4L80e doesn't downshift sometimes

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

RKCRLR

Newbie
Joined
Dec 7, 2019
Messages
10
Reaction score
4
Location
Garden Valley, CA
I have a 2000 7.4L K3500 with a 4L80e transmission.
Sometimes when I'm hauling a heavy load (truck camper) up a grade the truck won't downshift into a lower gear even if I move the gearshift lever to a lower position. The engine temperature may be a little above the middle mark on the gauge but it doesn't seem to be overheating. It only seems to happen after it's been driven for a while (and it doesn't always happen). The check engine light doesn't come on and there no codes stored after it happens. The truck/transmission runs fine other than this.

Any suggestions where to look?
I have Torque Pro and OBDLink MX+ adapter. I was going to monitor transmission temperature, input shaft speed, output shaft speed, and torque converter slip. Anything else I should look at?

Thanks
 

Supercharged111

Truly Awesome
Joined
Aug 20, 2015
Messages
12,617
Reaction score
15,246
I'd have to look, but these trucks do have a hot shift pattern where it will shift differently in an attempt to cool the trans off. It wouldn't surprise me if this is what's happening because my stock tune was retarded, motor was always zinged out and converter never locked which absolutely cooks the fluid.
 

RKCRLR

Newbie
Joined
Dec 7, 2019
Messages
10
Reaction score
4
Location
Garden Valley, CA
^^^
That would be appreciated! It would help me focus on what to monitor. My truck doesn't seem to run hot but maybe a faulty sensor.
Thanks
 

Supercharged111

Truly Awesome
Joined
Aug 20, 2015
Messages
12,617
Reaction score
15,246
The only thing I can't see is what temp it switches over to the hot shift pattern. I can see it definitely avoids downshifts more when it is hot.
 

RKCRLR

Newbie
Joined
Dec 7, 2019
Messages
10
Reaction score
4
Location
Garden Valley, CA
^^^
I'm assuming you mean the one in the transmission pan (I didn't know there was one there). I have an option to read transmission fluid temperature so I guess it uses that sensor.
Thanks
 

andy396

GMT400 Enthusiast
Joined
Jan 30, 2014
Messages
538
Reaction score
391
Location
Maryland
The only thing I can't see is what temp it switches over to the hot shift pattern. I can see it definitely avoids downshifts more when it is hot.

Set Overtemp = 266.9F
Clear Temp = 264.2F
Set Time = 600s
Clear Time = 5s
 
Top