[VAL] Trans temps

[Stan Nicholson <stan@xxxxxxxxxxx>]

Tom & Ed -

It's great that engineers can use today's electronics to protect 
transmissions from complete disaster, but I wouldn't use that as the 
first line of defense.  Heat kills transmissions by degrading the 
trans fluid, and the rate of degradation increases substantially with 
relatively small increases in temp. At the moment I can't lay my 
hands on a chart that plots the number of miles before you should 
change the fluid vs. measured temp, but it's pretty dramatic. IIRC, a 
trans that should be able to go 50,000 miles between changes at temps 
of 200 degrees should only go 10,000 at 225 degrees, or 500 miles at 
250 degrees -- don't hold me to the particular numbers, 'cause I'm 
just trying to illustrate how dramatic the relationship is. Of 
course, it also makes a difference how much time the trans temps 
spend elevated. A momentary spike to 250 degrees is far less 
worrisome than an hour spent at 250 degrees (or even than an hour 
spent at 225 degrees).

Bottom line - I'd keep an eye on the gauge, and try to keep the 
electronic safety measures from ever kicking in. If they ever DID 
kick in, things would have gotten so hot that I'd change the fluid at 
the first opportunity.

Ed, I believe my tow vehicle ('96 Suburban 454, 4L80E trans) has the 
same transmission as your new Suburban. I installed a B&M aluminum 
trans pan, and tapped it for a gauge, when I bought it. I also 
replaced the factory tow package trans cooler with one twice as big. 
So it's been interesting to see that the temp never goes above 200 
degrees using the Sub solo, or when towing on the flat, but even 
small grades can cause notable rises. It was an uncomfortable 
surprise to see that a few miles of 6% grade could produce a reading 
of 240 degrees, and to see how long it takes to dissipate that heat 
after reaching the crest. Be nice to your tranny, err on the side of 
caution in your fluid change intervals, and it'll take care of you 
for a very long time. At least until GM has the sense to overhaul the 
Suburban chassis to permit the Allison 1000 to fit.

Stan
'72 Overlander