Chevy Silverado 1500 Towing Question

[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.

[RockyMtnRay]

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.

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.

[B_and_D]

Oh my, RMR, you've given me a lot of homework on this one.

I tried to find out what the axle ratio was, but I don't know and can't find it on the net.

It is a 5.3L V8, 4 speed transmission, with 4 wheel drive, 4 door. We just had the transmission serviced 8,000 miles ago (and supposedly they checked the fluid level last time we had the oil changed, we try to take good care of our truck). But you know, now that you pointed it out, last time I had my Cad done there, the fluid was low right after they serviced it, and I had to add a lot of fluid one day. They said sometimes the system had to "burp". I wished they'd "burped" it for us. I will check the tranny fluid next time we take the truck out.

The temperature was probably about 65 degrees, grade, ??? If I look at a protractor it looks like maybe 5 to 10 degrees (but it's so hard to tell while you're driving). The engine temperature never varies much. Don't have a transmission temp gauge or cooler.

I wasn't driving, so I can't tell you what the RPM's were. I just felt the truck's transmission "slipping" out of 3rd whenever DH put his foot on the gas hard.

Thanks for your imput...

[RockyMtnRay]

Originally Posted by B_and_D

I wasn't driving, so I can't tell you what the RPM's were. I just felt the truck's transmission "slipping" out of 3rd whenever DH put his foot on the gas hard.

True automatic transmission "slippage" is a sharp rise/fall in the RPM during the shift. When an automatic transmission shifts gears, the set of bands and clutches that are used for the original gear are released, then another set of bands and clutches needed for the new gear are tightened. If the new set of bands/clutches don't engage promptly, there will be a very noticeable delay between the gears and the engine will rev up sharply until the new set of clutches finally take hold, then the engine RPM will drop and hold steady for the new gear. A transmission that has really bad clutches/bands will also have some slippage if the throttle is sharply opened in a gear...the RPMs will rise by as much as 1000 RPM...very similar to a manual transmission car with a really bad clutch. For a really good explanation of how an automatic transmission works, go here.

My guess is what you were experiencing was a quick downshift (no noticeable delay or spiky RPMs) when DH stompled the throttle, followed by a rapid rise in engine RPM in the lower gear for a few seconds, then a quick, possibly jerky upshift back to the gear you were in when the throttle was stomped. I'd also guess that the period of time spent in the lower gear was longer and the engine was rising to higher RPMs before it upshifted when you used Tow/Haul mode. Given the 20-30 mph speeds, I'd also guess that the transmission was going from 2nd (maybe 3rd) all the way down to 1st, then back to 2nd when it hit its upshift point.

If these guesses are correct, then what you were experiencing was not actually "slippage" but rather the quirky "hunting" of a GM transmission when it is very close to its programmed upshift points under high load and wide open throttle. If this quirky behavior was occuring mostly when you were not far from Yosemite (IOW, at elevations around 4000 feet), you were also experiencing the deleterious effect that altitude has on engine performance. 4000 isn't very high but it is high enough to cause a 10 to 15 percent drop in performance which in turn is enough to produce effects you're not used to in near-sea level operations.

If my guesses are not correct, please advise with more details.

[MikeD]

B_and_D,

I think that Ray's explanation is the right one. I just returned from a trip to Yosemite over Highway 120. I recall that the grades are all in the 6% to 7% range. I haul my 2720 with a Chevy Tahoe (5.3L, 3.83 axel, 4 wheel drive) - so the comparison is good with your Silverado. I used the Tow/Haul selector, but experience the same engine surge which climbing as you do - as the transmission hunts down to 2nd from 3rd and back to 3rd when I depress the throttle. I usually shift manually, leaving it in 3rd unless I slow too much, and then dropping down to 2nd. I find that I can do a much better job of gear selection than the tranny brain. For going downhill, I spent a lot of time in 2nd, to take advantge of the engine braking to keep my speed under control without using my brakes.

MikeD

[B_and_D]

I appreciate your answers - sounds like it's fairly normal for this to happen. We were concerned that maybe there was something wrong with the transmission. Next time we do that much climbing we'll use the manual shifting technique, and I'm also going to check the fluid next time we get the truck warmed up.