kusman

While wheeling recently I noticed something iffy in the sense that I do not know if this is the right behavior or not and I can't seem to find an answer in google.

Basically, when the jeep is in HI on the transfer case and there is high load on the vehicle (e.g recovery of other vehicle or deep and steep sand hill) the engine would rev up to about 2k RPM even if gas pedal is floored, but there is no wheel movement. This happens in 1-2 gear, in 3 and in D.
If I switch it to LO, then it starts to go.
On HWY, the engine revs past 2k RPM without issue.

Is this behavior normal?

DFlintstone

Sounds like you are getting enough traction to over-whelm the torque/power you have in high. You need less traction!, (or more power). Just a thought.

kusman

Yes, this is exactly what seems to be happening, but shouldn't the engine be able to rev pas 2k RPM in this case? I am simply not sure if something is broken or it should be like this.

Donnie_K

If someone in stuck up to their body line in mud... pulling them out in 4 hi is a hell of a lot to ask of a vehicle.

razor02097

The High and Low refers to the gear not the load.

4 hi should be used on trails and loose surface conditions in which you are going forward at any sort of speed.
4 lo is used for crawling up or down hills, pulling and any situation where you need maximum traction and torque.

Donnie_K

Doesn't that in itself mean low load you can use 4 hi, high load you need 4 low?

razor02097

Yes

VTJeep

Same issue on occasion. Kinda bogs and seems like it's running rich.. I have a bad wire somewhere.. The PCM is also possible at this point, too. It's goin under the wrench on tuesday... And were gonna find the bastard, this time... Either way, I've got this hesitation issue that is messing with the o2's, so that could be the runnin rich issue... Idk...

kusman

Well yes, this is understandable. I guess I would just expect the engine to rev more if there is no power. The gearbox already does this anyway: when you are starting from a steep hill, you need to give more gas (higher rpm) to start moving. Following that logic, even in high, it should rev up until there is enough power, not just get stuck on 2k rpm. I am not sure if there is a limiter there somewhere that causes it to do this? Is this normal for an auto transmission or should it rev up like I am assuming?


Well for me it was not an occasional thing. Happens all the time in high-load situations. The reason I think something is artificially limiting the engine is because it pulls great on tarmac, so there is plenty of power. I mean the engine sound does not change, nor is there anything coming out of the exhaust when it happens. Its just like it revs up and hits a wall at around 2k RPM. I don't have another automatic car to test it, maybe it should be like this?

razor02097

All automatic transmissions have a torque converter. All torque converters have a rated stall speed. A stall speed is pretty much what it sounds like... you can spin it up to that speed and either the output moves and the RPMs continue to climb or it isn't going to spin any faster causing the RPMs to "stall" at that speed. The idea is you have the stall speed at where the engine makes power.

What it sounds like happened is you maxed out the available power either binding the drive train or pulling something. Using 4lo will multiply the torque using the gearing in the transfer case. However keep in mind maxing out the pulling power of your Jeep all the time... you WILL break parts.

kusman

Thanks, I see, this clarifies a lot then. I just though there was more power, but could be because of the tire change. I was wondering this because some time ago I was going up the same sand hills on road tires and there were no issues on HI, but now with mud tyres I guess it just digs to deep and requires too much power to spin the wheels.

Thanks, knowing this, I'll try to avoid situations like so that I don't break parts.

DFlintstone