I have this sweet 1999 GMC Suburban 2500 4x4 with the 7.4 & 4L80e that i picked up a few years back. 245k when i got it, 258k now. Mainly used as a towing vehicle for my cars, as i have things that don't get single digit fuel mileage that i drive for errands around town...
Truck has been fantastic, have made a drive from southern Utah to Louisiana, Louisiana to Denver, CO twice now, and much more with no issues and the A/C blasting.
Recently, it has developed a very random, violent powertrain jolt anywhere from 50-80mph. I'll be driving along, then "clunk!" and an aggressive, split second jolt from what feels like the front end, maybe transmission. It's hard to catch on video because it's so random. It's not between gear changes, and seems to only be in 4th, when cruising. Hasn't done it on hard acceleration or deceleration. RPM doesn't change, and it seems to be felt and heard from something lower than the engine bay. This started i would say ~4,000 miles ago. For example, i recently drove 1100 miles in one sitting, and it didn't do it at all. On the 1100 miles run back, it let off about 2-4 of these clunks in a ~15 second time frame, then on for another 500 miles, no clunks, followed by 2-3 more as i got within 100 miles of home. Has never done it at any slower speeds. I'm unsure if it does it unloaded, as it has been hooked up to my car trailer every time i've driven the truck in the past few months. Last night, i had to drop off a car about 45 miles from home. On the way back, it let off about 8-12 clunks in a short 10-15 second window, which is worse than its ever done.
I'm not taking the risk of driving it long distance anymore with this issue, and need some help maybe solving what the deal is. The truck has the Auto4wd/autotrac t-case with the pushbutton unit on the dash. I haven't noticed if i see a change in the lights on that pushbutton unit, because they don't light up half of the time. I rarely use 4wd. I know it will go into 4hi with no problems, but it has never gone into 4lo for me. The previous owner replaced the transmission ~11 years ago, and said 4lo hasn't worked since. I've checked the trans and T-case fluid, both are at acceptable levels, the trans fluid has good color to it, and the t-case fluid looks blue, which IIRC, is correct. Any help would be greatly appreciated.
Truck has been fantastic, have made a drive from southern Utah to Louisiana, Louisiana to Denver, CO twice now, and much more with no issues and the A/C blasting.
Recently, it has developed a very random, violent powertrain jolt anywhere from 50-80mph. I'll be driving along, then "clunk!" and an aggressive, split second jolt from what feels like the front end, maybe transmission. It's hard to catch on video because it's so random. It's not between gear changes, and seems to only be in 4th, when cruising. Hasn't done it on hard acceleration or deceleration. RPM doesn't change, and it seems to be felt and heard from something lower than the engine bay. This started i would say ~4,000 miles ago. For example, i recently drove 1100 miles in one sitting, and it didn't do it at all. On the 1100 miles run back, it let off about 2-4 of these clunks in a ~15 second time frame, then on for another 500 miles, no clunks, followed by 2-3 more as i got within 100 miles of home. Has never done it at any slower speeds. I'm unsure if it does it unloaded, as it has been hooked up to my car trailer every time i've driven the truck in the past few months. Last night, i had to drop off a car about 45 miles from home. On the way back, it let off about 8-12 clunks in a short 10-15 second window, which is worse than its ever done.
I'm not taking the risk of driving it long distance anymore with this issue, and need some help maybe solving what the deal is. The truck has the Auto4wd/autotrac t-case with the pushbutton unit on the dash. I haven't noticed if i see a change in the lights on that pushbutton unit, because they don't light up half of the time. I rarely use 4wd. I know it will go into 4hi with no problems, but it has never gone into 4lo for me. The previous owner replaced the transmission ~11 years ago, and said 4lo hasn't worked since. I've checked the trans and T-case fluid, both are at acceptable levels, the trans fluid has good color to it, and the t-case fluid looks blue, which IIRC, is correct. Any help would be greatly appreciated.
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