The Stovebolt Site Forums The Parking Lot Big Bolts SM420 and 2 Speed Not Split Shifting Correctly

'64 C60 freshly overhauled and rebuild with 292, SM420, and 2 speed rear.

The issue at hand: I can split shift up to 3 hi, and then I cannot shift to 4 lo, it just grinds away without wanting to engage, regardless of rpm range. I am stuck with 3 hi to 4 hi shift which is a big gap and bogs like a son of a gun, or 3 lo to 4 lo and then 4 hi. The 3 lo to 4 lo again is a big jump, but it doesn't seem to be as bad as the 3 hi to 4 hi jump. Its weird because all the gears work great, just not in proper order.

Up to this point it has been a "whatever" issue, but with the new log bed complete at now empty +1k lbs, it makes a noticeable difference, so it can be assumed that at actual GVW it will be significant. That and, I just overhauled everything else, so the axle not cooperating correctly leaves me wanting to fix it.

At this point I have ruled out every possibility outside of the diff. All new air lines on every square inch of the truck. Brand new rebuilt 292 and SM420. I cleaned and rebuilt the shifter with a new diaphragm and ensured all was sealed well. I checked the actuator and all appeared to function perfectly. So what I am left with is that it seems to be something mechanical in the diff, but that seems counter intuitive as the axle doesn't care what gear the trans is in, simply that the rpms are where they need to be to shift properly. So before I open up the diff in the coming 2 weeks (need to anyways for a cleaning and refresh of gear oil and seals), does anyone have any hypothesis on what the problem could be or what I need to pay close attention to?

As a random thought, do I need to try and shift the trans from 3 hi to neutral, shift the axle to low, and then shift the trans to 4 lo and see if that works? Is it possible that someone who previously worked on the truck could have reassembled something incorrectly to cause this?

If one tries to downshift, need to slow down so the faster engine RPM can catch up to and move the two speed into low, otherwise just grinds like you mentioned, takes a while to get used to slowing down, don't need to plan to slow down if on hill already slowing down, for example.

Ed

Totally understand Ed, but its the acceleration where the issue is occurring not the downshift. As I split shift up, the axle refuses to shift up from 3 hi and into 4 lo. I know certain setups have very particular shift patterns, but this setup, as per dash and manual instruction, is 1 lo - hi, 2 lo - hi, 3 lo - hi, 4 lo - hi and it is that jump from 3 hi to 4 lo where it simply grinds away.

Hypothesis is that something was installed improper, but again, it is only this junction that does it, and theoretically if something in the diff was wonky, it would do this at all 3 shift jumps (1 hi - 2 lo, 2 hi-3 lo, 3 hi - 4 lo) but it only does it at the latter.

What if you go 3-lo, 4-lo, 4-hi?

Ed

The truck is fine...I believe this is operator error.

It won't work right with an empty truck driving on a flat road, your ground speed is to fast and there's no load on the rear. Find a long steep hill and see if that makes a difference. If the rear works in 2nd and 3rd than it will work in 4th...it doesn't know what gear the trans is in.

With my 2-speed rear trucks I drive them all the same...start in 2-LO, 3-LO, 4-Lo and then shift the rear to 4-HI. Split shifting is only needed when the truck is really laboring to get moving.

Mike B

This is what I am currently doing at the moment while rolling empty. It works beautifully and i've changed driving habit to roll in lo gears whenever empty and treat 4 hi like OD.


I fully admit that this is not out of the realm of possibility, and the thought has been in my mind. I've been running her through numerous paces over the past few weeks though, on the hills and flat roads, shifting at high and low rpms and speeds, and it seems to be constant across the board regardless of situation. But, I am rolling at a weight of only 9k lbs.

I have the new seals coming in shortly and will open up the diff for inspection and cleaning/maintenance. Ill give everything the once over to ensure all looks good. I will hold off until first load is hauled to try any more testing just to see though. I know it is just small increments of speed differences between the gears, but even just the weight of the bed makes that 3 hi - 4 hi gap pretty strong. Let me take a look this coming weekend/week and get back here to report on it.

Appreciate the info.

Just a thought here . Are we certain its a sm420 , in some of the bigger trucks 1st and 2nd were in the normal position but 3rd was in the 4th gear location and 4 th was in the 3rd location . the reason was 3rd was direct drive and 4th was an o/d . this could explain the unwillingness to enter the next gear as its could actually be dropping a gear . If the trans. had been changed it might not match the tag on the dash for shift patterns .

John

Haven't seen a 4 speed like that, however NP540, yes.

Is the np540 a 4 speed? I like the thought of a 4 speed OD trans.

When I pulled the trans, all the numbers matched that of a SM420, and the guy I have rebuild it indicated that he had to source pretty far and wide for the needed 420 parts. In the 400 or so miles I have put on her thus far, 4th has got to be the direct drive gear. 3 hi wound out on the engine produces about 25 mph, and I haven't even tried to push 4 hi beyond the 55 mark. 55 is about all she wrote.

Also, The trans shifts perfectly between 3-4, its just that 3 hi to 4 lo axle shift that's wonky

It's a 5-speed..

Mike B

NP-540 series trans have 4 shift rails, Al top cover, look just like 435 except 4 instead of 3 shift rails, 1 and R are to the side and fwd, longer throw, non synchronized.

Ed

Does the np 540 have an e-brake drum on it? I guess i could google all this stuff, but actually like da conversation? And what bell housing does it use? Or could it be adapted to a 305E?

The NP-540 IS synchronized in all gears except 1st and Reverse. Shift pattern has 1st, far left and up while Reverse is far right and up.

Mike B

Yes, the NP-540 would have a 9-1/2" x 2-1/2" band brake...

The NP-540 will bolt in anywhere a SM-420 would go...it was an option on the 5, 6 & 7000 Series trucks starting in 1956.

Mike B

Thanks for the detailed info Mike B. It might be exactly what i need for my 1ton. Why do we not hear more about this option for a trans change choice? Is there a bad side to this swap?

Be careful, 540 series trans came with long and short input shafts, new shafts are no longer available, short input shafts are harder to find. They came on IHC, Chevy, GMC, Ford, Dodge, Stude, etc, all with different mounting bolt patterns. Most yards don't know what they are, just need to get in the yard and search for oneself. Al top cover, 4 shift rails quicker sway to ID. Top of Al housing, steel pins shifthandle pivots on, pins get loose in housing-watch for.

Nice close ratio trans. Some available as OD in 5th, however, then large gap in ratios, awkward w/o 2 speed rear.

I put one in an early Bronco, worked really well, much better than the original 3 speed.


Ed

I see you have a np540 in your 47 gmc 1ton. Is it an od trans? Do you recommend it?

Oh Ed i just reread your last post. So the 5 speed is only the same as the 4 with an extra gear? No od.

Some come with OD, but that leaves a big gap in ratios where 3rd used to be, not so bad if one has a 2 speed rear, though.

Ed

Gotcha, thanks. Probably wont go that route unless i fall into 1. Was getting all excited to hear of a bolt in OD trans without mods. Lol