Transmission SLAM

[bamaspiveys]

At the end of a 3 hour interstate cruise I pulled on to a exit ramp ( a long uphill exit ramp). As I coasted up the hill and slowed down the transmission slammed down a gear. Scared the hell out of both of us, I thought He engine fell out!
Stopped at the light then pulled off normally. Got up to 3rd before we had to slow down for the next light, this time I was watching and when it shifted down to second it cluncked. Not a slam this time but not abnormal down shift. We were headed into a target parking lot and had to go through two mor lights with downshifts into 2nd that were normal.

I checked the transmission fluid once we got stopped. It was above the "hot" line. Had the engine at idle in neutral.

We are stopped for dinner with friends and have about another 2 hours to drive tonight.

Not sure what else to check any ideas? Thoughts?

[Butte64]

Was the engine brake in High position?

[brobox]

Michael, shut the engine battery disconnect off. It might reset the Allison memory then try driving it again. I am a firm believer to "reboot" anything electrical on our units. The Allison's before Transynd would do that all the time.

[MoHoGo]

X2 on the question...was the engine brake on. I have experienced this on a couple of occasions, now when I get off the highway I turn the brake off. Has never happen when the brake was off.

Not sure what the reason is for the transmission bucking like this and I agree with you that the first time it happens it sound like you dropped the tranny.

Good question for Alisson.

[Sneelock]

x2 on trying a reboot.

I don't have an Entegra but if your rig has an Allison transmission you might be able to query any stored trouble codes through the shift pad. Sometimes a loose ground or a bad speed sensor will cause that slamming shift action.

hope it turns out to be something simple.

[Dtwallace]

I've seen this happen a few times on truck application, sometimes when the trans goes to a down shift, the torqu converter is still in the lock mode.

When this happens, there is no cushion effect as there would be when the converter is unlocked and allowing slippage.

This is a condition that comes and goes, and I've never gotten a good reason why it happens from any Allison tech.

I've never had or heard of a transmission failure from this issue.

Dtw

[bamaspiveys]

Thanks for the replies, dinner took forever so we just buckled down there for the night. I am up early this morning, so I will do the reboot.

Yes the engine brake was on but in the lower of the two settings, seldom if ever run it in the higher setting.

The DW said she remembers it happening one time on our Phaeton, I don't....tried to forget everything about that RV.

[Matt D]

Michael, this happens when we are slowing but I’m actually feeding increasing fuel to the engine.

The vehicle speed is decreasing, and if I attempt to increase RPMs too rapidly before the transmission has downshifted, it will slam.

This situation is most likely to occur when going on a steep uphill in 2nd gear, and gravity is slowing us rapidly, and I get too aggressive with the accelerator pedal = BANG! Every time.

My guess is that you had the engine brake slowing you too rapidly on the uphill exit, and right when it was going to downshift to 1st, you increased throttle.

-Matt

[Rocky Larson]

My allison 1000 trans will go into limp mode, similar to what you experienced, when it senses overheating. Has happened on long uphill grades while towing heavy. Only allows enough power to pull over to the side to cool down. Good to go with no damage after 20 to 30 minutes. Not a good habit to get into.

[bamaspiveys]

Originally Posted by Matt D

Michael, this happens when we are slowing but I’m actually feeding increasing fuel to the engine.

The vehicle speed is decreasing, and if I attempt to increase RPMs too rapidly before the transmission has downshifted, it will slam.

This situation is most likely to occur when going on a steep uphill in 2nd gear, and gravity is slowing us rapidly, and I get too aggressive with the accelerator pedal = BANG! Every time.

My guess is that you had the engine brake slowing you too rapidly on the uphill exit, and right when it was going to downshift to 1st, you increased throttle.

-Matt

Matt, I think you are onto something here, I was applying the throttle to disengage the engine break because we were slowing down quickly and still needed to get up the hill. That is exactly when the slam occurred, I looked at the Allison display and it was in second so maybe in this case it was from 3rd to 2nd.

THanks

[757driver]

Originally Posted by bamaspiveys

At the end of a 3 hour interstate cruise I pulled on to a exit ramp ( a long uphill exit ramp). As I coasted up the hill and slowed down the transmission slammed down a gear. Scared the hell out of both of us, I thought He engine fell out!
Stopped at the light then pulled off normally. Got up to 3rd before we had to slow down for the next light, this time I was watching and when it shifted down to second it cluncked. Not a slam this time but not abnormal down shift. We were headed into a target parking lot and had to go through two mor lights with downshifts into 2nd that were normal.

I checked the transmission fluid once we got stopped. It was above the "hot" line. Had the engine at idle in neutral.

We are stopped for dinner with friends and have about another 2 hours to drive tonight.

Not sure what else to check any ideas? Thoughts?

Michael,

According to Spartan class the transmission dipstick is there to verify if there is fluid in the transmission. The best place to check the fluid level, trouble codes, filter life, and overall health is with the shift pad diagnostics.

[bamaspiveys]

Originally Posted by 757driver

Michael,

According to Spartan class the transmission dipstick is there to verify if there is fluid in the transmission. The best place to check the fluid level, trouble codes, filter life, and overall health is with the shift pad diagnostics.

Good point Don, I will do that now that as well.

Thanks