Transmission slips when offroad

Cindy Wood · October 23, 2023

I'm traveling in Baja California. Just had the pan dropped and transmission filter/fluid changed up in California. Driving on pavement transmission acts normal, but after about 10 miles of off road driving in heavy washboard, the tach started revving to 5000 rpm and no forward motion. The worse the washboard got, the worse the problem. When driving and switching gears manually it seemed to work OK. Only had problems when shifting automatically. Constantly revving. I worked my way back to the pavement and once I reached the pavement all was working fine again. My best guess is that when they changed the fluid, they didn't add enough and agitating it off road is allowing air into the lines? Of course no way to check how much fluid is in there (at least that I know of). Tomorrow I plan to drain the fluid and see how much there is and replace with the correct amount.

Vehicle is a 2020 Silverado HD. 6.6 gasoline engine.

My questions:

Anyone know how much fluid I should put it without having dropped the pan?

Can you think of any other faults that could be causing the transmission to slip only offroad?

Thank you for any help you might be able to offer.

Jettech1 · October 23, 2023

25 minutes ago, Cindy Wood said:

I'm traveling in Baja California. Just had the pan dropped and transmission filter/fluid changed up in California. Driving on pavement transmission acts normal, but after about 10 miles of off road driving in heavy washboard, the tach started revving to 5000 rpm and no forward motion. The worse the washboard got, the worse the problem. When driving and switching gears manually it seemed to work OK. Only had problems when shifting automatically. Constantly revving. I worked my way back to the pavement and once I reached the pavement all was working fine again. My best guess is that when they changed the fluid, they didn't add enough and agitating it off road is allowing air into the lines? Of course no way to check how much fluid is in there (at least that I know of). Tomorrow I plan to drain the fluid and see how much there is and replace with the correct amount.

Vehicle is a 2020 Silverado HD. 6.6 gasoline engine.

My questions:

Anyone know how much fluid I should put it without having dropped the pan?

Can you think of any other faults that could be causing the transmission to slip only offroad?

Thank you for any help you might be able to offer.

It definitely sounds like you are low on fluid. Please keep us updated on what you find.

Cindy Wood · October 23, 2023

Thank you Jettch1! Hopefully that's it. I'll find out tomorrow.

Still...anyone know how much less to fluid to put in if just change the fluid without dropping the pan? Thank you!

newdude · October 23, 2023

Pan drop on a 6L90 is typically 6-7 quarts. These have a thermostat on them too so to properly set the level it has to open and fill the cooler and lines and then cool down and then have the level checked and set.

However...a 6L90 (and 6L80) not moving in drive? An all too common sight these days. Typically a sign of converter or fluid pump failure.

What was your transmission temp while driving? If the thermostat hasn't been changed to the lower temp one, it should still run at 194F when its full opened.

EDIT. Before jumping to conclusions as I did (late night last night), Pryme has the best simple explanation that could be what happened, especially since you went back to regular roads and it drove fine. Plus as I read it again, you shifted in manual mode and it would drive better. You short shift, it will reduce the amount of spinning going on by cutting power/RPM lower.

Edited October 23, 2023 by newdude

Jettech1 · October 23, 2023

12 minutes ago, newdude said:

Pan drop on a 6L90 is typically 6-7 quarts. These have a thermostat on them too so to properly set the level it has to open and fill the cooler and lines and then cool down and then have the level checked and set.

However...a 6L90 (and 6L80) not moving in drive? An all too common sight these days. Typically a sign of converter or fluid pump failure.

What was your transmission temp while driving? If the thermostat hasn't been changed to the lower temp one, it should still run at 194F when its full opened.

There we go, the God of all has spoken. Thank you @newdude!! I'm being serious here of course, not mocking you sir!

Pryme · October 23, 2023

Do the trans dipstick mod so you can keep an eye on the level.

im sure this is a dumb question but are you certain you’re not getting wheel spin in soft sand and getting the tach to climb based on that? Kind of like ice causes wheel spin and the tach to rise?

