I maintained throttle in 3rd or 4th and pressed the brakes enough to throw the relay then the TC should unlock and I would jump up a few hundred rpms. I tried that to no avail... Only other thought is that this pcm has not been tuned yet. Is that a function that can be turned on/off and might just be off at the moment?

[BR3W CITY]

Originally Posted by Billett

Got my TCC wired up the other day hoping to improve the 14.5 mpg I calculated on my last trip. Filled up yesterday at an estimated 12.2mpg... now I try not to consider myself an mpg freak, but I could have done 12 with my 350/ granny gears. Anyway...This is a 2 question post:

1: what kind of rpms are you guys turning at highway speeds? I know I am running with 3.73 rear but 2400 at 70mph seems high compared to what I've read here. It calculates out about right with my 28" tires but I've read under 2000 from some on here...

2: how can I test that the TCC is even functioning? Or that I have one? 4l80 is from a 95 van, pcm is 2003 van. I heard that if I maintained throttle in 3rd or 4th and pressed the brakes enough to throw the relay then the TC should unlock and I would jump up a few hundred rpms. I tried that to no avail... Only other thought is that this pcm has not been tuned yet. Is that a function that can be turned on/off and might just be off at the moment?

I have a 3:73 and I turn about 24-2600 at 72. I've had folks tell me that was high, yet all signs point to the tcc working. (I get 18ish freeway locked up). I just "count" the gears. So if you go into 4th, and then 2 seconds later you feel another 2-300rpm drop, its locked up. I had a number of issues with lockup initially, bouncing in and out because the Misfire Detection was thinking the big cam was actually misfire, and kicking me out of lockup for safety.

[Billett]

The MT DOTs you mentioned in the other thread are roughly the same diameter as mine and we are running about the same rpms, but I never feel the TCC kick in... should be in 3rd and 4th if I've read right. Maybe the line pressures in the un-tuned trans are so much smoother that I don't notice? That would only be for shifting though not the TC...

I've also used the torque app to test the rpms during acceleration up to 50 or so. I notice about 4 shifts, or distinct drops in rpm, but not the other one or two from the converter.

[BR3W CITY]

Mine never locks up in third afaik. Its set to not use lock until 49mph because of how heavy my drivetrain is, and I don't think I've ever noticed lockup in 3rd unless under very very light throttle.

[68GMCCustom]

I don't think I ever noticed mine locking in 3rd. I am also running 3.73's...haven't taken mpg tho. My TBSS with LS2/4L70e gets about 14-15mpg.....as it has 4.10's and thus runs higher rpms on the highway...70's at 2400rpm or so?

Interestingly....I pulled my Caddy back this weekend from Houston with the TBSS and got 11.1mpg on one tank pulling 6800lbs at 65-70mph or more...haven't checked yet but think the other tank was even better.

[Wasted Income]

A lot of times, the TCC on these trucks is applied via PWM, so the lock is a lot softer than most people are used to feeling since it slips the converter clutch as it applies. The only way to know for sure is to hook a scan tool up and see what the PCM says as far as lockup goes.

[Blackpony]

My truck with 3.73 and 28" tires, I am around 2100 ish at 70 mph. I have the converter set to lockup in 4th only and i adjusted my tcc pressures to 80% minimal and 100% max. It feels like a 5th gear when it locks up. doing that helps keep the heat down a bit since there is very little to no tcc slip, I have noticed about a 10* change since doing this. Sounds like you may need to verify your connections on the tcc relay.

[Billett]

Just did the relay the other day so I know the relay side of it works, but not sure on the actual trans side. I think it's just soft to lock up because I'm pretty sure I feel it unlocking when coasting on the highway right around 45 and 60 mph or so.

Also, my tune shop said they've been having problems with the truck so that has probably been affecting my mileage as well.

[Jon01]

See if you can have your tuner go for a ride-along with you. They can monitor lockup state with the VCM Scanner.

Mine has heads, cam and intake plus 3700 converter, 4.10's, -80E and 28" tires and gets 19-20 on the highway running 75-80.
There's no reason you shouldn't get better than that.