NP203 fluid and which driveline oils confirmation

[Jordi]

So i know i have the T350 auto box and i think i have an NP205 transfer case?

Edit: thanks for the responses. Confirmed as an NP203

Just gearing up to change the driveline oils and making sure i put the right stuff back in.

Its a frankentruck 83 K30, 6.2, Dana 60 front, 14bolt chevy rear.

You must be registered for see images attach

You must be registered for see images attach

Another edit…Please correct me if the below is incorrect as it can be a very expensive mistake.

Transmission TH350 is Dexron II
Transfer case (NP203 with part time conversion) is SAE30
Dana 60 is SAE 80W90 GL5
Chevy 14 bolt (with LSD) is SAE 80W90 NOT GL5 with LSD additive

Thanks
Jordi

 

[legopnuematic]

Looks to be an NP203.

 

[nvrenuf]

Definitely a 203. The required oil depends on if it’s still full time 4wd or converted to part time.

 

[Jordi]

Looks to be an NP203.

Further googling of “identify an NP203” and i think you are correct. There is no 3 bolt idler on mine as fitted to the 205.

 

[Jordi]

I have read about this conversion but have no idea if it’s been done.

It does have FWH fitted but may just have been driven in high lock position.

Is there a simple way to identify if the conversion has been done?

 

[nvrenuf]

FWH? Is that a reference to locking hubs?

Put the tcase in 2wd hi with the locking hubs unlocked, if it acts like it’s in neutral it’s still full time 4wd.

 

[nvrenuf]

To clarify, with the locking hubs unlocked, if you have to put the tcase in a “LOC” position to get it to move it is still full time 4wd.

You need to test it like this because lots of trucks had the drive flanges swapped for locking hubs without the part time conversion installed in the tcase because people didn’t want to take the tcase apart.

 

[Jordi]

FWH? Is that a reference to locking hubs?

Put the tcase in 2wd hi with the locking hubs unlocked, if it acts like it’s in neutral it’s still full time 4wd.

Yeah. Free wheel hubs / locking hubs.

I have messaged the previous owner asking if he knows, but all the work he had done was by a garage and they went bust many years ago.

Possibly done in America before it was imported to the UK

I’ll do some checks on it, although the lever doesn’t have a particularly positive feel for which gear its in.

 

[75gmck25]

GM recommended using 30W or 10/30 oil in the NP203. I use 10/30 full synthetic.

The stock NP203 is a full-time 4WD (or AWD) transfer case, with a front/rear differential built into the transfer case. In the Lo-Loc or Hi-Loc positions it locks the case differential and gives you conventional 4WD.

The NP203 was used by Chevy/Ford/Dodge in the early 70’s and 80’s, and was often blamed for the poor gas mileage of the trucks (although new smog devices should also be blamed). Milemarker then developed an inexpensive (about $125?) part-time case conversion that allowed you to switch between 2WD and 4WD. With the part time conversion, most folks also added front hubs that you could unlock to disengage the front axle.

In the Milemarker docs they told owners that after conversion they should run the truck at least every month in 4WD with locked hubs so that oil would be fully circulated through the case. I doubt that many ever paid attention to those directions.

If the NP203 shifts hard or imprecisely, you can usually fix it by careful cleaning and adjustment of the shifter linkage. Although many folks complain that it’s a chain driven case (implying that it’s less robust), internal breakage is uncommon. For example, I have never seen a post on this site in the last 10-15 years that was asking how to fix my completely broken NP203. It’s always questions about shifter adjustments, what oil, chain noise, has it been converted, etc.

 

[Jordi]

Some absolutely great info chaps.

This thing is a whole new ball game to me. Especially as its a mix and match truck, appears to have had mods done to it and has had quite a major rebuild about 16 years ago so I am approaching it with the thought, nothing may be as it seems.

I’m on cleaning and lubing the mechanism as it was very sticky and not very positive as to what position its in.

Gonna get it selecting nicely and then check if its had the conversion done or if i just happen to have locking front hubs.

 

[Jordi]

I have driven it with the front prop off but it may have been in Hi lock as the shifter was vague at best, given it a good clean, degunge, bathed it in WD40 and its a lot more positive.

So, basic test with it on stands as the front prop is off.

Spinning the rear prop

Hi lock (4Hi) - front flange turns
Hi (2WD) - front flange does not turn
Lo (2WD Lo) - front flange does not turn
Lo lock (4WD Lo) - front flange turns

Part time conversion done? Or because it’s essentially an open diff in Hi / Lo it would do that anyway?

Take it the best / definitive check is, even with the front prop still off, try and drive it in Hi / Lo. If it moves it’s converted, if the front flange just spins it isn’t converted?

 

[Jordi]

Smoked the shed out in the process.

In hi it moves with no prop on!

100% 2WD converted!

 

[Jordi]

So, as mentioned above I have done various tests. On various posts you guys have confirmed what my drive line is.

I’ve done the part time 4WD check, no prop on, in Hi it drives.

Had it jacked up and span the rear wheel, both turn in the same direction, LSD fitted.

I have been through my Chilton manual, searched posts and google, blah blah blah.

I just need lots of you to confirm the following is correct:

Transmission TH350 is Dexron II

Transfer case (NP203 with part time conversion) is SAE30

Dana 60 (could this also have a LSD???) is SAE 80W90 GL5

Chevy 14 bolt (with LSD) is SAE 80W90 NOT GL5 with LSD additive

Thanks

 

[idahovette]

Didn't know Dex II was still available...........I use DEX III or DEX VI. GM said to use whatever is in the engine for the t-case.....kept it simple.

 

[Jordi]

Didn't know Dex II was still available...........I use DEX III or DEX VI. GM said to use whatever is in the engine for the t-case.....kept it simple.

I can still get Dex II in the UK. Got a great oil & lubricants place near me.