Cindy Wood · October 23, 2023

Thank you for the insight. Driving on pavement the temp is normal, even up long steep hills. No slippage, Also, no it wasn't' sand slippage. RPM's up to 5000 and no forward motion or tire spin. Let off accelerator and when slowed down drove normal for a bit then repeat. Put it in manual and was able to creep back to pavement. Not a single issue on pavement. Still have a 2 hr drive this morning to a supposedly good mechanic. So far so all good driving on pavement, almost 700 miles on pavement and no issues. Drove about 550 miles on pavement after trans service and didn't even know there was an issue until hit heavy offroad conditions. Have driven another 150 miles on pavement since and have another 150 to go to a good mechanic. So far so good. Weird stuff.

Cindy Wood · October 25, 2023

Unfortunately we had to drive just short of another 250 miles to find a mechanic who knew transmissions. He determined we were three quarts low!!! He didn't have any experience with this truck/transmission but found a small sticker on the transmission, so small it was unreadable without taking a picture of it. It initially didn't make sense. Then after some research we found that in order to check the level you have to heat the engine to the temps in the sticker. Cycle through N,D,R,P, leave the engine running and open a plug on the bottom right forward area of the transmission. If fluid drips out it's full, if it drains out, it's too full. If nothing comes out you need to fill until some fluid starts to drain out. What I'm not sure of yet if the plug is the regular drain plug or a separate plug. Have go get under the truck and see for future reference.

The mechanic devised an ingenious device to fill the trans. He fashioned a plug to fit into the fill hole, cored it out so a tube could enter it, then made a funnel cap to screw on the trans fluid bottle and put the trans fluid through the tube and into the transmission. Took three quarts.

I think the U.S. Mechanics just arbitrarily put in 6 quarts cold (or maybe got distracted and put in less) and never checked the level. Cost was $355.00 The guy in Mexico spent probably twice as much time on the truck trying to figure it out and charged me $15.00. To think....I had the service done in the U.S. so it would be done right! Fortunately we did the off road trek or we would have never known there was a problem, maybe until something burnt up.

All is well, we made it to our destination, albeit a bit late

Thank you for the help and insights!

Pryme · October 25, 2023

2 hours ago, Cindy Wood said:

Unfortunately we had to drive just short of another 250 miles to find a mechanic who knew transmissions. He determined we were three quarts low!!! He didn't have any experience with this truck/transmission but found a small sticker on the transmission, so small it was unreadable without taking a picture of it. It initially didn't make sense. Then after some research we found that in order to check the level you have to heat the engine to the temps in the sticker. Cycle through N,D,R,P, leave the engine running and open a plug on the bottom right forward area of the transmission. If fluid drips out it's full, if it drains out, it's too full. If nothing comes out you need to fill until some fluid starts to drain out. What I'm not sure of yet if the plug is the regular drain plug or a separate plug. Have go get under the truck and see for future reference.

The mechanic devised an ingenious device to fill the trans. He fashioned a plug to fit into the fill hole, cored it out so a tube could enter it, then made a funnel cap to screw on the trans fluid bottle and put the trans fluid through the tube and into the transmission. Took three quarts.

I think the U.S. Mechanics just arbitrarily put in 6 quarts cold (or maybe got distracted and put in less) and never checked the level. Cost was $355.00 The guy in Mexico spent probably twice as much time on the truck trying to figure it out and charged me $15.00. To think....I had the service done in the U.S. so it would be done right! Fortunately we did the off road trek or we would have never known there was a problem, maybe until something burnt up.

All is well, we made it to our destination, albeit a bit late

Thank you for the help and insights!

Glad they got it figured out. I wonder if irreversible damage has occurred that might pop up later? Hopefully not.

Jettech1 · October 25, 2023

4 hours ago, Pryme said:

Glad they got it figured out. I wonder if irreversible damage has occurred that might pop up later? Hopefully not.

I would highly doubt it @Pryme. If they went thousands of miles with it being that low on fluid then I think they could have some burnt clutch packs, but with the little use they had before the problem was recognized and fixed, I would bet no harm done.