Bucking in T/H mode - Built trans

[NevadaLLY]

I'm getting some pretty severe bucking in tow haul mode that isn't present in regular drive loaded. The trans shifts in T/H since the build have always been really hard, now with the hard shift I'm getting bucking. It bucked before the full engine build but much worse now with the higher power level. Thoughts on where to start? I trust Mike's build and expected the T/H mode to be abrupt. I tow heavy and hate switching between regular/Tow haul just to avoid the buck. Hill climbs are especially tough not locking the TC.

I have run data logs and the fuel pressure looks perfect, CP3 and injectors are brand new (3500 miles) and the balance rates are all nicely in sync. I have read some old posts around a spring on the throttle pedal but I can confirm the buck happens when given adequate acceleration and doesn't appear to be hyper sensitivity of the pedal.

No CEL's. I have seen range shift inhibited twice in the last 4 weeks... maybe sign of a progressive internal trans issue?

 

[Bdsankey]

Have you called Mike directly? He would be where I'd start as he is arguably one of the most (if not the most) knowledgeable Allison guy.

When mine gave reoccurring range shift inhibited issues it was my poopy parts store NSBU letting go. I replaced it with a GM NSBU and it's been great since (5 years).

 

[Mike L.]

I would be looking at the tune.

 

[NevadaLLY]

Have you called Mike directly? He would be where I'd start as he is arguably one of the most (if not the most) knowledgeable Allison guy.

When mine gave reoccurring range shift inhibited issues it was my poopy parts store NSBU letting go. I replaced it with a GM NSBU and it's been great since (5 years).

I've never touched the NSBU... maybe something to look at. Wouldn't that throw a code?

 

[NevadaLLY]

I would be looking at the tune.

Hi Mike! Doing that as well. I have a stock tune that will go back in to check if that alleviates the issues. It's over with Andy here in Reno right now - we'll see what he says.

 

[Bdsankey]

I've never touched the NSBU... maybe something to look at. Wouldn't that throw a code?

Mine never thew a code, just range shift inhibited.

 

[Mike L.]

Hi Mike! Doing that as well. I have a stock tune that will go back in to check if that alleviates the issues. It's over with Andy here in Reno right now - we'll see what he says.

I just got off the phone with Andy and he says there is a sure in the engine. He will do some further tests this afernoon. I'm in touch with him.

 

[NevadaLLY]

I just got off the phone with Andy and he says there is a sure in the engine. He will do some further tests this afernoon. I'm in touch with him.

Thanks Mike. Range shift would be unrelated? Also as I mentioned the tow haul mode never smoothed out. Drove like it did when you and I did the test drive after the build. Shifts hit hard in TH.

 

[gmduramax]

If you accelerate with cruise control does the bucking go away?

 

[NevadaLLY]

If you accelerate with cruise control does the bucking go away?

I haven’t tried.. it seems to really happen between 1-2 and 2-3 shifts the worst. 4-5 is fine… pulls nice in 4 and 5.

 

[NevadaLLY]

Andy said here at A1 (Mike's buddy) that he really doesn't think it was the trans. The bucking is quite violent at times in T/H but he also was able to get it to do it very subtlety in regular drive. Either way, he thinks tuning is doing. I'm going to load a new (stock) tune into the truck and see what that does. This tune has only modification to support the pump and scaled the MAF for the turbo mods. Everything else would be stock.

 

[NevadaLLY]

Did a trans fluid change to amsoil torque drive a few nights ago and added the PPE deep pan. The shifts have drastically improved but the hard hits in TH are still there, just less prominent. Surge is still an issue. I’m working out some new tunes.

However, I found a ton of metal fiber on the magnet and on the magnet sticky in the pan. Has 12k on the trans since the build. No
Chunks but the oil was pretty nasty dark.

 

[gmduramax]

I’m betting it’s the tune. Does it do it on a stock tune? Does it do it using cruise control to accelerate

 

[NevadaLLY]

I’m betting it’s the tune. Does it do it on a stock tune? Does it do it using cruise control to accelerate

Did it in a stock tune** had to adjust for 12mm LBZ cp3 and different intake but otherwise stock. Reduced power level so it was a reduced hit but it would still surge. New turbo in there with the build but one log shows the turbo not following commanded vane position very well. Next log looks fine. Still a puzzling case

 

[gmduramax]

What’s the maf sensor doing during the surging?