Hard Downshift

[Joseph Garcia]

Hi Folks,

I'm looking for some initial thoughts here. Also including @NickTransmissions to the discussion.

As part of my supercharger project, I purchased a new GM 6 speed transmission and torque converter, as I thought that new equipment would be better to handle the increased power over the 165,000 mile equipment in the truck at the time. To my knowledge, a stock GM tune was put on the new transmission.

The transmission performs flawlessly, except for a very hard, and usually unsuccessful downshift. Let me explain the downshift issue, and classify it as a 'soft' downshift situation, without me trying to force it. I will be traveling in semi-city traffic, going around 30-40 mph. I give the truck a bit more gas to slightly accelerate (softly pressing the gas pedal slightly), and the truck will suddenly try to downshift (to 2nd gear, I think), but it hits the downshift hard, but it fails to lock into the lower gear, and when I take my foot off the gas, it reverts back to presumably the gear that it was originally in. The same thing happens, if I am going up a slight incline, and I give the truck a bit more gas to slightly accelerate.

This hard downshift has occurred since day 1 with the new transmission.

What are your initial thoughts?

 

[Doubeleive]

sounds like a clutch set problem to me, when I had mine built I had a similar issue the builder ended up using a different clutch set and it resolved most of it. Depending on "how I drive" I can still create a hard upshift to 2nd gear which is not harmful it's been like that for 70k now, downshifts are perfect.
a transmission tune may help without having to pull it. In my case it was just a problem of the clutch set not having enough clearance.
but no telling, it could be something else entirely.....

 

[NickTransmissions]

Hi Folks,

I'm looking for some initial thoughts here. Also including @NickTransmissions to the discussion.

As part of my supercharger project, I purchased a new GM 6 speed transmission and torque converter, as I thought that new equipment would be better to handle the increased power over the 165,000 mile equipment in the truck at the time. To my knowledge, a stock GM tune was put on the new transmission.

The transmission performs flawlessly, except for a very hard, and usually unsuccessful downshift. Let me explain the downshift issue, and classify it as a 'soft' downshift situation, without me trying to force it. I will be traveling in semi-city traffic, going around 30-40 mph. I give the truck a bit more gas to slightly accelerate (softly pressing the gas pedal slightly), and the truck will suddenly try to downshift (to 2nd gear, I think), but it hits the downshift hard, but it fails to lock into the lower gear, and when I take my foot off the gas, it reverts back to presumably the gear that it was originally in. The same thing happens, if I am going up a slight incline, and I give the truck a bit more gas to slightly accelerate.

This hard downshift has occurred since day 1 with the new transmission.

What are your initial thoughts?

Have you put it on a scanner and taken it for a drive to replicate the condition and see what the transmission is doing and values are for the various transmission-related PIDs? A priori, I'd look at pressure switch data as well as line pressure to see if there is either an anomalous line pressure spike or delay in pressurizing the circuit due to a lame switch response. 2-6 clutch regulator valve and associated VF solenoid may also warrant scrutiny, since it only seems to happen in 2nd on a 3-2 downshift (at least that is what I'm gathering - correct me if I am wrong).

 

[Joseph Garcia]

Have you put it on a scanner and taken it for a drive to replicate the condition and see what the transmission is doing and values are for the various transmission-related PIDs? A priori, I'd look at pressure switch data as well as line pressure to see if there is either an anomalous line pressure spike or delay in pressurizing the circuit due to a lame switch response. 2-6 clutch regulator valve and associated VF solenoid may also warrant scrutiny, since it only seems to happen in 2nd on a 3-2 downshift (at least that is what I'm gathering - correct me if I am wrong).

Thank you for your quick reply, Nick. Much appreciated. The transmission is a deep dark hole for me with no applied wrenching experience. I'll have to research up on what I can see on my Tech 2 regarding transmission operations.

I believe that the truck is in a higher gear while cruising 30-40 mph, so I think that the attempted downshift may actually be starting at a higher gear like 4th gear.

 

[strutaeng]

Do you know of someone that has HP Tuners? You can do a datalog while driving, which captures and records just about all the sensor data, rpm, speed, gear, and whatnot.

Then you can playback the data and see if you have an anomaly or something wacky.

I'm assuming you got a tune for the supercharger? I know some guys will tweak some parameters on the 6 speed, but I'm not saavy enough on tbe tuning side myself...

 

[Joseph Garcia]

Do you know of someone that has HP Tuners? You can do a datalog while driving, which captures and records just about all the sensor data, rpm, speed, gear, and whatnot.

Then you can playback the data and see if you have an anomaly or something wacky.

I'm assuming you got a tune for the supercharger? I know some guys will tweak some parameters on the 6 speed, but I'm not saavy enough on tbe tuning side myself...

Yes, I do, but I'm not sure about their skill set in using it for that purpose. I'll certainly look into it, though.

Yes, the supercharger came with a tune from Whipple for the ECM, but not the TCM. Whipple said that the TCM did not have to be re-tuned.

I'll be reaching out to BlackBear soon and sending them an AutoCal datalog of the ECM and TCM for their review and potential tweaking.