Cheap Fix for TCC/4th Gear Problem

My 1995 Transport developed "the common" problem of losing TCC/OD when the trans. fluid got hot. First I installed a small trans. cooler in front of the rad/AC cond. Tried piping it both ways-with and w/o the rad. cooler. It worked much better in series, with the fluid going thru the add-on last.

But the problem remained-it just took longer to lose the TCC/OD. Did a LOT of research into the problem. Seems some moron trans. engineer in GM managed to design the TCC apply valve such that the slightest leakage which got past the valve (steel valve sliding in the soft alum. valve body) would cause the TCC to disengage. The "stupid computer" would then sense the RPM mismatch-it had commanded the TCC to apply-so it then decides to prevent 4th gear (OD) from working. I live in Phoenix and the trans. temp. sensor never got hot enough to set a code-the problem was the trans. fluid getting thinner as it got hot.

I believe Trans-go or Sonnax makes a kit to replace this valve and add a steel bushing (what the trans. should have had originally), but it appears that the trans. has to be removed to install it. But there is another answer.

I decided to take a chance and use a thicker trans. fluid. I knew I was going to have to pull the trans. anyway if it didn't work, and then I'd have it rebuilt with the stronger heat-treated stuff etc. that also should have been used originally. I also wanted to regear it lower, because even with a good-running 3.8 the trans. was way overgeared-at least for me.

So, after making MANY phone calls, etc. I bought a 5 gal. pail of 50 wt. "Powershift" trans./hydraulic clutch fluid from a Caterpiller dealer. This fluid is used in earthmoving equip., and also comes in 30 and 10 weight. I dropped the pan and refilled the trans.-you can only get ~50-70% of the fluid out. Refilling with the 50 wt. gives about 30 wt. with the fluid that remains in the converter. What all this means is that I gained ~30-40 deg. F on the viscosity of the trans. fluid, and the problem disappeared. It has worked fine for ~3 years and 20,000 miles so far.

The fluid is not expensive-I think I paid $35 for the 5 gal. They don't sell it in smaller amounts.

Several cautions besides the "do this at your own risk". Because I live in Phoenix, the lowest temp. is ~30 F. That means the trans. fluid is about like reg. ATF at 0 or -10 F--no big deal. But if I lived where it got colder I'd have used the 30 wt. for ~15-20 F "advantage".

The trans. works fine now, but I still put up with the gearing that I don't like. I'm just waiting for something else to break so I can remove it and rebuild it right!

Excellent report!!! I'd thought about trying just the trans cooler thing, but it didn't occur to me until reading your post that some transmissions now have "thermostats" that will get the fluid in the control system up to a specific temp somewhat regardless of how much cooling capacity you add. Tricking it by making your oil thicker at the regulated temp sounds like a great idea, and your success should give a lot of people the confidence to try it before spending tons of money on mechanical fixes. Thanks again!

Very interesting indeed. Naturally, all of this is unheard of to me because the lowest temperature in my place is -30, so this minus makes it almost impossible here to experience an overheat problem even in our highest temperatures. Several have complained of same problam as yours and I hope they read your experience..and that they have a Caterpillar dealer in their vincinity.

'bout gear ratio: I know the Bonneville runs a little faster, like 1850 at 60mph. However, I do like my 1600rpm purr at 60 mph but I kindly save the toll on the gearing by downshifting in advance when a significant uphill is coming.

The overall ratio in table below is counted in 3rd gear - does not count the overdrive -As seen here
http://www.avigex.ca/xport/ratio4t60.jpg there are 2 variables: chain sprockets and final frive - equivalent to differential. I have not checked the transmission code but i,d guess it is one with the 35-35 sprocket drive...and then the only alternative to increase the ratio would be the final drive within the transmission....allowing switching from 2.84 to 3.06, or to 3.33 but at great expense of time and hardware.
If it were a 37-33 then it could be changed easily to 35-35 providing a 12% rpm increase...a significant torque boost at the wheels!

>"I have not checked the transmission code but i,d guess it is one with the 35-35 sprocket drive...and then the only alternative to increase the ratio would be the final drive within the transmission....allowing switching from 2.84 to 3.06, a 7% rpm increase, but at great expense of time and hardware.
If it were a 37-33 then it could be changed easily to 35-35 providing a 12% rpm increase...a significant torque boost at the wheels!"<

I think I have the 35/35 chain setup and the 3.06 final drive ratio. If I ever get around to pulling the trans. I'd use a 33 driving a 37 and the 3.33 final.

But first I have to find a weird electrical problem. The local dealer couldn't find it. It's intermittent--I strongly suspect a bad ground somewhere (under dash???). The voltage will suddenly drop down, causing the speedo to jump way up, and some of the dash warning lights light up. It also causes the trans. to go to neutral. It is all over in a second or two, then everything is back to normal for a while.

If it were a short somewhere I don't think it would "fix itself" so easily. So far I've checked/cleaned the battery cables and a few easy-to-reach junction points. Also removed and cleaned most of the fuses--after I managed to find them inside the glove comp.

Unhappily, the OEM 37/33 drive and driven sprockets cannot be switched one for another to make a 33/37 ratio because they are very different in their design ..maybe performance shops have these and other options.