High-Low clunk

[TK THE TRUCKER]

One of our trucks has a tranny issue. Everything shifts right except when your going from high to low. From low to high it's ok but if your, let's say slowing down and go from high range to low range it clunks in, rather loud and you can definitely feel it. The only time it does not do it is when your completely stopped or moving very slow, like 2 mph, then it sounds normal. We have had someone tell us it might be some sort of pressure regulator in the tranny that is only supposed to get about 60 psi but might be messed up and not delivering the right pressure to the system. It's a 1993 Pete 378 425 CAT Eaton 18 speed. We just found out the truck had a 18 speed. It originally had a 13 speed because everything on the truck states that and also my boss bought it like that but the driver and I were talking and he mentioned he could split the low side and I happen to drive it a few days later and sure enough you can. I told my boss this and he wouldn't believe me so we crawled under the truck to look at the tranny tag. He believes now. I've driven it quite a bit when the driver is out and my boss has driven it a lot in the past but we both assumed it had a 13 speed so we never attempted to split the low side. The gears the truck has you can hardly tell the difference on the low side splitting gears. They're so close it's not much use. Anyways, any other ideas on the high to low clunk ?

[allan5oh]

well first the most simple thing, is the range selector being pre-selected or is someone just hitting it in neutral to go into the basement?

Always pre-select.

[silvan]

One of our trucks has a tranny issue. Everything shifts right except when your going from high to low. From low to high it's ok but if your, let's say slowing down and go from high range to low range it clunks in, rather loud and you can definitely feel it. The only time it does not do it is when your completely stopped or moving very slow, like 2 mph, then it sounds normal.

I'm not sure what causes this, but I experienced it, and know exactly what you're talking about. I used to coast in neutral a little way until I was going slow enough to make the range change without the big clunk. Whatever this was, my old boss wasn't interested in fixing it. He knew what it was, and said it would probably go a long time like that. He was right. It lasted another 2-3 years, and we didn't scrap the truck over the tranny.

That was in a low weight stick hauling application though. YMMV hauling real freight. I think he said it was the synchronizers going out. The range change box does have complicated synchronizers.

[TK THE TRUCKER]

It's been doing it for awhile. I usually don't drive it but after driving it the other day my boss and I were talking about what it could be. We're going to take the tranny out of the wrecked truck and have it looked over and then replace the messed up one and have that repaired so we have an extra tranny around.

I wait till I'm stopped or moving really slow to put it in low range but sometimes your approaching a red light and it turns green and you gotta go into low and let it slam. It hasn't broken yet so we're gonna run it till it can be swapped out. If it ain't broke......wait..... if it ain't completely broke don't fix it 8) 8)

[Kranky]

Bad low range synchronizer.

It will keep working (with clunking & grinding) for a while, then once the teeth get worn down enough from all that grinding it will just jump out of low range if you try to put any power to it.

At that point, the tranny must come apart & get fixed.

[SoCal79]

Mine will do that also,some times it will hit so hard like a sledge hammer. most of the time it is when I roll up to a light or to a stop in high range and drop into low to get moving again. I found that feathering the fuel pedal just a bit when selecting low range eliminates it.