Hello all,

I've recently encountered an issue with the transmission on my 76 F150 4WD with the NP435:

A couple weeks ago, after getting underway and shifting from 3rd to 4th for the first time, I got a severe grind/rake and refusal to go into gear. After returning to neutral, disengaging/re-engaging the clutch, it then went into 4th. It only did it on the first time going into 4th and shifted fine after that.

On subsequent drive, it shifted fine the entire time.

A few days ago, the same problem arose again, but his time it occurred more than the first time shifting into 4th.

I changed the fluid a couple months ago. I drained and refilled with Valvoline straight 50 weight.

I was inclined to believe it was the 3-4 synchro going out. But I've now have come upon some new info that makes me think otherwise. I found a shop manual for the NP435 (GM version) that talks about this issue. It says heavy oil in colder weather can cause the synchro to not work properly. It goes on to say that the synchro should return to working once the oil warms up.

The weather here has started getting cooler, but not cold. Temps in the Houston area seem to go back and forth this time of year. The straight 50 oil was pretty thick when I put it in during much warmer weather. So I can imagine it's much thicker now. All my research says straight 50 was the proper fluid for the NP435.

Has anyone else experienced problems like this that were resolved with a switch to regular 80-90W gear oil? I suppose it's worth trying another oil change before diving into a rebuild (which will be new territory for me).

Thank you in advance for any input.

 

Hey RJK671, I'm near Katy too!

I just put 80-90W in my transmission and transfer case (both of which I just rebuilt). I've yet to drive it as I am finishing up my exhaust but should be on the road very soon and I'll let ya know how its working.

I've read a bunch of threads on the oil too and there just seemed to be plenty of thumbs up for the 80-90w... So I just went with that... the cheap stuff from O'Reilly's.

I did read you don't want to use synthetic.

Post up a picture of your truck!

 

KIKKUP,

I'm not opposed to synthetic, especially if that was a solution, but I'm not sure it's worth the money in a NP435. They're not known for heat or other issues where synthetic can help - or so I've read.

I went with the straight 50 as it has no additives that can affect the yellow metals said to be in these gearboxes. Or am I confusing it with the t-case? Is there anything to avoid when selecting a gear oil?

I believe I've got some pics of my truck in my gallery. I haven't figured out how to put pics in a post.

 

You can run SOME synthetics in old gear boxes, it depends on the type of sulfates they use. Amsoil and Valvoline both work, Mobil 1 will not. I'm not sure on any of the other brands.

Most guys who rebuild them recommend 80w90.

I actually had 50wt in my np435 for a while and it shifted ok, but it was too thin for the tired old bearings. I put in 85w140 and the bearings were much quieter and it shifted smoother. It was grumpy for the first couple shifts in the morning, but after that it was night and day better.

I'd check the clutch an make sure it isn't dragging too.

Good luck!

Sam

 

sounds like a weak synchro or possibly a broken roll pin on shifter rail I have seen a broken roll pin in the shifter rail cause hard shifting .Heavy oil would effect more than one gear .I would still be leaning towards a bad 3rd gear synchro

 

That's correct, though most guys just run 90w gear oil (50w motor oil and 90w gear oil are the very same viscosity).

Avoid the use of GL-5 and use GL-4 instead (GL-5 has much higher levels of sulfur and phosphorus which can corrode the yellow metals in your tranny/t-case).

 

I'm running valvoline 85-140 in mine which works fine; but now I'm concerned because it says it's gl-5/gl-4 and I thought I was doing right by using the 85-140 limited slip valvoline.

 

Is the MT-1 certification on the label?

MT-1 indicates gear oil intended for NON-synchronized manual transmissions.

 

I read the pdf file it says "superior sulfur-phosphorus"...,
BUT on the main page it says
"All hypoid differentials (conventional and limited slip) and manual transmissions in passenger cars, light trucks, sport utility vehicles, vans and heavy-duty trucks where an API-GL5 or MT-1 fluid is specified."
so I'm not sure if it's good or not. Good info I have to pass more rep around or I'd add more.