Awful Automatic
Question: We've got a '96 GMC Sierra 2500 that is completely stock with the exception of the front bumper and the neck-over trailer ball in the bed. The problem is we've owned this truck since 1999 and we are about to have the fifth (yes, fifth) 4L60E automatic transmission put in. Mostly the truck is up and down the highway and slow idle through pastures. But we do pull with it. By pull, I mean we hook up a 28-foot flatbed and then proceed to put about 12,000 pounds of hay on it. We don't do this all the time, but when we do, it is for 3-7 days in a row making trips of about 12 miles loaded, and then 12 empty.
I'm wondering why this trans doesn't seem to hold up, because we also have three other trucks that never have any trans-related problems doing the same kinds of loads. And two of those trucks will see that same load only we are dragging said load through rutted pasture roads that are usually about halfway up the 16-inch wheels in mud. The other trucks are '79 F-350 CrewCab, '79 F-250 SuperCab, and '96 Dodge 2500. I realize that the C6 autos in the Fords are a different animal than the 4L60E, but that different? And the Dodge is all stock and auto as well. To be clear, the trans gets to where it won't shift out of Second gear and into Third.
Any suggestions or ideas as to what's going on would be appreciated. Also, we are not taking the truck to the same shop when we have the trans done. It will be at a fourth shop this time. None of the shops seem to have an answer other than not to pull serious loads with the truck. I'd be more prone to accept this advice if I weren't doing it with the other trucks and having no problems.
Wade Bozeman
Benton, LA
Answer: The first thing I notice is that your truck won't come out of Second gear. This could be a very simple repair. First check to make sure all your wiring is properly attached. My electronically controlled 4L80E had a similar problem when one of the speed-sensor wires was broken and it went into limp mode, which didn't let it shift out of Second gear to protect it. If you are getting a check engine light on the dash you may want to have a local shop or dealer run the codes, this could be a quick fix. However, it may be more than that.
The other problem is that at some point the OEMs started making light-duty and heavy-duty 3/4-ton trucks. If your transmission is indeed a 4L60E then you have a light-duty 3/4-ton and not a heavy-duty 2500 that came with a beefier drivetrain including a 454 big-block engine, 4L80E transmission, and 14-bolt full-floating rear axle. The 4L60E is basically a 1/2-ton TH350 with an overdrive gear, and to put it bluntly, you're towing too much weight with a light-duty transmission. In fact, your truck is more parallel to a 1/2-ton where your Ford and Dodge are probably closer to being 1-tons since the Ford C6 and (what I assume you have) Dodge 47RE transmission are also found in 1-tons.
The GMC should have a 305 V-8 and a semi-floating 14-bolt as well as a lighter-duty frame, all of which are being over-taxed by that 12,000-pound trailer full of hay. If you are dead-set on keeping that truck I would at the very least upgrade to a TH400, which is closer to the Ford and Dodge automatics with a massive auxiliary cooler or an old four-speed manual transmission and lower axle gearing of around 4.56. This upgrade in strength will, however, cause you to lose your overdrive, but you shouldn't be towing in overdrive anyways, a fact that may be leading to your many dead 4L60s.
Another option is to swap in a 4L80E, which is basically a TH400 with an overdrive, but even still you are towing a weight that is more akin to late-model 8.1L big-block gas or Duramax diesel 1-ton limits and those come with the massive Allison transmission. I hate to say it, but I think you need another truck, and to be on the safe side I would step up to a true 1-ton.
Tech Letter Of The Month
It's The Least We Can Do
Question: I'm currently assigned to Camp Liberty, Iraq. My wife sends me an issue of 4-Wheel & Off-Road in every month's care package. I was just wondering by chance if I could get or buy a license plate from y'all that I can put on my Humvee. The commander even approved it if I could get one from y'all. I'll send a picture of the license plate, the mag, and myself to you if you send me one. Thanks for a great mag and keep wheelin...you can wheel in Iraq too, you might just have to keep your head down.
Spc. Zachary Stilson, Camp Liberty, Iraq
Answer: Every month I pick a Tech Letter of the Month that covers a question many of you have. But this month as I'm sitting here in my cluttered office, I imagined all you troops over there sleeping in the desert, and figured it's time to sent out a thanks. This month's prize goes to you, Zachary. I know there doesn't appear to be any tech questions in your letter but I think I found one. It seems you are wondering how to make your Humvee more capable off-road while at the same time conveying just one of the great advantages of a democracy to the people of that war-torn country, and I have to agree that a 4-Wheel & Off-Road license plate along with some special edition stickers, maybe a coozie or two, and whatever other cool four-wheel stuff I can find around the office to send you would definitely make it work that much better, and help convince the locals that freedom is very valuable. I'm sure whenever all us Americans get bummed that our jobs stink or we don't have enough time to go wheeling, we just need to think of you and the troops out there getting shot at and be thankful that we can go wheeling at all. I'll keep locking it in low range and you keep safe over there. Thanks for all the hard work you and your fellow service people are doing and when you get back here let's go find some mud, rocks, sand, or snow to hammer down in.
Cheers to ya!