4l80 engine braking

Hummermark

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Hi have a complete standard m1113 Humvee turbo diesel with the 4 speed 4l80
When you release the throttle pedal at say 60 mph the engine goes to what seems idle and you just coast along with no engine over run drag un like the th400 3 speed in the Humvee which has engine over run drag when you release the throttle pedal at 55 mph .
Is this normal on the 4l80 in a military Humvee ?
 

Retiredwarhorses

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The way to test the TC lockup is tap the brake pedal at about 45-50mph, you should have a surge of Rpm’s as the TC unlocks, Keep your foot on the gas pedal at the same time you tap the brake.
 
Last edited:

Milcommoguy

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Rosamond, CA
4l80E TC should have locked when in fourth gear (depending on set-up 45-50 mph should do it) Tap the brakes to UN-lock.

Slight RPM increase due to un-locking of TC & power transfer losses with same throttle applied. That's why the for lock-up, power transfer, mileage, overall less trans / engine heating, efficiency, etc.

Letting off the throttle and out of lock-up should produce some level of drag. Not as much as a three speed in comparison with the extra shift point to get down past. Down to idle would seem something is ?? TPS / computer could be set wrong? Can one manual shift up and down thur the gears and get the drag / loading effect? Hard shift up.... and back down and see what happens. Don't break it drag race style.

TC have been known to go BAD, Transmissions, Those too. 🤔 , CAMO
 
Last edited:

Hummermark

Member
194
11
18
Location
London uk
4l80E TC should have locked when in fourth gear (depending on set-up 45-50 mph should do it) Tap the brakes to UN-lock.

Slight RPM increase due to un-locking of TC & power transfer losses with same throttle applied. That's why the for lock-up, power transfer, mileage, overall less trans / engine heating, efficiency, etc.

Letting off the throttle and out of lock-up should produce some level of drag. Not as much as a three speed in comparison with the extra shift point to get down past. Down to idle would seem something is ?? TPS / computer could be set wrong? Can one manual shift up and down thur the gears and get the drag / loading effect? Hard shift up.... and back down and see what happens. Don't break it drag race style.

TC have been known to go BAD, Transmissions, Those too. 🤔 , CAMO
 

REF

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Location
Porterville Calif
I have found that on my truck (M1123) that if I slow down i can feel the engine braking , as soon as I lift clear off the throttle the torque converter releases and feels like free wheeling. So it will unlock if you touch the brakes or lift your foot off the throttle. Both the brake switch and throttle position sensor input to the trans. Try running at speed then slowly lift your foot you will feel the TC unlock at a certain throttle position then remember that position, or at least that's the way my truck works.
 

Hummermark

Member
194
11
18
Location
London uk
I have found that on my truck (M1123) that if I slow down i can feel the engine braking , as soon as I lift clear off the throttle the torque converter releases and feels like free wheeling. So it will unlock if you touch the brakes or lift your foot off the throttle. Both the brake switch and throttle position sensor input to the trans. Try running at speed then slowly lift your foot you will feel the TC unlock at a certain throttle position then remember that position, or at least that's the way my truck works.
The way you one is operating is exactly how my one is operating lift of throttle completely feels like freewheeling slightly touch throttle then feels like engine is dragging so might be normal?
But will try the previous posts tests.
I have put a meter on the torque converter (tc) switch connected to the brake pedal and it is normally close until you press the brake pedal and I have wiggled the pedal on it's free play it stays closed circuit ,switch only goes open circuit with a proper brake application after free play has been taken up.
 
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