Transmission shift points/roughness

[Enforced_Leo44]

'Evening everyone,

Ever since I bought my CUCV, I always felt a bit of a hard shifting behaviour, which ultimately got traced down to one of the POs installed a transgo shift kit and got crazy with the holes in the valve body plate, drilling them to max recommended size, so that's kind of taken care of

But, I have a couple of question also regarding the Th400 shifting behaviour, especially the downshifting. In my case, my transmission doesn't downshift unless: I go at wide open throttle and engage the kickdown switch or I go really slow

For example: the truck downshifts from 3rd to 2nd at around 15mph and from 2nd to 1st at 5mph give or take, no matter the throttle position, I could be at 80% throttle at slightly above 15mph and it will stay in 3rd gear, unless as I said I go at WOT and engage the kickdown

Is this normal for this transmission on these trucks? Or am I missing something? I think I read on the Transgo shiftkit manual that by removing a spring in front of the vacuum modulator, it gets rid of part throttle downshifts, could this be the cause?

Thanks in advance, any advice is appreciated!
Leo

 

[WWRD99]

Is it a truck or blazer?
I assume truck. This sounds like they have the springs and weights to light in the governor and it's keeping line pressure to high for the throttle to overcome. On the test side what is the vacuum at the modulator set at. Could be to low.

 

[Enforced_Leo44]

Is it a truck or blazer?
I assume truck. This sounds like they have the springs and weights to light in the governor and it's keeping line pressure to high for the throttle to overcome. On the test side what is the vacuum at the modulator set at. Could be to low.

Yes, M1008 or M1028, not really too sure.
Never really tested the vacuum... my bad. I should really get the whole vacuum system checked, hoses, pump, VRV, all the goodies.

 

[WWRD99]

Yes, M1008 or M1028, not really too sure.
Never really tested the vacuum... my bad. I should really get the whole vacuum system checked, hoses, pump, VRV, all the goodies.

Yes, start there first!

 

[Enforced_Leo44]

Yes, start there first!

The reason that I didn't check that to being with is that the upshifts seem fine, other than being a bit harsh, 1-2 at 15mph and 2-3 at 25mph, so I didn't go directly to the vacuum part of the system. It's the vacuum modulator that dictates the non-WOT upshifts, right?

 

[WWRD99]

The reason that I didn't check that to being with is that the upshifts seem fine, other than being a bit harsh, 1-2 at 15mph and 2-3 at 25mph, so I didn't go directly to the vacuum part of the system. It's the vacuum modulator that dictates the non-WOT upshifts, right?

Yes the vacuum does the normal load up shifting points from the throttle. So when I tuned in my last vacuum modulator...I got an adjustable one...I set the no load upshift to 2nd at 23 mph and the third at 45 mph. Granted the 45 can be a pain if the speed limit is 35 but I just push it a touch and it'll upshift. Most of the vacuum tuning is done at the throttle vrv though...you will need a vacuum gauge to set it accurately...that valve changes things pretty crazy when moving it on the IP. You can fine tune the shift with the adjustable modulator. My concern with yours being so low is it probably has no power shifting into 3rd at 25...to me that's way to low. So...it could be that both need a little timing and tuning to get the shift timing where you want it between the governor and the modulator. I have the hunch someone upped the line pressure with the shift kit to make it last longer or whatever,,,and that will throw off the governor speed shifting to be very low since throttle pressure will overcome the governor that much faster...as well as downshift at such low speeds. Or the vacuum is set so high it shifts very early...no vacuum means very high shift and usually very harsh. So quick ref on what each does....the governor is strictly a speed related part....the faster you go the more the weights spring out and force a shift...the modulator tries to overcome the governor pressure with the throttle to simulate a load...they fight each other constantly to win....all depending on line pressure...whichever one can make the most. This is a very broad statement since neither make line pressure they just allow pressure to flow to shift a gear, governor, or hold the gear, modulator. The harsh shift isn't something I would try to tune out until you get the shift timing correct as the line pressure is my guess has been raised...if that's the case it may not be possible to tune that down. Do you have an adjustable modulator on the trans now? I would have to look up the shift kit they put in it to see if leaving a spring out for the modulator valve is something they do. You happen to know which one they used? If you have what's left of the kit usually they keep the old parts they took out or didn't use,,,ie the extra governor weights and springs they didn't use.

