Chevy & GMC Vans

So we're heading home from Sedona on I75 north somewhere between Daytona and Toledo.

We stop for a fill up and my wife takes over. Back on the highway and there is this Ka-Thunk that feels and sounds like we just drove over a big ass road seam.

It happens 3 more times, I am thinking the rear end is going and this is going to cost us big time and we're 8 hrs from home.

We pull off and find and found an auto parts store. I go in and ask if we are good to work on it in the parking lot, all good. I phone our mech back home to deliberate about what it could be.

A few suggestions later, and some investigation on my end. Thank God I brought toos, jack stands and a bottle jack. Removed the back wheels, checked the rear brakes for potential lock up, nope, checked for possible rear axle seal, nope.

I think one of the Spyder gears or something in the posi might be broken and hopping around, drained the rear diff, removed the cover. No issue. WTF now!

Phoned our mech back, more discussion. I decided we should try pulling the transfer case control modual connector thinking it might be a fault somewhere causing the transfer case to engage and disengage, I am new to this electronic transfercase thingie.

3hrs in the parking lot, some fluid brake cleaner, and drain pan purchases from the parts store and we were on our way.

Back on the highway, no issues at all. Smooth sailing for the next 8hrs.

Man thank God we didn't have the energy to pull the straight drive through non stop that we had been hoping for, we might have been in a real jam in the middle of nowhere at night.

Anyone have any ideas what the actual cause is?

I am not really concerne as I plan to swap in an np236 this year.

I will add I had all fluids changed in both axles, tranny and TC 14k miles prior to this.

let me know if you find it out ??? I get that once or twice a year for at least 7 of the last 10 years. I too figured it was the diff eating itself but they look fine every time they are serviced ( every 3 years ). I have been told it could be everything from the vss to the fuel pump to the torque convertor acting up ( assuming that something is letting the driveline slack then spin again to catch up ). Mine is like a "neutral drop" only it happens at highway speeds. It bucks a couple times then goes away. '03 awd in my case

Interesting.

If we are getting it to happen at highway speeds then it has to be a more common problem.
I am surprised more owners haven't posted it up about at least having herd about it.

I will most definitely post up if/when I find a remedy or cause.

I have heard a similar thing on rare occasion back when I was still AWD-equipped. Mine was reliably doing it on an up-hill, off-camber, left turn, after a red-light (near my house). Something about that combination of wheel speeds made it do it 1x or 2x a week for a couple of months.

[quote="truC"]So we're heading home from Sedona on I75 north somewhere between Daytona and Toledo.

We stop for a fill up and my wife takes over. Back on the highway and there is this Ka-Thunk that feels and sounds like we just drove over a big ass road seam.

So were you JUST getting on the highway as in trans still up shifting? Mine did something similar yesterday getting on the highway. It was an uphill grade. I kicked it into passing gear and merged, lifted off the throttle and when it up shifted to OD the van started almost bucking. Thuds in rapid succession. The last shift felt a little softer than usual
Took my foot off the gas and moved to the right. Crested the hill stepped on the gas and all was fine. Check engine lite started flashing and then stayed on. No repeat during rest of the trip. Random misfire code was noted.
I thought torque converter was locking/ unlocking but no way to prove it

With a misfire/P0300 code, the PCM won't command TCC apply so that is perhaps what you were feeling.

If you get the rapid 'check engine' light flashing, you should stop driving and resolve the issue as that is indicating a serious misfire condition that can damage the engine.

Ya I've never seen the check engine flashing before and as soon as the shudder quit the light quit flashing and went to steady on. Hasn't been back since reset. Bizarre set of circumstances.

Capt'nC, It was happening after we were traveling at highway speed at 80mph no up shifting on a level highway, it continued to happen once we slowed to 60mph to exit the highway.

Stovelander, No check engine light for us.

As I mentioned, once I disconnected the lead for the TCCM going into the TC we never experienced it again. We drove for another 6hrs at 80mph, it never occurred again.

No feeling of a misfire or stumble either.


It wasn't a shutter either. We felt a clear thud, and a very very slight forward slowing down pitch. Which is what lead me to try the disconnect.


Herbie, now that is strange. But now that you have dumped the 136 you no longer have it? Which clearly indicates a fault in the the TC or the TCCM system.

It's funny how we get pre-conceived notions on what our problems might be. In your case you logically checked the rear brakes, and looked at the rear-end, which all turned out to be fine. I had rear brakes that would bang and lock up.. the first time it happened I thought I had been rear-ended.

I once had an issue with severe bucking and erratic operation while cruising the highway at part throttle.. it felt like the transmission was failing. It turned out to be a worn distributor.

I'm not sure if AWD engagement at high speeds could cause this issue... since everything is already turning on both ends of the transfer case anyway. One simple solution to engagement on 99-up is to remove the fuse for the AWD controller... which you probably already know.

Good luck tracking this one down!

Thanks,

Chasing an intermittent problem like this can be a real challenged or sure.

Until our trip I had been chasing a vibration in the drive train. I now know it is coming from the front diff. I had checked the fluid level during our trip and found it to be low. I decided to buy some 85-140 gear oil and over fill the diff. I put the passenger front wheel up on a rock so the diff was at a pretty good angle allowing me to over fill the diff. Once back on the road the pitch of the vibration changed.
During cold mornings and having cold fluid and running gear when first getting on the highway and getting up to 80mph the vibration was virtually gone. As soon as temps came up it was back.


As for removing the fuse, Idon't/didn't know which fuse was for the TCCM. It was just easier to pull the connector.