PWM Question

[stutaeng]

I was talking to my brother the other day about his 90 RCSB and he mentioned something about TC locking up/rpm drop. I honestly can't say that I know when the TC locks on all the trucks that I've owned, but they have all been "E" tranmissions. Last I drove Dad's 89 Sierra was like 20 years ago. ...too young to know about anything, lol.

Anyway, why did manufacturers go to PWM? Is it only so that you don't feel the on-off operation? Isn't PWM standard now for all automatics?

And why do guys block off the valve that does this? For durability? Firmness? Would you even block it off for a daily driver with stock drivetrain?

Here's what I'm talking about. At around the 7:45 mark he talks about it...

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[Schurkey]

The point of PWM for torque converter clutch engagement was to smooth-out the "bump" when the clutch locked. Can't be upsetting Grandma with all that violent shifting, you know.

So Granny is happy, but the extra slipping of the clutch as it slowly (relatively speaking) engages isn't doing the friction material, or the fluid any favors.

GM has been...obsessed...with "smoothness" of shifting ever since Buick refused to use the original Hydra-Matic. The chief engineer for Buick called it "Hydra-Jerk". Buick (and Chevy) went so far as to design NON-SHIFTING "automatic" transmissions; the earliest versions (the original Dynaflow and Powerglide) just started in high gear so performance really sucked unless you slammed the shift lever down to "L". And if you have to manually shift...it's not really an "automatic" transmission. Later versions of Powerglide abandoned that principle; starting in first, then shifting to "high". Dynaflow decendants (and the Chevy Turboglide) got tricky with two or even three turbines in the torque converter each attached to a different gear ratio so that there was no "shifting" except by the action of the hydraulic fluid in the torque converter. So there were progressive gear ratios, but they hydraulicly overlapped without a perceptible change from one to another. Essentially, a hydraulic CVT.

 

[stutaeng]

I read a bit of this article, related to the topic. It talks about the "On/Off" lockup, as well as the PWM, and EC³ (I didn't know what this is...)

According to the Sonnax article, there could be bad things from blocking off that valve...well, maybe I'm thinking they are blocking it off, but looks they are actually modifying it to convert it to an On/Off? Instead, Sonnax recommends a kit for dealing with the valve bore wear on the TCC regulator. Looks like a reamer to enlarge the bore on the casting and an insert sleeve with a new valve. I'm assuming the new valve/sleeve are better materials to stand up to the oscillations?

20 Years Later: What’s New with 4L60-E PWM Modifications

Transmission builders are no strangers to modification. Making changes to increase performance, to prevent known issues from cropping up or to overcome existing

www.sonnax.com

https://www.transtar1.com/TranstarIndustries/media/Transtar1/technical/bulletins/TLT-TC-035.pdf

This second article has a table of different torque converter friction materials and type of lockup based on years. Indeed, the PWM+EC³ looks like came out in 97 or 98. Also interesting information.