 

[Enforced_Leo44]

Yes the vacuum does the normal load up shifting points from the throttle. So when I tuned in my last vacuum modulator...I got an adjustable one...I set the no load upshift to 2nd at 23 mph and the third at 45 mph. Granted the 45 can be a pain if the speed limit is 35 but I just push it a touch and it'll upshift. Most of the vacuum tuning is done at the throttle vrv though...you will need a vacuum gauge to set it accurately...that valve changes things pretty crazy when moving it on the IP. You can fine tune the shift with the adjustable modulator. My concern with yours being so low is it probably has no power shifting into 3rd at 25...to me that's way to low. So...it could be that both need a little timing and tuning to get the shift timing where you want it between the governor and the modulator. I have the hunch someone upped the line pressure with the shift kit to make it last longer or whatever,,,and that will throw off the governor speed shifting to be very low since throttle pressure will overcome the governor that much faster...as well as downshift at such low speeds. Or the vacuum is set so high it shifts very early...no vacuum means very high shift and usually very harsh. So quick ref on what each does....the governor is strictly a speed related part....the faster you go the more the weights spring out and force a shift...the modulator tries to overcome the governor pressure with the throttle to simulate a load...they fight each other constantly to win....all depending on line pressure...whichever one can make the most. This is a very broad statement since neither make line pressure they just allow pressure to flow to shift a gear, governor, or hold the gear, modulator. The harsh shift isn't something I would try to tune out until you get the shift timing correct as the line pressure is my guess has been raised...if that's the case it may not be possible to tune that down. Do you have an adjustable modulator on the trans now? I would have to look up the shift kit they put in it to see if leaving a spring out for the modulator valve is something they do. You happen to know which one they used? If you have what's left of the kit usually they keep the old parts they took out or didn't use,,,ie the extra governor weights and springs they didn't use.

The shift points don’t seem low to me considering that the RPMs sound pretty high when shifting, maybe you are thinking about M1009s shift points? The M1009 has 3.08 final ratio vs M1008/M1028 having 4.56
I think I’ve read somewhere that M1008 upshifts should happen at 15mph for 1-2 and 25mph for 2-3, without engaging the kickdown of course

for the shift kit remaining pieces… I should have a pic of it laying around, I’ll look for it

 

[WWRD99]

The shift points don’t seem low to me considering that the RPMs sound pretty high when shifting, maybe you are thinking about M1009s shift points? The M1009 has 3.08 final ratio vs M1008/M1028 having 4.56
I think I’ve read somewhere that M1008 upshifts should happen at 15mph for 1-2 and 25mph for 2-3, without engaging the kickdown of course

for the shift kit remaining pieces… I should have a pic of it laying around, I’ll look for it

No I'm thinking of my 1028 not my 1009. It isn't revving high but bogging down if shifting into 3rd at 25 mph. Way to low. But I guess if youre happy with that low shift point just leave the trans how it is. Any change you make will only raise shift points as I don't think you can make them any lower.

Sent from my SM-S906U using Tapatalk

 

[Enforced_Leo44]

No I'm thinking of my 1028 not my 1009. It isn't revving high but bogging down if shifting into 3rd at 25 mph. Way to low. But I guess if youre happy with that low shift point just leave the trans how it is. Any change you make will only raise shift points as I don't think you can make them any lower.

Sent from my SM-S906U using Tapatalk

Maybe I'm getting things mixed up then, let's see if I got it now: my problem could be related to the VRV and/or vacuum modulator because, for instance, I'm getting too much vacuum at the modulator at any given throttle position and the transmission "thinks" that I'm in a no-load situation, hence why it always upshifts at the slowest speeds possible if I'm not at WOT engaging the kickdown

All this excluding the possibility of the TransGo shiftkit messing with the operation of the valvebody

Also, something I missed to specify in the previous posts, when I'm at WOT (kickdown engaged), the shift points are around 25mph for 1-2 and 40-45 for 2-3, does that sound right?