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Originally Posted by B_and_D
We towed our TM up to Yosemite last weekend. We encountered some slower moving vehicles that we got stuck behind at low speeds on steep, curvy roads, and when we tried to get around them in the passing lanes, it seemed that the transmission would try to downshift, but wouldn't stay more than one gear down. It would momentarily downshift into the 2nd gear down, but then it seemed like it was slipping, wouldn't stay down in that gear. We didn't try to use the manual downshift on the steering column.
I think the engine has plenty of power, and when we used the "tow" mode it seemed to help. Has anyone experienced this when towing uphill at slow speeds? If we were going over 35 - 40 it wasn't a problem, it just happened while we were going 20 - 30.
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What's your engine's size and what's the axle ratio? What is the redline RPM on your engine? What RPMs were showing on the tachometer during these downshift/upshift episodes in regular and in tow mode? About how steep (% grade) were the climbs where this was happening? What was the outside temperature and what was showing on the engine temperature gauge? Do you have a transmission temperature gauge and, if so, what temperature was it displaying?
20-30 mph is possibly near the upper end of your transmission's 1st gear RPM range. What behavior do you normally experience at WOT (wide open throttle) on flat land towing while doing maximum accleration in this speed range (e.g. accelerating up a freeway onramp)?
On electronically controlled transmissions like yours, clutch/band slippage at WOT is sometimes an indicator that the transmission fluid needs to be replaced. How many miles have you gone since the last transmission fluid flush?
Among other things, "Tow/Haul mode" on GM transmissions changes the upshift RPM points to keep the engine in its best power band. In "Tow mode" at WOT the transmission should not have had an upshift until the engine was at or near redline RPM.
Tip: As someone who mostly tows on steep, high altitude, mountain roads, I use the manual gear selector lever a
lot on both climbs and descents.
First, it allows
me (not the transmission's "brain") to control when the transmission will upshift. On a lot of long (7 to 10 mile) climbs on multi-lane highways, I find I have to momentarily "feather" the throttle because some goombah slowpoke has gotten out in my lane...if I have the selector in Drive (3rd on my tranny), the transmission will immediately upshift from 2nd to 3rd gear...and then I'll have to force a downshift when I go back to WOT.
Every shift, especially those at or near WOT causes wear on the transmission's clutches/bands. But if I lock the transmission in 2nd with the selector, no unwanted upshifts.
Second, if I know there's a passing situation coming up and I'm shortly going to want a downshift and full power, I move the selector down one gear
before opening the throttle. This prevents the strain on the transmission that comes from a full throttle "passing gear" downshift.